Rezolvarea ecuaţiilor şi sistemelor de ecuaţii diferenţiale ordinare (II)

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Rezolvarea ecuaţiilor şi sistemelor de ecuaţii diferenţiale ordinare (II) Metode multipas Prof.dr.ing. Universitatea "Politehnica" Bucureşti, Facultatea de Inginerie Electrică Suport didactic pentru disciplina Metode numerice, 2016-2017 1/30

Cuprins Introducere 1 Introducere Unipas vs. multipas Formule de integrare numerica Newton-Cotes 2 Metode Milne explicite Metode Adams-Bashforth 3 Metode Milne implicite Metode Adams-Moulton BDF (metoda lui Gear) 4 COMSOL Matlab 2/30

Unipas vs. multipas Unipas vs. multipas Formule de integrare numerica Newton-Cotes dx dt = f(x, t) x(t B ) x(t A ) = tb t A f(x, t) dt tb x(t B ) = x(t A )+ f(x, t) dt t A Soluţia se va calcula numeric în punctele discrete t 0 = t0, t 1,...,t n = T Unipas t A = t j t A = t j+1 Multipas t A = t j t A = t j+m tj+1 x(t j+1 ) = x(t j )+ f(x, t) dt t j tj+m x(t j+m ) = x(t j )+ f(x, t) dt t j x j+1 = x j + I I tj+1 f(x, t) dt x j+m = x j + I I tj+m f(x, t) dt t j t j j = 0, n 1, x 0 este cunoscut; j = 0, n m, x 0 este cunoscut; 3/30

Unipas vs. multipas Unipas vs. multipas Formule de integrare numerica Newton-Cotes Metode unipas - folosesc informaţii din intervalul [t j, t j+1 ] Metode θ x j + x j+1 = h [ θf(x j, t j )+(1 θ)f(x j+1, t j+1 ) ] Dacă θ = 1 - metoda este explicită (Euler explicit) Dacă θ 1 - metoda este implicită (θ = 0 - Euler implicit, θ = 1/2 - trapeze); necesită în general rezolvarea unei ecuaţii algebrice neliniare la fiecare pas de integrare. Metode Runge-Kