Universität Stuttgart
|
|
- Reginald Richards
- 6 years ago
- Views:
Transcription
1 Universität Stuttgart Second Order Lagrange Multiplier Spaces for Mortar Finite Elements in 3D Bishnu P. Lamichhane, Barbara I. Wohlmuth Berichte aus dem Institut für Angewandte Analysis und Numerische Simulation Preprint 2003/007
2
3 Universität Stuttgart Second Order Lagrange Multiplier Spaces for Mortar Finite Elements in 3D Bishnu P. Lamichhane, Barbara I. Wohlmuth Berichte aus dem Institut für Angewandte Analysis und Numerische Simulation Preprint 2003/007
4 Institut für Angewandte Analysis und Numerische Simulation (IANS) Fakultät Mathematik und Physik Fachbereich Mathematik Pfaffenwaldring 57 D Stuttgart WWW: ISSN c Alle Rechte vorbehalten. Nachdruck nur mit Genehmigung des Autors. IANS-Logo: Andreas Klimke. L A T E X-Style: Winfried Geis, Thomas Merkle.
5 Second Order Lagrange Multiplier Spaces for Mortar Finite Elements in 3D BishnuP.Lamichhane andbarbarai.wohlmuth Abstract Domain decomposition techniques provide a flexible tool for the numerical approximation of partial differential equations. Here, we consider mortar techniques for quadratic finite elements in 3D with different Lagrange multiplier spaces. In particular, we focus on Lagrange multiplier spaces which yield optimal discretization schemes and a locally supported basis for the associated constrained mortar spaces. As a result, standard efficient iterative solvers as multigrid methods can be easily adapted to the nonconforming situation. We present the discretization errors in different norms for linear and quadratic mortar finite elements with different Lagrange multiplier spaces. Numerical results illustrate the performance of our approach. Key words. mortar finite elements, Lagrange multiplier, dual space, domain decomposition, nonmatching triangulation. AMS subject classification. 65N30, 65N55. Introduction The coupling of different discretization schemes or of nonmatching triangulations can be analyzed within the framework of mortar methods. These nonconforming domain decomposition techniques provide a more flexible approach than standard conforming approaches. The nonconforming approach is of particular interest in many situations, for example, in problems with discontinuous diffusion coefficients and local anisotropies, when different parameters dominate different parts of the simulation domain or different discretization schemes are used in different subdomains. A complex global domain can be decomposed into several small subdomains of simple structure, and these subdomains can be meshed independently. To obtain a stable and optimal discretization scheme for the global problem, the information transfer among the subdomains has to be analyzed. Mortar methods were originally introduced to couple spectral and finite element approximations,see[bdm90,bmp93].anoptimalaprioriestimateinthe H -normforthemortar finite element method has been established in[bmp93, BMP94, Ben99, BM97]. The analysis of three-dimensional mortar finite elements is given in[bm97, KLPV0, BD98], and a hpversionisstudiedin[ss00].inthelinear3dcase,thestabilityofthemortarprojection for the standard Lagrange multiplier space is established in[bd98], where also a multigrid ThisworkwassupportedinpartbytheDeutscheForschungsgemeinschaft,SFB404,C2. 5
6 methodforthesaddlepointproblemisdiscussed.themainideaofthemortartechniqueis to replace the strong continuity condition of the solution across the interface by a weak one. Here,weconsidermortarmethodsforsecondorderfiniteelementsin3D.WefocusonLagrange multiplier spaces which yield locally supported basis functions of the constrained mortar space. Thepaperisorganizedasfollows:Intherestofthissection,wepresentourmodelproblem and briefly review the mortar method. In Section 2, we give some sufficient conditions on Lagrange multiplier spaces for quadratic finite elements and show the optimality of the approach. In Section 3, we present some examples of Lagrange multiplier spaces for quadratic finite elements for hexahedral triangulations. In contrast to earlier approaches, we use Lagrange multiplier spaces yielding a sparse inverse of the mass matrix. We also use the so-called dual Lagrange multiplier spaces which are biorthogonal to the trace of the finite element space at the interface. Unfortunately, a locally defined dual Lagrange multiplier space containing the bilinear hat functions does not exist for the serendipity elements.inthatcase,weaugmentthespacebyfacebubblefunctions.finallyinsection4,we present some numerical results in 3D for different Lagrange multiplier spaces illustrating the flexibility and performance of our approach. In particular, we consider the discretizationerrorsinthe L 2 -norm,theenergynormandinaweighted L 2 -normforthelagrange multiplier. We consider the following elliptic second order boundary value problem div(a u) + cu = f, in Ω, u = 0, on Ω, () where 0 < a 0 a L (Ω), f L 2 (Ω), 0 c L (Ω),and Ω Ê 3 isabounded polyhedral domain. The domain Ω is decomposed into K non-overlapping polyhedral subdomains Ω k, k =,, K,suchthat Ω = K Ω k with Ω i Ω j = for i j. k= Here, we consider only geometrically conforming situations where the intersection betweentheboundariesofanytwodifferentsubdomains Ω l Ω k, k l,iseitherempty, acommonedgeoraface. Wedefine Γ kl := Ω k Ω l, k, l K,theintersection of the boundaries of two subdomains and select only disjoint and non-empty interfaces γ k, k N.Moreover,each γ k canbeassociatedwithacouple k < k 2 Ksuch that γ k = Ω k Ω k2.oneachsubdomain,wedefine H (Ω k ) := {v H (Ω k ), v Ω Ωk = 0}, k =,, K and consider the unconstrained product space X := K H (Ω k). k= To impose a weak matching condition on the interface, a Lagrange multiplier space M := N k= H /2 (γ k )isintroducedontheskeleton Γ = N γ k. k= 6
7 Then,theweakmatchingconditionontheskeleton Γisrealizedintermsofthe L 2 -orthogonality of the jump [u] of the solution across the interface and the Lagrange multiplier space M [u] µ dσ = 0, µ M. Γ Eachsubdomain Ω k isassociatedwithashaperegularfamilyofhexahedraltriangulations T k;hk,themeshsizeofwhichisboundedby h k.wedenotethediscretespaceofconforming piecewisetriquadraticfiniteelementsorofserendipityelementson Ω k associatedwith T k;hk by X hk H (Ω k ).Now,theunconstrainedfiniteelementspace X h canbewrittenas X h := K X hk. k= Eachinterface γ k inheritsatwo-dimensionaltriangulation S k;hk eitherfrom T k;h k or T k2;h k2. The subdomain from which the interface inherits its triangulation is called slave or nonmortarside,theoppositeonemasterormortarside. Inthefollowing,wedenotethe indexoftheslavesideby s(k)andtheoneofthemastersideby m(k). Hence,theelementsof S k;hk areboundaryfacesof T s(k);hs(k) withameshsizeboundedby h s(k). Furthermore, we assume that the mesh on Γ is globally quasi-uniform, and each element in S k;hk, k =,, N,canbeaffinelymappedtothereferenceelement ˆT := (0, ) (0, ). ThediscreteLagrangemultiplierspace M h on Γisdefinedas M h := N k= M h(γ k ).Then, thediscreteweakmatchingconditionfor v h X h canbewrittenas [v h ] µ i dσ = 0, i n k, k N, (2) γ k where n k := dimm h (γ k )and {µ i } i nk formsabasisof M h (γ k ). Here, [v h ]isthejump ofthefunction v h on γ k fromthemastersidetotheslaveside. Asusual, s,ωk and (, ) s,ωk denotethenormandthecorrespondinginnerproducton H s (Ω k ),respectively, and s,ωk standsfortheseminorm.thenormon H /2 00 (γ k)anditsdualspace H /2 (γ k ) willbedenotedby /2 H 00 (γ k) and /2,γ k,respectively.wedefinethebrokennorm s on Xandthebrokendualnorm M on Mby K u 2 s := u 2 s,ω k, k= N and µ 2 M := µ 2 /2,γ k, respectively. Therearetwomainapproachestoobtainthemortarsolution u h X h of a discrete variational problem. The first one is based on the positive definite variational problemontheconstrainedfiniteelementspacewhichisgivenbymeansoftheglobal Lagrangemultiplierspace M h V h := {v h X h b(v h, µ h ) = 0, µ h M h }, where b(v h, µ h ) := N k= γ k [v h ] µ h dσ.weremarkthattheelementsofthespace V h satisfy a weak continuity condition on the skeleton Γ in terms of discrete Lagrange multiplier space M h.however, V h is,ingeneral,notasubspaceof H0 (Ω).Then,thevariationalfor- mulationofthemortarmethodcanbegivenintermsoftheconstrainedspace V h : find u h V h suchthat a(u h, v h ) = (f, v h ) 0, v h V h. (3) k= 7
8 Here,thebilinearform a(, )isdefinedas a(v, w) := K k= Ω k a v w + cv w dx. The second approach is based on enforcing the weak continuity condition on the skeleton Γasanadditionalvariationalequationwhichleadstoasaddlepointproblemonthe unconstrainedproductspace X h,see[ben99]:find (u h, λ h ) X h M h suchthat a(u h, v h )+ b(v h, λ h ) = (f, v h ) 0, v h X h, b(u h, µ h ) = 0, µ h M h. (4) ItisclearthatthechoiceofthediscreteLagrangemultiplierspace M h playsanessential role for the stability of the saddle point problem and the optimality of the discretization scheme. The Lagrange multiplier space has to be large enough to obtain an optimal consistencyerror,andithastobesmallenoughtogetanoptimalbestapproximationerroranda suitable discrete inf-sup condition. In the next section, we state sufficient conditions on the Lagrange multiplier space for quadratic finite elements to get optimal a priori estimates. Here, the nodal Lagrange multiplier basis functions are defined locally and are associated withtheinteriornodesofthemeshon γ k, k =,, N. Now,wegroupthedegreesof freedomof X h associatedwiththeskeleton Γintotwogroups u h Γ := (u m, u s ),where u m containsallnodalvaluesof u h onthemastersidesandallnodalvaluesatthenodesonthe boundaryoftheinterface γ k ontheslavesides,and u s consistsofallnodalvaluesof u h at theinteriornodesof γ k ontheslavesides, k N,seeFigure. Theassociatedsets ofnodesarecalled N m and N s,respectively.furthermore,wedenoteby N h thesetofall nodesin X h andweset N i := N h \(N m N s ).Thecorrespondingnodalvaluesof u h in N i willbedenotedbyablockvector u i.then(2)canbewritteninitsalgebraicformas M s u s + M m u m = 0. (5) Theentriesofthemassmatricesaregivenby m ij := γ k [φ j ] µ i dσ,where φ j arethefinite u m u s master side slave side Figure:Decompositioninto u m and u s fortheserendipityelements elementbasisfunctionscorrespondingtothedifferentgroupsofnodes,and µ i denotethe basisfunctionsof M h. Sincethebasisfunctionshavealocalsupport,themassmatrices aresparse. Formally,wecanobtainthevaluesontheslavesideas u s = Ms M m u m. Although M s isasparsematrix,theinversionof M s is,ingeneral,expensive,and Ms is 8
9 dense. This observation motivates our interest in Lagrange multiplier spaces which yield a sparseinverseofthemassmatrix M s.anaturalchoiceisaduallagrangemultiplierspace, see,e.g.,[woh0],havingadiagonalmassmatrix M s.then,thebasisfunctions {µ i } i nk of M h (γ k )and {ϕ i } i nk ofthetracespace W 0,h (γ k )havingthezeroboundarycondition on γ k satisfythebiorthogonalityrelation µ i ϕ j dσ = δ ij ϕ j dσ, i, j n k. (6) γ k γ k If M s isasparsetriangularmatrix,theinverseof M s isalsoasparsetriangularmatrix. Hence,togetasparseinverseofmassmatrix M s,itisenoughtoworkwithalagrange multiplierspacewhichyieldsasparsetriangularmassmatrix M s.wedefinetheproduct space W 0,h as N W 0,h := W 0,h (γ k ), k= andthebroken H /2 00 -normon W 0,hisdenotedby v 2 W := N k= v 2 H /2 00 (γ k). 2 A priori estimates In this section, we give some assumptions on quadratic Lagrange multiplier spaces which guarantee optimal a priori estimates. Following a similar approach as in[klpv0], we impose the following assumptions on the discrete Lagrange multiplier spaces for quadratic finite elements [P0] dimw 0,h (γ k ) = dimm h (γ k ), k N. [P] There is a constant C independent of the triangulation such that inf µ M h (γ k ) v µ 0,γ k Ch 2 s(k) v 2,γ k, v H 2 (γ k ), k N. [P2] There is a constant C independent of the triangulation such that θ 0,γk C sup µ M h (γ k )\{0} (θ, µ) 0,γk µ 0,γk, θ W 0,h (γ k ), k N. Itfollowsfromassumption[P]that P (γ k ) M h (γ k )forall k =,, N,where P (γ k ) isthespaceoflinearfunctionson γ k. Foreach γ k,themortarprojection Π k : L 2 (γ k ) W 0,h (γ k )isdefinedas Π k v µ dσ = γ k v µ dσ, µ M h (γ k ). γ k (7) The stability of the mortar projection is essential for the optimality of the best approximation error. 9
10 Lemma.Undertheassumptions[P0]and[P2],themortarprojection(7)isstableinthe L 2 -norm. Furthermore,if w H 0(γ k ) Π k w,γk C w,γk. Proof: Byassumption[P2],wefindthatif v W 0,h (γ k )satisfies (v, µ) 0,γk = 0forall µ M h (γ k ),then v = 0.Hence,themortarprojectioniswell-definedbytheassumptions [P0]and[P2].The L 2 -stabilityof Π k followsfrom Π k w 0,γk C sup µ M h (γ k )\{0} (Π k w, µ) 0,γk (w, µ) 0,γk = C sup C w 0,γk. µ 0,γk µ M h (γ k )\{0} µ 0,γk Now,usingthe L 2 -stabilityandaninverseestimate,wefindfor w H0 (γ k) ( ) Π k w,γk Π k w Pw,γk + Pw,γk C Π k (w Pw) 0,γk + w,γk h s(k) ( ) C w Pw 0,γk + w,γk C w,γk, h s(k) where Pdenotesthe L 2 -projectiononto W 0,h (γ k ). UsingLemmaandaninterpolationargument,weobtainfor w H /2 00 (γ k), Π k w H /2 00 (γ k) C w H /2 00 (γ k). Inanextstep,weprovidethebestapproximationpropertyofthespace V h. Weusethe ideas and techniques introduced in[bm97, BMP93]. Lemma2. Assumethattheassumptions[P0] [P2]hold.If u H 0 (Ω)and u Ω k H 3 (Ω k )for all k =,, K,thenthereexistsaconstant Cdependingonlyontheratioofthemeshsizesofthe master and slave sides such that inf u h V h u u h 2 C K h 4 k u 2 3,Ω k. k= Proof: Since W 0,h (γ k ) H /2 00 (γ k),each v W 0,h (γ k )cantriviallybeextendedtoafunction ṽ H /2 ( Ω s(k) ). Let H h ṽ H (Ω s(k) )bethediscreteharmonicextensionof ṽon Ω s(k). Then, H h ṽ,ωs(k) C ṽ H /2 ( Ω s(k) ) C v H /2 00 (γ k). Bymeansofthisdiscrete harmonicextension,wedefineadiscreteextensionoperator E k : W 0,h (γ k ) X h foreach γ k as E k v := H h ṽon Ω s(k),and E k v := 0elsewhere.Then E k v C v H /2 00 (γ k), v W 0,h(γ k ). (8) Let l h u X h bethelagrangeinterpolantof uin X h. Itiseasytoseethat v := l h u + N k= E kπ k [l h u]isanelementof V h.then,wefind N u v u l h u + E k Π k [l h u]. 0 k=
11 Byusing(8)andacoloringargument,wehave N K E k Π k [l h u] 2 = N N E k Π k [l h u] 2,Ω l C Π k [l h u] 2. H /2 00 (γ k) k= l= k= Wenotethattheconstant Cdoesnotdependonthenumberofsubdomains.Applyingthe L 2 (γ k )-stabilityof Π k andaninverseestimate,weget Π k [l h u] 2 H /2 00 (γ k) C h s(k) [l h u] 2 0,γ k k= k= k= C ( ) (u l h u) Ωm(k) 2 0,γ h k + (u l h u) Ωs(k) 2 0,γ k s(k) C ( ) h 5 m(k) h u 2 3,Ω m(k) + h 5 s(k) u 2 3,Ω s(k). s(k) Summingoverall k =,, N,weobtain ( ) N N N E k Π k [l h u] 2 C h mr h 4 m(k) u 2 3,Ω m(k) + h 4 s(k) u 2 3,Ω s(k), where { hm(k) h mr := max, h s(k) k= } k N. Finally,thelemmafollowsbyusingtheinterpolationpropertyof l h u. Weremarkthatincontrasttoaconvergencetheoryofmortarfiniteelementsin2D,the constantintherighthandsidedependsontheratioofthemeshsizesofmasterandslave sides.however,ifthemeshonthewirebasketisconformingand uiscontinuous,wefind [l h u] H /2 00 (γ k)andthusthe H /2 00 -stabilityof Π kcanbedirectlyapplied. Inthatcase, the ratio does not enter in the upper bound, see[klpv0]. Working with mesh dependent normsandatrivialextensionshowsthattheglobalratio h mr canbereplacedbyalocal one. Theorem3. Let uand u h bethesolutionsoftheproblems()and(3),respectively.assumethat u H0(Ω), u Ωk H 3 (Ω k )for k =,, K,and [a u n ] = 0on Γ. Undertheassumptions [P0] [P2], there exists a constant C depending only on the ratio of the meshsizes of the master and the slave sides such that K u u h 2 C h 4 k u 2 3,Ω k. Proof: Thebilinearform a(, )iscontinuouson X,anditiscoerciveon { } B := v v H (Ω k ), k K,and [v] dσ = 0, k N, γ k k= see[bmp93].hence,assumption[p]assuresthat V h B.Thus,Strang slemma[bs94] canbeapplied,andweget ( ) a(u u h, v h ) u u h C inf u v h + sup. (9) v h V h v h v h V h \{0}
12 Thefirsttermintherightsideof(9)denotesthebestapproximationerrorandthesecond one stands for the consistency error. Lemma 2 guarantees the required order for the best approximation error. Thus it is sufficient to consider the consistency error in more detail. Now, a(u u h, v h )canbewrittenas a(u u h, v h ) = Γ a u n [v h] dσ = N k= ( a u ), [v h ], v h V h. n k 0,γ k Here, u u nistheoutwardnormalderivativeof uon Γfromthemasterside,and n = u n k on γ k.wetake µ M h (γ k ),thenthedefinitionof V h yields ( a u ) (, [v h ] = a u ) µ, [v h ]. n k 0,γ k n k 0,γ k Since µ M h (γ k )isarbitrary,wecanboundtheconsistencyerrorby ( a u ) u, [v h ] inf a µ n k 0,γ µ M h (γ k ) k n (H /2 (γ k k )) [v h] /2,γk. Let Q : L 2 (γ k ) M h (γ k )bethe L 2 -projectiononto M h (γ k ).Usingassumption[P2]andan interpolationbetween L 2 (γ k )and H 2 (γ k ),weobtain w Qw 0,γk Ch 3/2 s(k) w 3/2,γ k, w H 3/2 (γ k ). In terms of a standard Aubin Nitsche trick and the previous estimate, we get w Qw (H /2 (γ k )) Ch/2 s(k) w Qw 0,γ k Ch 2 s(k) w 3/2,γ k. Finally, the trace theorem yields ( a u ), [v h ] Ch 2 u s(k) n k 0,γ k n k [v h ] /2,γk 3/2,γk Ch 2 s(k) u 3,Ω s(k) ( vh,ωm(k) + v h,ωs(k) ). Now,usingtheCauchy Schwarzinequalityandsummingoverall k =,, N,weobtain ( K ) /2 a(u u h, v h ) C v h h 4 k u 2 3,Ω k. k= To obtain an a priori estimate for the Lagrange multipliers, we follow exactly the same procedures as in[ben99]. Lemma4.AssumethattheLagrangemultiplierspace M h satisfiesassumptions[p0] [P2].Then, forevery µ M h,thereexistsav µ X h suchthat v µ C µ M, µ 2 M Cb(v µ, µ) and [v µ ] W C µ M. 2
13 Proof: Bymeansofthestabilityofthemortarprojection,wegetfor µ M h (γ k ) µ /2,γk = sup ϕ H /2 00 (γ k)\{0} = C sup ϕ W 0,h (γ k )\{0} (µ, ϕ) 0,γk ϕ /2 H 00 (γ k) (µ, ϕ) 0,γk ϕ H /2 00 (γ k) C sup ϕ H /2 00 (γ k)\{0} (µ, Π k ϕ) 0,γk Π k ϕ H /2 00 (γ k) C(µ, ϕ k ) 0,γk (0) forsome ϕ k W 0,h (γ k )with ϕ k H /2 00 (γ k) =.Now,weextend ϕ k W 0,h (γ k )to X h by usingtheextensionoperator E k asdefinedinlemma2toget E k ϕ k =: v k X h.then,we have v k C ϕ k H /2 00 (γ k) and 0 (µ, ϕ k ) 0,γk = b(v k, µ). Setting v µ := N k= b(v k, µ)v k andusingthefactthat v k C,weget N K N v µ 2 = b(v k, µ)v k 2 = b(v k, µ)v k 2,Ω l k= k= l= k= N N C b(v k, µ) 2 C µ 2 /2,γ k = C µ 2 M. This proves the first assertion. Using a coloring argument, the constant C does not depend on the number of subdomains. Furthermore, summing the equation(0) over all interfaces γ k, k =,, N,weobtainthesecondassertion µ 2 M C k= N b(v k, µ) 2 = Cb(v µ, µ). k= Finally, the third assertion follows from [v µ ] 2 W = N b(v k, µ) 2 [v k ] 2 = N b(v H /2 00 (γ k, µ) 2 k) k= k= N k= µ 2 /2,γ k [v k ] 2 H /2 00 (γ k) = µ 2 M. The best approximation property, Lemma 4, assumption[p2] and the first equation of the saddle point problem give an a priori estimate for the Lagrange multipliers. Lemma5.UndertheassumptionsofTheorem3,wehave λ λ h 2 M C K h 4 k u 2 3,Ω k. k= 3 Quadratic Lagrange multiplier spaces in 3D In this section, we consider different possibilities for Lagrange multiplier spaces in 3D for quadraticfiniteelementswithsuppϕ i =suppµ i.inparticular,wefocusonthestandard finite elements and the serendipity elements and restrict ourselves to hexahedral triangulations. These two finite elements have different degrees of freedom on the interface and therefore, the Lagrange multiplier spaces should be considered separately. 3
14 3. A dual Lagrange multiplier space for the standard triquadratic finite elements In the case of a hexahedral triangulation, a dual Lagrange multiplier space in 3D for trilinearandtriquadraticfiniteelementscanbeformedbytakingthetensorproductofthedual Lagrangemultiplierspacein2D.Let ˆϕ 0, ˆϕ and ˆϕ 2 bethenodalquadraticfiniteelement basisfunctionsonthereferenceelement (0, )inonedimension,where ˆϕ 0 and ˆϕ arethe basis functions corresponding to the left and the right vertices of the reference element, and ˆϕ 2 isthebasisfunctioncorrespondingtothemidpointofthereferenceelement.then,the quadratic dual Lagrange multiplier basis functions on the reference element are defined by ˆλ 0 (t) := ˆϕ 0 (t) 3 4 ˆϕ 2(t) + 2, ˆλ (t) := ˆϕ (t) 3 4 ˆϕ 2(t) + 2 and ˆλ 2 (t) := 5 2 ˆϕ 2(t). The Lagrange multiplier basis functions for the element touching a crosspoint should be modified.inparticular,if t = 0isacrosspoint,wehave andif t = isacrosspoint,weset ˆλ 2 (t) := 2t + 2, ˆλ (t) := 2t, ˆλ 2 (t) := 2t, ˆλ0 (t) := 2t. Furthermore,forahatfunction φ l pataninteriorvertex p,wefind φ l p(t) = µ p (t) + 2 (µ e(t) + µ e2 (t)), () where µ p isalagrangemultipliercorrespondingtothevertex pand µ e and µ e2 arethe basis functions associated with the midpoints of the two adjacent edges. If p is a crosspoint, wehave φ l p(t) = 2 µ e(t),where µ e isthelagrangemultiplierbasisfunctioncorresponding to the midpoint of the edge containing the crosspoint. Then, the Lagrange multiplier basis functionsonthereferenceelement ˆT = (0, ) (0, )havingatensorproductstructureare defined as ˆλ ij (x, y) := ˆλ i (x)ˆλ j (y). Here, ˆλ 00 (x, y), ˆλ 0 (x, y), ˆλ (x, y)and ˆλ 0 (x, y)arethelagrangemultiplierscorrespondingtothefourvertices (0, 0), (, 0), (, )and (0, ),and ˆλ 20 (x, y), ˆλ2 (x, y), ˆλ2 (x, y)and ˆλ 02 (x, y)aretheonescorrespondingtothemidpoints (0.5, 0), (, 0.5), (0.5, )and (0, 0.5) ofthefouredges,respectively,andfinally ˆλ 22 (x, y)istheonecorrespondingtothecenter of gravity (0.5, 0.5) of the reference element. The Lagrange multiplier basis functions are associated with the vertices, midpoints of the edges and the center of gravity of elements in S k;hk, k N.Theglobalbasisfunctions µ i areobtainedbyusinganaffinemapping andgluingthelocalonestogether.allnodesontheboundary γ k of γ k arecrosspointsand donotcarryadegreeoffreedomforthelagrangemultiplierspace.wenotethatwehave to use the modification at the crosspoints to compute the tensor product for the Lagrange multiplierscorrespondingtotheelementstouching γ k.observing(),wefindthatthe bilinearhatfunctionateachvertexiscontainedinthelagrangemultiplierspace M h (γ k ). Wepointoutthatthisisalsovalidon γ k,althoughtherearenodegreesoffreedom.hence, assumption[p] is satisfied. Assumption[P0] is trivially satisfied by construction. Now, 4
15 weverifyassumption[p2].let ϕ := n k k= a kϕ k bein W 0,hk (γ k )andset µ := n k k= a kµ k.in thefollowing,weassumethat ˆϕ i and ˆµ i areobtainedfrom ϕ i and µ i byanaffinemapping fromtheelement Ttothereferenceelement ˆT.Now,byusingthebiorthogonalityrelation (6) and the quasi-uniformity assumption, we get (ϕ, µ) 0,γk = n k i,j= n k a i a j (ϕ i, µ j ) 0,γk = i= a 2 i n k ϕ i dσ C a 2 i h2 s(k) C ϕ 2 0,γ k. γ k Takingintoaccountthefactthat ϕ 2 0,γ k µ 2 0,γ k n k i= a2 i h2 s(k),wefindthatassumption [P2] is satisfied. Figure 2 shows the three different types of Lagrange multipliers on the reference element. i= Figure 2: The Lagrange multipliers corresponding to a vertex(left), to an edge(middle) andtothecenterofgravity(left) 3.2 Serendipity elements and a dual Lagrange multiplier space Here, we give a non-existence result for a dual Lagrange multiplier space for the serendipity elements. A similar result for simplicial triangulations and quadratic finite elements is givenin[lw02].wedenoteby W h (γ k)thefiniteelementspaceofpiecewisebilinearhat functionson γ k. Incaseofstandardtriquadraticfiniteelements,thedualLagrangemultiplierspacewithtensorproductstructurecontains W h (γ k). Unfortunately,thereexists no dual Lagrange multiplier space for the serendipity elements yielding optimal a priori estimateswithsuppϕ i =suppµ i,where ϕ i aretheserendipitynodalfiniteelementbasis functionsontheinterface γ k. Lemma6.Undertheassumptionthatsuppϕ i =suppµ i,thereexistsnoduallagrangemultiplier space M h (γ k )suchthat W h (γ k) M h (γ k ). Proof: We prove this by contradiction. Assume that α i µ i = φ l p, (2) i 5
16 where φ l pisthebilinearhatfunctionassociatedwiththeinteriorvertex phavingthecoordinates (0, 0),seeFigure3.Supposethecoordinatesofthefourcornersoftheelement T be (, 0), (0, 0), (0, )and (, ),andoftheelement T 2 be (0, 0), (, 0), (, )and (0, ). ϕ j0 at (0, ) T T 2 φ l pat (0, 0) Figure 3: 2D interface of 3D hexahedral triangulation Becauseoftheduality,thefunctions µ i arebiorthogonaltothefiniteelementbasisfunctions ϕ i ontheinterface.hence,aftermultiplying(2)bysomefiniteelementbasisfunction ϕ j andintegratingovertheinterface γ k,weget α j = γ k ϕ j φ l p dσ γ k ϕ j dσ. Let j 0 betheinteriorvertexwithcoordinates (0, )suchthat j 0 and pshareoneedge,see Figure3.Then,wecanwrite ϕ j0 φ l p dσ = ϕ j0 φ l p dσ + ϕ j0 φ l p dσ = γ k T T 2 8 andthus α j0 0.Sincethebasisfunctions µ i arelocallylinearlyindependent,weobtain suppµ j0 supp i α iµ i.byconstruction,wefindsuppµ j0 suppφ l p,whichcontradicts (2). 3.3 Lagrange multiplier spaces for the serendipity elements The previous subsection shows that there does not exist a dual Lagrange multiplier space for the serendipity elements containing the bilinear hat function at each vertex and satisfyingsuppϕ i =suppµ i. Here,weconsidertwodifferentLagrangemultiplierspacesfor the serendipity elements. The essential point is that the Lagrange multiplier space should lead to an optimal and stable discretization scheme. For this purpose, the assumptions [P0] [P2] are crucial. The first idea is to choose a standard Lagrange multiplier space, see [BMP93,BMP94].Inthiscase,thebasisfunctionsforeachinteriorelement T S k;hk ofthe interface γ k (i.e., T S k;hk with T γ k = )aretheserendipityelementsin2d.allnodes on γ k donotcarryadegreeoffreedomforthelagrangemultipliers.therefore,inorder to satisfy assumption[p], it is necessary to modify the definition of the basis functions for theelementstouchingtheboundary γ k oftheinterface γ k.supposeanelement T S k;hk with T γ k has ndegreesoffreedomforthelagrangemultipliers. Then,thelocalLagrangemultiplierbasisfunction µ i atanode x i of Tischosentobeapolynomial 6
17 ofminimaldegreesuchthat µ i (x j ) = δ ij forall x j, j =,, n. Here, δ ij isthekronecker delta. These Lagrange multiplier basis functions are continuous. Since, in general, M h (γ k ) ( H /2 (γ k ) ),wecanworkwithdiscontinuouslagrangemultipliers. Working with a continuous Lagrange multiplier space which locally contains the linear functions has the advantage that assumption[p] is satisfied. Here too, assumption[p0] is trivially satisfiedbyconstruction.toverifyassumption[p2],wetake ϕ := n k k= a kϕ k in W 0,h (γ k ) anddefine µ := n k k= a kµ k.then (ϕ, µ) 0,γk = M ˆT = n k i,j= a i a j (ϕ i, µ j ) 0,γk = n k i,j= a i a j γ k ϕ i µ j dσ. Thelocalmassmatrix M ˆTforanelementhavingalltheverticesin γ k isgivenby Similarly, computing the local mass matrices for the different boundary cases, we find that alltheeigenvaluesofthelocalmassmatricesaregreaterthan 00 andsmallerthan 6.Then (ϕ, µ) 0,γk, ϕ 2 0,γ k and µ 2 0,γ k areequivalentto n k i= h2 s(k) a2 i,whichguaranteesassumption [P2].Thecouplingofthelocalmassmatricesyieldaglobalmassmatrixwhichissparsebut hasabandstructureofband-width O(/h).Thus,theinverseoftheglobalmassmatrix M s ontheslavesideisdense.asaconsequence,weobtainastiffnessmatrixassociatedwith the variational problem(3), which is not sparse. Then we cannot apply static condensation, and the multigrid method discussed in[wk0] cannot be used. To overcome this difficulty, we introduce a new approach for the serendipity elements. The ideaistousealagrangemultiplierspacewhichyieldsasparseinverseoftheglobalmass matrix M s ontheslaveside. Inthiscase,weusethetensorproductLagrangemultiplier spaceintroducedinsubsection3.. Tosatisfythecondition dimw 0,h (γ k ) = dimm h (γ k ), eachnon-emptyface f T Γoftheelement T oftheslavesideisenrichedwitha bubblefunction.thebubblefunction b H (T)correspondingtotheface fof Thasthe propertythat b T \f = 0and f b dσ 0.Wedefine K s := {T T s(k);hs(k), k N T Γcontainsatleastafaceof T },thesetofelementsontheslavesideeachofwhichhasat leastoneface fon Γ.Now,thespaceofbubblefunctions B h isformedby N s bubbles,where N s isthenumberofelementsin N k= S k;h k,andeachofthemiscorrespondingtoaface f ofanelement T K s,where f T Γ.Thisleadstooneadditionaldegreeoffreedom foreachnon-emptyface f T Γof T K s. Therearemanypossibilitiestodefine such a bubble function. Here, the triquadratic nodal finite element function associated withthecenterofgravityofthefaceisusedasabubblefunctioncorrespondingtothis face. Although we need only the restriction of bubble functions to the associated face to satisfy assumption[p0], each bubble function is supported on the whole element. Now, the
18 modifiedunconstrainedproductspace Xh tcanbewrittenas Xt h = Xs h B h,where Xh sis the unconstrained product space associated with the serendipity elements. In the sequel, thespace Xh twillbecalledaugmentedserendipityspaceandthecorrespondingelements augmentedserendipityelements.thisleadstoamassmatrix M s ontheslavesidehavinga specialstructure.suppose ˆϕ i and ˆλ i, ( i 9)bethelocalbasisfunctionsofthestandard triquadratic finite elements and their dual Lagrange multipliers, respectively. Here, the first four basis functions correspond to the vertices, the second four ones correspond to themidpointsoftheedges,andthelastonecorrespondstothecenterofgravityofthe referenceelement ˆT. Then,thelocalbasisfunctionsoftheserendipityelementscanbe writtenas ˆϕ s i = ˆϕ i + α i ˆϕ 9, ( i 8),where α i = 4 for ( i 4)and α i = 2 for (5 i 8).Usingthebiorthogonalityof ˆϕ i and ˆλ i,wehave ˆϕ s ˆλ i j dσ = (ˆϕ i + α i ˆϕ 9 ) ˆλ j dσ = δ ij ˆϕ i dσ + α i δ 9j ˆϕ 9 dσ. ˆT ˆT ˆT Infact,themassmatrixonthereferenceelement ˆTis M ˆT = (3) To show the consequence of our new Lagrange multiplier space, we consider the global massmatrix M s ontheslavesideinmoredetail.inthefollowing,weusethesamenotation forthevectorrepresentationofthesolutionandthesolutionasanelementin Xh tand M h. Thematrix Aisthestiffnessmatrixassociatedwiththebilinearform a(, )on Xh t Xt h,and thematrices Band B T areassociatedwiththebilinearform b(, )on Xh t M h.then,the algebraic formulation of the saddle point problem(4) is given by [ ] [ ] [ ] A B T uh fh =. (4) B 0 λ h 0 Werecallthegroupingofthedegreesoffreedomof Xh t introducedinsection. After augmentingtheserendipityspacewiththespaceofbubblefunctions B h,wefurtherdecomposethedegreesoffreedomassociatedwiththeinteriornodesof γ k, k N,on theslavesideintotwogroups (u s, u b ).Here,theblockvector u s containsnodalvaluesof uattheinteriornodesof γ k, k N,correspondingtotheverticesandedgesonthe slaveside,and u b standsforallnodalvaluescorrespondingtothebubblefunctionsonthe slaveside.withthisdecomposition,wecanwrite u T h = (ut i, ut m, u T s, u T b ).Theblockvector λ h containingthenodalvaluesofthelagrangemultiplierissimilarlydecomposedwith λ T h = (λt s, λt b ). Intermsofthisdecomposition,wecanrewritethealgebraicformofthe ˆT 8
19 saddle point problem(4) as A ii A im A is A ib 0 0 A mi A mm A ms A mb Mm T MT bm A si A sm A ss A sb D s Mbs T A bi A bm A bs A bb 0 D b 0 M m D s M bm M bs D b 0 0 u i u m u s u b λ s λ b = f i f m f s f b 0 0. (5) Recallingthealgebraicstructure(5)ofthebilinearform b(, )restrictedto Xh t M h,we have [ ] Mm D s 0 B =, M bm M bs D b where D b and D s arediagonalmatrices,and M bs, M m and M bm arerectangularmatrices. Thematrix D b isdiagonalduetothefactthatthebubblefunctionsaresupportedonly inoneelement,andthediagonalformof D s followsfromthestructureofthelocalmass matrix,see(3).hence,theglobalmassmatrix M s ontheslavesidecanbewrittenas [ ] Ds 0 M s =. M bs ThegreatbenefitofthisLagrangemultiplierspaceisthattheinverseofthemassmatrix M s canbecomputedveryeasily,andtheinverseissparse.infact,theinverseofthemass matrix M s is [ ] M s = D s D b D b M bs Ds 0 D b Thus,thesolutionattheslavesidedependslocallyonthesolutionatthemasterside.Here, wehavetoinvertonlytwodiagonalmatricesandscale M bs tocomputetheinverseofthe massmatrix M s.thestiffnessmatrixassociatedwiththevariationalproblem(3)issparse, and efficient iterative solver like multigrid can easily be adapted to the nonconforming situation. Additionally, the condition number of the mass matrix is better compared to the standard Lagrange multiplier space. Furthermore, the degree of freedom corresponding to thebubblefunctionscanlocallybeeliminatedbystaticcondensation.sincethematrix D b isdiagonal,thesixthandthefourthlineofthesystem(5)give u b = D b (M bm u m + M bs u s ), and λ b = D [ b fb A bi u i (A bm A bb D b M bm )u m (A bs A bb D ] b M bs )u s. Now,weeliminate u b and λ b fromthesystem(5)andobtainanewsystem Âû h = ˆF h, where û T h = (ut i, ut m, ut s, λt s ).Defining M := D b M bm and M 2 := D b M bs wehave A ii A im A ib M A is A ib M 2 0 Â = A mi M T A bi A mm A mb M M T (A bm A bb M ) A ms A mb M 2 M T (A bs A bb M 2) Mm T A si M2 T A bi A sm A sb M M2 T (A bm A bb M ) A ss A sb M 2 M2 T (A bs A bb M 2) D s, 0 M m D s 0. 9
20 andtherighthandsidecanbewrittenas ˆF h = f i f m M T f b f s M T 2 f b 0 Weobservethatthematrix Âissymmetric,if Aissymmetricandithasexactlythesame structure as the saddle point matrix arising from mortar finite element method with a dual Lagrange multiplier space, see[woh0]. Because of this structure of the algebraic system, we can apply the multigrid method proposed in[wk0]. Remark7.Thereisalsoapossibilitytousewaveletstogetamassmatrixofspecialstructuresothat the inversion can be cheaper, and the inverse is sparse. In[Ste00], locally supported and piecewise polynomial wavelets are studied on non-uniform meshes which give a lower triangular mass matrix with higher order finite elements in triangular meshes.. 4 Numericalresults Here, we present some numerical examples in 3D for linear and quadratic mortar finite elements. We consider three different cases for quadratic mortar finite elements. The first one is the standard triquadratic finite element space with the dual Lagrange multiplier space introduced in Subsection 3.. The second one is the serendipity space with a standard Lagrange multiplier space given in Subsection 3.2. Finally, the third one is the augmented serendipity space associated with the tensor product Lagrange multiplier space. Our numerical results show the same asymptotic behaviour as predicted by the theory. Theimplementationisbasedonthefiniteelementtoolboxug,[BBJ + 97]. Wedonotdiscuss and analyze an iterative solver for the arising linear systems. Working with dual Lagrange multiplier spaces has the advantage that the flux can locally be eliminated, and static condensation yields a positive definite system on the unconstrained product space. In[WK0,Woh0],themodificationofthesystemhasbeencarriedoutandalocalmodification of the transfer operators of lower complexity has been proposed. The introduced multigrid has a level-independent convergence rate and is of optimal complexity. Unfortunately, in the case of a standard Lagrange multiplier space no local elimination of the flux can be carried out. Following the approach in[wk0], the sparsity of the modified systemandtheefficiencyofthemultigridsolverislost.inthatcase,weapplyamultigrid method for saddle point problems. This technique has been considered for mortar elements in[ww98]andfurtheranalyzedin[bdw99,ww02].itturnsoutthatwedonothaveto workinapositivedefinitesubspace,andthesmoothercanberealizedinaninnerandouter iteration scheme. As in the other approach, level-independent multigrid convergence rates can be established. However, the numerical solution process is slower if we have to work with the saddle point approach. We point out that the more efficient multigrid method for themodifiedpositivedefinitesystemcanonlybeappliedwhentheinverseof M s issparse, whereasthesaddlepointmultigridmethodismoregeneral.althoughwedonothavea dual Lagrange multiplier space for the serendipity elements, the tensor product Lagrange multiplier space in combination with the introduction of bubble functions on the slave side of the interface yield an optimal discretization scheme. We present some numerical results in 3D illustrating the performance of the different Lagrange multiplier spaces. In particular, 20
21 wecomparethediscretizationerrorsinthe L 2 -and H -normforthesolutionforlinearand quadratic mortar finite elements. The discretization errors in the flux across the interface are compared in a mesh-dependent Lagrange multiplier norm, which is defined by µ µ h 2 h := N m= T S m;hm h T µ µ h 2 0,T, where h T isthediameteroftheelement T.Forallourexamples,wehaveuseduniformrefinement. In each refinement step, one element is refined into eight subelements. We denote by X l h and Xf h theunconstrainedfiniteelementspacesassociatedwiththestandardfinite elementspacesforthetrilinearandthetriquadraticcase,respectively. Similarly, X s h and X t h aretheunconstrainedfiniteelementspacesassociatedwiththeserendipityelements and the augmented serendipity elements as defined in the previous section, respectively. Thecorrespondingsolutionsaredenotedby u l h, uf h, us h and ut h,respectively. Remark 8. We note that the concept of dual Lagrange multiplier spaces can be generalized to distorted hexahedral meshes. In that case, the mapping between the actual element and the reference elementhasanon-constantjacobian. Asaconsequence,wehavetocomputeforeachfaceonthe interface a biorthogonal basis with respect to local nodal one. This can be easily done by solving a local mass matrix system. By construction, the sum of the local dual Lagrange multiplier basis functions is one. Defining the global Lagrange multiplier basis functions by gluing the local ones together, we find that the constants are included in the Lagrange multiplier space. As a consequence, itiseasytoverifythatthediscretizationerrorisoforder hforthelowestorderconformingfinite elements. Ourfirstexampleisgivenby u = f on Ω := (0, ) 2 (0, 2),where Ωisdecomposedintotwosubdomains Ω := (0, ) 3 and Ω 2 := (0, ) 2 (, 2). Therighthand side fandtheboundaryconditionsarechosensuchthattheexactsolutionisgivenby u(x, y, z) = 2 exp( x 2 y 2 z 2 )sin(2 yx)(x + y + z)(x y z).infigure4,thedecomposition of the domain, the initial triangulation and the isolines of the solution at the interface z = are shown. We have used a nonconforming initial triangulation, where the Figure 4: Decomposition of the domain and initial triangulation(left), isolines of the solution at the interface(right), Example lowercubeisrefinedonce,andtheuppercubeisleftasitis.theerrorsinthe L 2 -, H -and the weighted Lagrange multiplier norms are given in Tables 3. The tables show that we getthepredictedasymptoticconvergencerates.forthequadraticcase,theerrorsinthe L 2-2
22 and H -normareoforder h 3 and h 2,whereastheyareoforder h 2 and hforthelinearcase. The errors in the weighted Lagrange multiplier norm for the quadratic and linear cases areoforder h 5/2 and h 3/2,respectively.Theoretically,theerrorsintheweightedLagrange multipliernormforquadraticandlinearcaseareexpectedtobeoforder h 2 and h,respectively. Thebetterconvergenceratesareduetothefactthattheerrorinthe H -normis equallydistributedandthelagrangemultiplierspacehasan O(h 5/2 )and O(h 3/2 )approximation property in the considered norm. For the three different second order approaches, thequantitativeresultsareasymptoticallyalmostthesameinthe L 2 -and H -norm.only in the weighted Lagrange multiplier norm, the full triquadratic yields better results. Table:Discretizationerrorsinthe L 2 -norm,(example) level #elem. u u l h 0 u u f h 0 u u s h 0 u u t h e e-0 9.2e e e e e e e e e e e e e e e e e e-04 Table2:Discretizationerrorsinthe H -norm,(example) level #elem. u u l h u u f h u u s h u u t h e e e e e e e e e e e e e e e e e e e e-02 Table 3: Discretization errors in the weighted Lagrange multiplier norm,(example ) level #elem. λ λ l h h λ λ f h h λ λ s h h λ λ t h h e e e e e e e e e e e e e e e e e e e e-02 InExample2,wechooseaL-shapeddomain.Thedomain Ω := ((0, ) 2 (0, 2)) ([, 2) (0, ) 2 )isdecomposedintothreecubes, Ω := (0, ) 3, Ω 2 := (0, ) 2 (, 2)and Ω 3 := (, 2) (0, ) 2.Wehaveshownthedecompositionofthedomainandtheinitialtriangulationinthe leftpictureoffigure5,andtheisolinesofthesolutionattheinterface z = areshownin theright.asbefore,wesolveapoissonproblem u = f 22
23 with the right hand side function f and the Dirichlet boundary conditions determined by ( theexactsolution u(x, y, z) = (x ) 2 + (z ) 2) 5/6 ( cos 6 y 2 + x ). Figure 5: Decomposition of the domain and initial triangulation(left), isolines of the solutionattheinterface z = (right),example2 WehavetabulatedtheerrorsindifferentnormsinTables4 6. Here,thesolutionisnot H 3 -regular.therefore,wecannotexpectthesameorderofconvergenceasintheprevious example.sincethesolution u H 8/3 ǫ (Ω)for ǫ > 0,weexpectthesameorderofconvergence as in the previous example in the linear case. In the quadratic case, theoretically the ordersof h 8/3 andof h 5/3 areexpectedinthe L 2 -and H -norm,respectively.inallthree casesofquadraticfiniteelements,weobservetheasymptoticratesinthe L 2 -and H -norm, which are better than predicted by the theory. The quantitative results are also almost the sameinthesenorms. Heretoo,weobservethebetterconvergenceratesfortheerrorsin the weighted Lagrange multiplier norm. Table4:Discretizationerrorsinthe L 2 -norm,(example2) level #elem. u u l h 0 u u f h 0 u u s h 0 u u t h e e e e e e e e e e e e e e e e e e e e-04 InExamplesand2,thereisnotanysignificantdifferenceintheaccuracybetweenthedifferentquadraticmortarsolutionseitherinthe L 2 -normorinthe H -norm.however,there is some quantitative difference in the errors in the weighted Lagrange multiplier norm between different quadratic mortar solutions. In this norm, the standard triquadratic finite elements with the tensor product Lagrange multiplier space gives the best result, whereas the difference between the augmented serendipity elements with the tensor product Lagrange multiplier space and the serendipity elements with the standard Lagrange multiplier space is quite negligible in Example 2. In Example, the serendipity elements with the standard Lagrange multiplier space yields better result than the augmented serendipity elements with the tensor product Lagrange multiplier space. However, the difference is not 23
24 Table5:Discretizationerrorsinthe H -norm,(example2) level #elem. u u l h u u f h u u s h u u t h e e e e e e e e e e e e e e e e e e e e-02 Table 6: Discretization errors in the weighted Lagrange multiplier norm,(example 2) level #elem. λ λ l h h λ λ f h h λ λ s h h λ λ t h h e e e e e e e e e e e e e e e e e e e e-02 essential, and the asymptotic convergence orders are the same. We remark that for both of theseexamplestheasymptoticphasestartsfromthethirdstep.thisisduetothefactthat the initial triangulation is very coarse. For the next two examples, we consider only the serendipity elements. In our third example,thedomain Ω := (0, ) 2 (0, 2.5)isdecomposedintothreesubdomains Ω := (0, ) 3, Ω 2 := (0, ) 2 (, 2),and Ω 3 := (0, ) 2 (2, 2.5). Therighthandside fandtheboundaryconditionsof u = farechosensuchthattheexactsolutionisgivenby u(x, y, z) = 5(z.4)((x 0.5) 2 + 4(y 0.3) 3 ) + z(z )sin(4πxy)(2(x y) 2 + (y + x ) 2 ).InFigure 6, we have shown the decomposition of the domain, the initial nonmatching triangulation andtheisolinesofthesolutionattheinterface z = 2. Figure 6: Decomposition of the domain and initial triangulation(left), isolines of the solutionattheinterface z = 2(right),Example3 Here,wehavethreesubdomainsandtwointerfaces.Themiddlecubeistakenastheslave 24
25 side. We start with a nonconforming coarse initial triangulation having 23 elements. The errors along with their order of convergence at every step of refinement in different norms aregivenintables7 9.Asbefore,wegetthecorrectasymptoticratesforbothcasesofthe serendipityelements,andtheerrorsinthe L 2 -and H -normarealmostthesameforboth approaches. In the weighted Lagrange multiplier norm the serendipity elements yield less errors than the augmented serendipity elements. However, the difference is quite negligible, and the asymptotic rate of convergence is optimal in both cases. Table7:Discretizationerrorsinthe L 2 -norm,(example3) level #elem. u u l h 0 u u s h 0 u u t h e e e e e e e e e e e e e e e Table8:Discretizationerrorsinthe H -norm,(example3) level #elem. u u l h u u s h u u t h e e e e e e e e e e e e e e e Table 9: Discretization errors in the weighted Lagrange multiplier norm,(example 3) level #elem. λ λ l h h λ λ s h h λ λ t h h e e e e e e e e e e e e e e e Inourlastexample,weconsideradomain Ω := (0, 2) (0, ) (0, 2),whichisdecomposed intofoursubdomains Ω := (0, ) 3, Ω 2 := (0, ) 2 (, 2), Ω 3 := (, 2) (0, ) 2 and Ω 4 := (, 2) (0, ) (, 2). Wehaveshownthedecompositionofthedomainandtheinitial triangulationintheleftpictureoffigure7,theisolinesofthesolutionontheplane y = 2 inthemiddle,andthefluxoftheexactsolutionattheinterface x = isshownintheright one.here, Ω 2 and Ω 3 aretakentobetheslavesidesandtherestarethemastersides.the problem for this example is given by a reaction-diffusion equation div(a u) + u = f in Ω, 25
26 where aischosentobe in Ω and Ω 4,and a = 0in Ω 2 and Ω 3.Wehavechosentheexact solution u(x, y, z) = (x )y (z )exp( (x ) 2 y 2 (z ) 2 )cos(2 x + 2 y + 2 z)/a and the right hand side f and the Dirichlet boundary conditions are determined from the exact solution. We remark that the exact solution u has a jump in the normal derivative Figure 7: Decomposition of the domain and initial triangulation(left), isolines of the solutionattheplane y = 2 (middle)andfluxoftheexactsolutionattheinterface x = (right), Example 4 across the interface, whereas the flux is continuous. We have given the errors together with their order of convergence in every step of refinement in different norms in Tables 0 2.Asinotherexamples,wegetthesameasymptoticratesforthe L 2 -and H -normand better convergence rates in the weighted Lagrange multiplier norm. In contrast to other examples, we see that the asymptotic of errors in the weighted Lagrange multiplier norm forthequadraticcaseisevenbetterthan O(h 3 ). Table0:Discretizationerrorsinthe L 2 -norm,(example4) level #elem. u u l h 0 u u s h 0 u u t h e e e e e e e e e e e e e e e Table:Discretizationerrorsinthe H -norm,(example4) level #elem. u u l h u u s h u u t h e e e e e e e e e e e e e e e InbothExamples 3and 4,thediscretizationerrorsinthe L 2 -and H -normarealmostthe same for different approaches. Concerning the discretization errors in the weighted Lagrange multiplier norm, although we see some quantitative difference in Examples 3 and 26
27 Table 2: Discretization errors in the weighted Lagrange multiplier norm,(example 4) level #elem. λ λ l h h λ λ s h h λ λ t h h e e e e e e e e e e e e e e e , we observe the better convergence rate for both approaches. In this norm, the serendipity elements with the standard Lagrange multiplier space gives better result in both examples. However, in Example 3, the order of convergence is almost the same for both approaches, whereas the augmented serendipity elements show the better order of convergence in Example 4. We already observe the better rate of convergence from the augmented serendipity elements from the second step of refinement. In all our examples, we observe the optimal asymptotic convergence rates as predicted by the theory. Although we see the same qualitative behaviour, some quantitative differences can be observed. However, there is not any essential difference in the discretization errors between different quadratic mortar solutions. Because of the addition of bubble functions attheskeleton Γ, X t h hasmoredegreeoffreedomthan Xs h.however,thesebubblefunctions can locally be eliminated from the algebraic formulation of the saddle point problem leadingtoasystemmatrix,whichissimilartothealgebraicformofthesaddlepointproblem arising from the mortar discretization with a dual Lagrange multiplier space. On the otherhand,thegrowthrateofthenumberofbubblefunctionsisonlyafactoroffourin each refinement step, and restricted to the skeleton. This is negligible since we can work with the efficient multigrid solver in case of the augmented serendipity space with the tensor product Lagrange multiplier space. Although we can work with the efficient multigrid solver in case of standard triquadratic finite elements, the approach is not as optimal as the augmented serendipity approach due to the more degree of freedom and the growth rate offactoreightineachrefinementstep.itturnsoutthatthemostefficientapproachisthe one given by the augmented serendipity elements. The discretization errors are as good as in the other cases, and the numerical solution is cheaper. References [BBJ + 97] P.Bastian,K.Birken,K.Johannsen,S.Lang,N.Neuß,H.Rentz Reichert,and C. Wieners. UG a flexible software toolbox for solving partial differential equations. Computing and Visualization in Science, :27 40, 997. [BD98] D. Braess and W. Dahmen. Stability estimates of the mortar finite element method for 3 dimensional problems. East West J. Numer. Math., 6: , 998. [BDM90] C. Bernardi, N. Debit, and Y. Maday. Coupling finite element and spectral methods: First results. Math. Comp., 54:2 39, 990. [BDW99] D. Braess, W. Dahmen, and C. Wieners. A multigrid algorithm for the mortar finite element method. SIAM J. Numer. Anal., 37:48 69,
Universität Stuttgart
Universität Stuttgart Second Order Lagrange Multiplier Spaces for Mortar Finite Elements in 3D Bishnu P. Lamichhane, Barbara I. Wohlmuth Berichte aus dem Institut für Angewandte Analysis und Numerische
More informationAND BARBARA I. WOHLMUTH
A QUASI-DUAL LAGRANGE MULTIPLIER SPACE FOR SERENDIPITY MORTAR FINITE ELEMENTS IN 3D BISHNU P. LAMICHHANE AND BARBARA I. WOHLMUTH Abstract. Domain decomposition techniques provide a flexible tool for the
More informationAn Iterative Substructuring Method for Mortar Nonconforming Discretization of a Fourth-Order Elliptic Problem in two dimensions
An Iterative Substructuring Method for Mortar Nonconforming Discretization of a Fourth-Order Elliptic Problem in two dimensions Leszek Marcinkowski Department of Mathematics, Warsaw University, Banacha
More informationUniversität Stuttgart
Universität Stuttgart Multilevel Additive Schwarz Preconditioner For Nonconforming Mortar Finite Element Methods Masymilian Dryja, Andreas Gantner, Olof B. Widlund, Barbara I. Wohlmuth Berichte aus dem
More informationETNA Kent State University
Electronic Transactions on Numerical Analysis. Volume 37, pp. 166-172, 2010. Copyright 2010,. ISSN 1068-9613. ETNA A GRADIENT RECOVERY OPERATOR BASED ON AN OBLIQUE PROJECTION BISHNU P. LAMICHHANE Abstract.
More informationNon-Conforming Finite Element Methods for Nonmatching Grids in Three Dimensions
Non-Conforming Finite Element Methods for Nonmatching Grids in Three Dimensions Wayne McGee and Padmanabhan Seshaiyer Texas Tech University, Mathematics and Statistics (padhu@math.ttu.edu) Summary. In
More informationPhD dissertation defense
Isogeometric mortar methods with applications in contact mechanics PhD dissertation defense Brivadis Ericka Supervisor: Annalisa Buffa Doctor of Philosophy in Computational Mechanics and Advanced Materials,
More informationDiscontinuous Galerkin Methods
Discontinuous Galerkin Methods Joachim Schöberl May 20, 206 Discontinuous Galerkin (DG) methods approximate the solution with piecewise functions (polynomials), which are discontinuous across element interfaces.
More informationTwo-scale Dirichlet-Neumann preconditioners for boundary refinements
Two-scale Dirichlet-Neumann preconditioners for boundary refinements Patrice Hauret 1 and Patrick Le Tallec 2 1 Graduate Aeronautical Laboratories, MS 25-45, California Institute of Technology Pasadena,
More informationThe Mortar Wavelet Method Silvia Bertoluzza Valerie Perrier y October 29, 1999 Abstract This paper deals with the construction of wavelet approximatio
The Mortar Wavelet Method Silvia Bertoluzza Valerie Perrier y October 9, 1999 Abstract This paper deals with the construction of wavelet approximation spaces, in the framework of the Mortar method. We
More informationA note on accurate and efficient higher order Galerkin time stepping schemes for the nonstationary Stokes equations
A note on accurate and efficient higher order Galerkin time stepping schemes for the nonstationary Stokes equations S. Hussain, F. Schieweck, S. Turek Abstract In this note, we extend our recent work for
More informationA FETI-DP Method for Mortar Finite Element Discretization of a Fourth Order Problem
A FETI-DP Method for Mortar Finite Element Discretization of a Fourth Order Problem Leszek Marcinkowski 1 and Nina Dokeva 2 1 Department of Mathematics, Warsaw University, Banacha 2, 02 097 Warszawa, Poland,
More informationNumerical Solutions to Partial Differential Equations
Numerical Solutions to Partial Differential Equations Zhiping Li LMAM and School of Mathematical Sciences Peking University Nonconformity and the Consistency Error First Strang Lemma Abstract Error Estimate
More informationA Domain Decomposition Method for Quasilinear Elliptic PDEs Using Mortar Finite Elements
W I S S E N T E C H N I K L E I D E N S C H A F T A Domain Decomposition Method for Quasilinear Elliptic PDEs Using Mortar Finite Elements Matthias Gsell and Olaf Steinbach Institute of Computational Mathematics
More informationPARTITION OF UNITY FOR THE STOKES PROBLEM ON NONMATCHING GRIDS
PARTITION OF UNITY FOR THE STOES PROBLEM ON NONMATCHING GRIDS CONSTANTIN BACUTA AND JINCHAO XU Abstract. We consider the Stokes Problem on a plane polygonal domain Ω R 2. We propose a finite element method
More informationYongdeok Kim and Seki Kim
J. Korean Math. Soc. 39 (00), No. 3, pp. 363 376 STABLE LOW ORDER NONCONFORMING QUADRILATERAL FINITE ELEMENTS FOR THE STOKES PROBLEM Yongdeok Kim and Seki Kim Abstract. Stability result is obtained for
More informationMultigrid Methods for Saddle Point Problems
Multigrid Methods for Saddle Point Problems Susanne C. Brenner Department of Mathematics and Center for Computation & Technology Louisiana State University Advances in Mathematics of Finite Elements (In
More informationThe mortar element method for quasilinear elliptic boundary value problems
The mortar element method for quasilinear elliptic boundary value problems Leszek Marcinkowski 1 Abstract We consider a discretization of quasilinear elliptic boundary value problems by the mortar version
More informationNumerical Solutions to Partial Differential Equations
Numerical Solutions to Partial Differential Equations Zhiping Li LMAM and School of Mathematical Sciences Peking University The Residual and Error of Finite Element Solutions Mixed BVP of Poisson Equation
More informationA LAGRANGE MULTIPLIER METHOD FOR ELLIPTIC INTERFACE PROBLEMS USING NON-MATCHING MESHES
A LAGRANGE MULTIPLIER METHOD FOR ELLIPTIC INTERFACE PROBLEMS USING NON-MATCHING MESHES P. HANSBO Department of Applied Mechanics, Chalmers University of Technology, S-4 96 Göteborg, Sweden E-mail: hansbo@solid.chalmers.se
More informationLecture Note III: Least-Squares Method
Lecture Note III: Least-Squares Method Zhiqiang Cai October 4, 004 In this chapter, we shall present least-squares methods for second-order scalar partial differential equations, elastic equations of solids,
More informationTechnische Universität Graz
Technische Universität Graz Stability of the Laplace single layer boundary integral operator in Sobolev spaces O. Steinbach Berichte aus dem Institut für Numerische Mathematik Bericht 2016/2 Technische
More informationHamburger Beiträge zur Angewandten Mathematik
Hamburger Beiträge zur Angewandten Mathematik Numerical analysis of a control and state constrained elliptic control problem with piecewise constant control approximations Klaus Deckelnick and Michael
More informationb i (x) u + c(x)u = f in Ω,
SIAM J. NUMER. ANAL. Vol. 39, No. 6, pp. 1938 1953 c 2002 Society for Industrial and Applied Mathematics SUBOPTIMAL AND OPTIMAL CONVERGENCE IN MIXED FINITE ELEMENT METHODS ALAN DEMLOW Abstract. An elliptic
More informationMortar finite elements for a heat transfer problem on sliding meshes
Calcolo manuscript No. (will be inserted by the editor) Mortar finite elements for a heat transfer problem on sliding meshes S. Falletta,, B.P. Lamichhane 2 Dip. Matematica-Politecnico di Torino, Corso
More informationThe All-floating BETI Method: Numerical Results
The All-floating BETI Method: Numerical Results Günther Of Institute of Computational Mathematics, Graz University of Technology, Steyrergasse 30, A-8010 Graz, Austria, of@tugraz.at Summary. The all-floating
More informationBasics and some applications of the mortar element method
GAMM-Mitt. 28, No. 2, 97 123 (2005) Basics and some applications of the mortar element method Christine Bernardi 1, Yvon Maday 1, and Francesca Rapetti 2 1 Laboratoire Jacques-Louis Lions, C.N.R.S. & université
More informationAMS subject classifications. Primary, 65N15, 65N30, 76D07; Secondary, 35B45, 35J50
A SIMPLE FINITE ELEMENT METHOD FOR THE STOKES EQUATIONS LIN MU AND XIU YE Abstract. The goal of this paper is to introduce a simple finite element method to solve the Stokes equations. This method is in
More informationHybridized Discontinuous Galerkin Methods
Hybridized Discontinuous Galerkin Methods Theory and Christian Waluga Joint work with Herbert Egger (Uni Graz) 1st DUNE User Meeting, Stuttgart Christian Waluga (AICES) HDG Methods October 6-8, 2010 1
More informationA gradient recovery method based on an oblique projection and boundary modification
ANZIAM J. 58 (CTAC2016) pp.c34 C45, 2017 C34 A gradient recovery method based on an oblique projection and boundary modification M. Ilyas 1 B. P. Lamichhane 2 M. H. Meylan 3 (Received 24 January 2017;
More informationAnalysis of Hybrid Discontinuous Galerkin Methods for Incompressible Flow Problems
Analysis of Hybrid Discontinuous Galerkin Methods for Incompressible Flow Problems Christian Waluga 1 advised by Prof. Herbert Egger 2 Prof. Wolfgang Dahmen 3 1 Aachen Institute for Advanced Study in Computational
More informationThe Mortar Boundary Element Method
The Mortar Boundary Element Method A Thesis submitted for the degree of Doctor of Philosophy by Martin Healey School of Information Systems, Computing and Mathematics Brunel University March 2010 Abstract
More informationAn a posteriori error estimate and a Comparison Theorem for the nonconforming P 1 element
Calcolo manuscript No. (will be inserted by the editor) An a posteriori error estimate and a Comparison Theorem for the nonconforming P 1 element Dietrich Braess Faculty of Mathematics, Ruhr-University
More informationDomain Decomposition Methods for Mortar Finite Elements
Domain Decomposition Methods for Mortar Finite Elements Dan Stefanica Courant Institute of Mathematical Sciences New York University September 1999 A dissertation in the Department of Mathematics Submitted
More informationA Framework for Analyzing and Constructing Hierarchical-Type A Posteriori Error Estimators
A Framework for Analyzing and Constructing Hierarchical-Type A Posteriori Error Estimators Jeff Ovall University of Kentucky Mathematics www.math.uky.edu/ jovall jovall@ms.uky.edu Kentucky Applied and
More informationLocal Mesh Refinement with the PCD Method
Advances in Dynamical Systems and Applications ISSN 0973-5321, Volume 8, Number 1, pp. 125 136 (2013) http://campus.mst.edu/adsa Local Mesh Refinement with the PCD Method Ahmed Tahiri Université Med Premier
More informationIsogeometric mortaring
Isogeometric mortaring E. Brivadis, A. Buffa, B. Wohlmuth, L. Wunderlich IMATI E. Magenes - Pavia Technical University of Munich A. Buffa (IMATI-CNR Italy) IGA mortaring 1 / 29 1 Introduction Splines Approximation
More informationA Posteriori Error Estimation Techniques for Finite Element Methods. Zhiqiang Cai Purdue University
A Posteriori Error Estimation Techniques for Finite Element Methods Zhiqiang Cai Purdue University Department of Mathematics, Purdue University Slide 1, March 16, 2017 Books Ainsworth & Oden, A posteriori
More informationDomain Decomposition Algorithms for an Indefinite Hypersingular Integral Equation in Three Dimensions
Domain Decomposition Algorithms for an Indefinite Hypersingular Integral Equation in Three Dimensions Ernst P. Stephan 1, Matthias Maischak 2, and Thanh Tran 3 1 Institut für Angewandte Mathematik, Leibniz
More informationA mixed finite element method for nonlinear and nearly incompressible elasticity based on biorthogonal systems
INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING Int. J. Numer. Meth. Engng 2000; 00:1 6 [Version: 2002/09/18 v2.02] A mixed finite element method for nonlinear and nearly incompressible elasticity
More informationChapter 5 A priori error estimates for nonconforming finite element approximations 5.1 Strang s first lemma
Chapter 5 A priori error estimates for nonconforming finite element approximations 51 Strang s first lemma We consider the variational equation (51 a(u, v = l(v, v V H 1 (Ω, and assume that the conditions
More informationLocal discontinuous Galerkin methods for elliptic problems
COMMUNICATIONS IN NUMERICAL METHODS IN ENGINEERING Commun. Numer. Meth. Engng 2002; 18:69 75 [Version: 2000/03/22 v1.0] Local discontinuous Galerkin methods for elliptic problems P. Castillo 1 B. Cockburn
More information20. A Dual-Primal FETI Method for solving Stokes/Navier-Stokes Equations
Fourteenth International Conference on Domain Decomposition Methods Editors: Ismael Herrera, David E. Keyes, Olof B. Widlund, Robert Yates c 23 DDM.org 2. A Dual-Primal FEI Method for solving Stokes/Navier-Stokes
More informationINTERGRID OPERATORS FOR THE CELL CENTERED FINITE DIFFERENCE MULTIGRID ALGORITHM ON RECTANGULAR GRIDS. 1. Introduction
Trends in Mathematics Information Center for Mathematical Sciences Volume 9 Number 2 December 2006 Pages 0 INTERGRID OPERATORS FOR THE CELL CENTERED FINITE DIFFERENCE MULTIGRID ALGORITHM ON RECTANGULAR
More informationA Balancing Algorithm for Mortar Methods
A Balancing Algorithm for Mortar Methods Dan Stefanica Baruch College, City University of New York, NY 11, USA Dan Stefanica@baruch.cuny.edu Summary. The balancing methods are hybrid nonoverlapping Schwarz
More informationOverlapping Schwarz preconditioners for Fekete spectral elements
Overlapping Schwarz preconditioners for Fekete spectral elements R. Pasquetti 1, L. F. Pavarino 2, F. Rapetti 1, and E. Zampieri 2 1 Laboratoire J.-A. Dieudonné, CNRS & Université de Nice et Sophia-Antipolis,
More informationTechnische Universität Graz
Technische Universität Graz Error Estimates for Neumann Boundary Control Problems with Energy Regularization T. Apel, O. Steinbach, M. Winkler Berichte aus dem Institut für Numerische Mathematik Bericht
More information8 On Polynomial Reproduction of Dual FE Bases
Thirteenth International Conference on Domain Decomposition Methods Editors: N. Debit, M.Garbey, R. Hoppe,. Périaux, D. Keyes, Y. Kuznetsov c 21 DDM.org 8 On Polynomial Reproduction of Dual FE Bases Peter
More informationTechnische Universität Graz
Technische Universität Graz A note on the stable coupling of finite and boundary elements O. Steinbach Berichte aus dem Institut für Numerische Mathematik Bericht 2009/4 Technische Universität Graz A
More informationMortar estimates independent of number of subdomains
Mortar estimates independent of number of subdomains Jayadeep Gopalakrishnan Abstract The stability and error estimates for the mortar finite element method are well established This work examines the
More informationMultigrid Methods for Elliptic Obstacle Problems on 2D Bisection Grids
Multigrid Methods for Elliptic Obstacle Problems on 2D Bisection Grids Long Chen 1, Ricardo H. Nochetto 2, and Chen-Song Zhang 3 1 Department of Mathematics, University of California at Irvine. chenlong@math.uci.edu
More informationBasic Concepts of Adaptive Finite Element Methods for Elliptic Boundary Value Problems
Basic Concepts of Adaptive Finite lement Methods for lliptic Boundary Value Problems Ronald H.W. Hoppe 1,2 1 Department of Mathematics, University of Houston 2 Institute of Mathematics, University of Augsburg
More informationNumerical analysis and a-posteriori error control for a new nonconforming quadrilateral linear finite element
Numerical analysis and a-posteriori error control for a new nonconforming quadrilateral linear finite element M. Grajewski, J. Hron, S. Turek Matthias.Grajewski@math.uni-dortmund.de, jaroslav.hron@math.uni-dortmund.de,
More informationHybridized DG methods
Hybridized DG methods University of Florida (Banff International Research Station, November 2007.) Collaborators: Bernardo Cockburn University of Minnesota Raytcho Lazarov Texas A&M University Thanks:
More informationINTRODUCTION TO FINITE ELEMENT METHODS
INTRODUCTION TO FINITE ELEMENT METHODS LONG CHEN Finite element methods are based on the variational formulation of partial differential equations which only need to compute the gradient of a function.
More informationAcceleration of a Domain Decomposition Method for Advection-Diffusion Problems
Acceleration of a Domain Decomposition Method for Advection-Diffusion Problems Gert Lube 1, Tobias Knopp 2, and Gerd Rapin 2 1 University of Göttingen, Institute of Numerical and Applied Mathematics (http://www.num.math.uni-goettingen.de/lube/)
More informationc 2008 Society for Industrial and Applied Mathematics
SIAM J. NUMER. ANAL. Vol. 46, No. 3, pp. 640 65 c 2008 Society for Industrial and Applied Mathematics A WEIGHTED H(div) LEAST-SQUARES METHOD FOR SECOND-ORDER ELLIPTIC PROBLEMS Z. CAI AND C. R. WESTPHAL
More informationMaximum norm estimates for energy-corrected finite element method
Maximum norm estimates for energy-corrected finite element method Piotr Swierczynski 1 and Barbara Wohlmuth 1 Technical University of Munich, Institute for Numerical Mathematics, piotr.swierczynski@ma.tum.de,
More informationNumerische Mathematik
Numer. Math. (1999) 82: 179 191 Numerische Mathematik c Springer-Verlag 1999 Electronic Edition A cascadic multigrid algorithm for the Stokes equations Dietrich Braess 1, Wolfgang Dahmen 2 1 Fakultät für
More informationComparison of V-cycle Multigrid Method for Cell-centered Finite Difference on Triangular Meshes
Comparison of V-cycle Multigrid Method for Cell-centered Finite Difference on Triangular Meshes Do Y. Kwak, 1 JunS.Lee 1 Department of Mathematics, KAIST, Taejon 305-701, Korea Department of Mathematics,
More informationTechnische Universität Graz
Technische Universität Graz A non-symmetric coupling of the finite volume method and the boundary element method C. Erath, G. Of, F. J. Sayas Berichte aus dem Institut für Numerische Mathematik Bericht
More informationA Robust Preconditioner for the Hessian System in Elliptic Optimal Control Problems
A Robust Preconditioner for the Hessian System in Elliptic Optimal Control Problems Etereldes Gonçalves 1, Tarek P. Mathew 1, Markus Sarkis 1,2, and Christian E. Schaerer 1 1 Instituto de Matemática Pura
More informationDomain decomposition methods via boundary integral equations
Domain decomposition methods via boundary integral equations G. C. Hsiao a O. Steinbach b W. L. Wendland b a Department of Mathematical Sciences, University of Delaware, Newark, Delaware 19716, USA. E
More informationOptimal Left and Right Additive Schwarz Preconditioning for Minimal Residual Methods with Euclidean and Energy Norms
Optimal Left and Right Additive Schwarz Preconditioning for Minimal Residual Methods with Euclidean and Energy Norms Marcus Sarkis Worcester Polytechnic Inst., Mass. and IMPA, Rio de Janeiro and Daniel
More informationarxiv: v1 [math.na] 1 May 2013
arxiv:3050089v [mathna] May 03 Approximation Properties of a Gradient Recovery Operator Using a Biorthogonal System Bishnu P Lamichhane and Adam McNeilly May, 03 Abstract A gradient recovery operator based
More informationA u + b u + cu = f in Ω, (1.1)
A WEIGHTED H(div) LEAST-SQUARES METHOD FOR SECOND-ORDER ELLIPTIC PROBLEMS Z. CAI AND C. R. WESTPHAL Abstract. This paper presents analysis of a weighted-norm least squares finite element method for elliptic
More informationHigh order, finite volume method, flux conservation, finite element method
FLUX-CONSERVING FINITE ELEMENT METHODS SHANGYOU ZHANG, ZHIMIN ZHANG, AND QINGSONG ZOU Abstract. We analyze the flux conservation property of the finite element method. It is shown that the finite element
More informationOn an Approximation Result for Piecewise Polynomial Functions. O. Karakashian
BULLETIN OF THE GREE MATHEMATICAL SOCIETY Volume 57, 010 (1 7) On an Approximation Result for Piecewise Polynomial Functions O. arakashian Abstract We provide a new approach for proving approximation results
More informationA local-structure-preserving local discontinuous Galerkin method for the Laplace equation
A local-structure-preserving local discontinuous Galerkin method for the Laplace equation Fengyan Li 1 and Chi-Wang Shu 2 Abstract In this paper, we present a local-structure-preserving local discontinuous
More informationA Neumann-Dirichlet Preconditioner for FETI-DP 2 Method for Mortar Discretization of a Fourth Order 3 Problems in 2D 4 UNCORRECTED PROOF
1 A Neumann-Dirichlet Preconditioner for FETI-DP 2 Method for Mortar Discretization of a Fourth Order 3 Problems in 2D 4 Leszek Marcinkowski * 5 Faculty of Mathematics, University of Warsaw, Banacha 2,
More informationTwo new enriched multiscale coarse spaces for the Additive Average Schwarz method
346 Two new enriched multiscale coarse spaces for the Additive Average Schwarz method Leszek Marcinkowski 1 and Talal Rahman 2 1 Introduction We propose additive Schwarz methods with spectrally enriched
More informationRemarks on the analysis of finite element methods on a Shishkin mesh: are Scott-Zhang interpolants applicable?
Remarks on the analysis of finite element methods on a Shishkin mesh: are Scott-Zhang interpolants applicable? Thomas Apel, Hans-G. Roos 22.7.2008 Abstract In the first part of the paper we discuss minimal
More informationMORTAR MULTISCALE FINITE ELEMENT METHODS FOR STOKES-DARCY FLOWS
MORTAR MULTISCALE FINITE ELEMENT METHODS FOR STOKES-DARCY FLOWS VIVETTE GIRAULT, DANAIL VASSILEV, AND IVAN YOTOV Abstract. We investigate mortar multiscale numerical methods for coupled Stokes and Darcy
More informationPriority Program 1253
Deutsche Forschungsgemeinschaft Priority Program 1253 Optimization with Partial Differential Equations Klaus Deckelnick and Michael Hinze A note on the approximation of elliptic control problems with bang-bang
More informationInstitut für Mathematik
U n i v e r s i t ä t A u g s b u r g Institut für Mathematik Xuejun Xu, Huangxin Chen, Ronald H.W. Hoppe Local Multilevel Methods for Adaptive Nonconforming Finite Element Methods Preprint Nr. 21/2009
More informationAdditive Average Schwarz Method for a Crouzeix-Raviart Finite Volume Element Discretization of Elliptic Problems
Additive Average Schwarz Method for a Crouzeix-Raviart Finite Volume Element Discretization of Elliptic Problems Atle Loneland 1, Leszek Marcinkowski 2, and Talal Rahman 3 1 Introduction In this paper
More informationA Mixed Nonconforming Finite Element for Linear Elasticity
A Mixed Nonconforming Finite Element for Linear Elasticity Zhiqiang Cai, 1 Xiu Ye 2 1 Department of Mathematics, Purdue University, West Lafayette, Indiana 47907-1395 2 Department of Mathematics and Statistics,
More information1. Introduction. The Stokes problem seeks unknown functions u and p satisfying
A DISCRETE DIVERGENCE FREE WEAK GALERKIN FINITE ELEMENT METHOD FOR THE STOKES EQUATIONS LIN MU, JUNPING WANG, AND XIU YE Abstract. A discrete divergence free weak Galerkin finite element method is developed
More informationR T (u H )v + (2.1) J S (u H )v v V, T (2.2) (2.3) H S J S (u H ) 2 L 2 (S). S T
2 R.H. NOCHETTO 2. Lecture 2. Adaptivity I: Design and Convergence of AFEM tarting with a conforming mesh T H, the adaptive procedure AFEM consists of loops of the form OLVE ETIMATE MARK REFINE to produce
More informationarxiv: v1 [math.na] 29 Feb 2016
EFFECTIVE IMPLEMENTATION OF THE WEAK GALERKIN FINITE ELEMENT METHODS FOR THE BIHARMONIC EQUATION LIN MU, JUNPING WANG, AND XIU YE Abstract. arxiv:1602.08817v1 [math.na] 29 Feb 2016 The weak Galerkin (WG)
More informationSpectral element agglomerate AMGe
Spectral element agglomerate AMGe T. Chartier 1, R. Falgout 2, V. E. Henson 2, J. E. Jones 4, T. A. Manteuffel 3, S. F. McCormick 3, J. W. Ruge 3, and P. S. Vassilevski 2 1 Department of Mathematics, Davidson
More informationarxiv: v1 [math.na] 11 Jul 2011
Multigrid Preconditioner for Nonconforming Discretization of Elliptic Problems with Jump Coefficients arxiv:07.260v [math.na] Jul 20 Blanca Ayuso De Dios, Michael Holst 2, Yunrong Zhu 2, and Ludmil Zikatanov
More informationSimple Examples on Rectangular Domains
84 Chapter 5 Simple Examples on Rectangular Domains In this chapter we consider simple elliptic boundary value problems in rectangular domains in R 2 or R 3 ; our prototype example is the Poisson equation
More informationTechnische Universität Graz
Technische Universität Graz Robust boundary element domain decomposition solvers in acoustics O. Steinbach, M. Windisch Berichte aus dem Institut für Numerische Mathematik Bericht 2009/9 Technische Universität
More informationENERGY NORM A POSTERIORI ERROR ESTIMATES FOR MIXED FINITE ELEMENT METHODS
ENERGY NORM A POSTERIORI ERROR ESTIMATES FOR MIXED FINITE ELEMENT METHODS CARLO LOVADINA AND ROLF STENBERG Abstract The paper deals with the a-posteriori error analysis of mixed finite element methods
More informationOptimal multilevel preconditioning of strongly anisotropic problems.part II: non-conforming FEM. p. 1/36
Optimal multilevel preconditioning of strongly anisotropic problems. Part II: non-conforming FEM. Svetozar Margenov margenov@parallel.bas.bg Institute for Parallel Processing, Bulgarian Academy of Sciences,
More informationSpace-time sparse discretization of linear parabolic equations
Space-time sparse discretization of linear parabolic equations Roman Andreev August 2, 200 Seminar for Applied Mathematics, ETH Zürich, Switzerland Support by SNF Grant No. PDFMP2-27034/ Part of PhD thesis
More information1. Introduction. We consider the model problem that seeks an unknown function u = u(x) satisfying
A SIMPLE FINITE ELEMENT METHOD FOR LINEAR HYPERBOLIC PROBLEMS LIN MU AND XIU YE Abstract. In this paper, we introduce a simple finite element method for solving first order hyperbolic equations with easy
More information18. Balancing Neumann-Neumann for (In)Compressible Linear Elasticity and (Generalized) Stokes Parallel Implementation
Fourteenth nternational Conference on Domain Decomposition Methods Editors: smael Herrera, David E Keyes, Olof B Widlund, Robert Yates c 23 DDMorg 18 Balancing Neumann-Neumann for (n)compressible Linear
More informationUnified Hybridization Of Discontinuous Galerkin, Mixed, And Continuous Galerkin Methods For Second Order Elliptic Problems.
Unified Hybridization Of Discontinuous Galerkin, Mixed, And Continuous Galerkin Methods For Second Order Elliptic Problems. Stefan Girke WWU Münster Institut für Numerische und Angewandte Mathematik 10th
More informationFINITE ELEMENT APPROXIMATION OF STOKES-LIKE SYSTEMS WITH IMPLICIT CONSTITUTIVE RELATION
Proceedings of ALGORITMY pp. 9 3 FINITE ELEMENT APPROXIMATION OF STOKES-LIKE SYSTEMS WITH IMPLICIT CONSTITUTIVE RELATION JAN STEBEL Abstract. The paper deals with the numerical simulations of steady flows
More informationOptimal Interface Conditions for an Arbitrary Decomposition into Subdomains
Optimal Interface Conditions for an Arbitrary Decomposition into Subdomains Martin J. Gander and Felix Kwok Section de mathématiques, Université de Genève, Geneva CH-1211, Switzerland, Martin.Gander@unige.ch;
More informationScientific Computing WS 2017/2018. Lecture 18. Jürgen Fuhrmann Lecture 18 Slide 1
Scientific Computing WS 2017/2018 Lecture 18 Jürgen Fuhrmann juergen.fuhrmann@wias-berlin.de Lecture 18 Slide 1 Lecture 18 Slide 2 Weak formulation of homogeneous Dirichlet problem Search u H0 1 (Ω) (here,
More informationarxiv: v2 [math.na] 23 Apr 2016
Improved ZZ A Posteriori Error Estimators for Diffusion Problems: Conforming Linear Elements arxiv:508.009v2 [math.na] 23 Apr 206 Zhiqiang Cai Cuiyu He Shun Zhang May 2, 208 Abstract. In [8], we introduced
More informationFETI domain decomposition method to solution of contact problems with large displacements
FETI domain decomposition method to solution of contact problems with large displacements Vít Vondrák 1, Zdeněk Dostál 1, Jiří Dobiáš 2, and Svatopluk Pták 2 1 Dept. of Appl. Math., Technical University
More informationAn Extended Finite Element Method for a Two-Phase Stokes problem
XFEM project An Extended Finite Element Method for a Two-Phase Stokes problem P. Lederer, C. Pfeiler, C. Wintersteiger Advisor: Dr. C. Lehrenfeld August 5, 2015 Contents 1 Problem description 2 1.1 Physics.........................................
More informationASM-BDDC Preconditioners with variable polynomial degree for CG- and DG-SEM
ASM-BDDC Preconditioners with variable polynomial degree for CG- and DG-SEM C. Canuto 1, L. F. Pavarino 2, and A. B. Pieri 3 1 Introduction Discontinuous Galerkin (DG) methods for partial differential
More informationSpace time finite and boundary element methods
Space time finite and boundary element methods Olaf Steinbach Institut für Numerische Mathematik, TU Graz http://www.numerik.math.tu-graz.ac.at based on joint work with M. Neumüller, H. Yang, M. Fleischhacker,
More informationA Priori Error Analysis for Space-Time Finite Element Discretization of Parabolic Optimal Control Problems
A Priori Error Analysis for Space-Time Finite Element Discretization of Parabolic Optimal Control Problems D. Meidner and B. Vexler Abstract In this article we discuss a priori error estimates for Galerkin
More informationScientific Computing WS 2018/2019. Lecture 15. Jürgen Fuhrmann Lecture 15 Slide 1
Scientific Computing WS 2018/2019 Lecture 15 Jürgen Fuhrmann juergen.fuhrmann@wias-berlin.de Lecture 15 Slide 1 Lecture 15 Slide 2 Problems with strong formulation Writing the PDE with divergence and gradient
More information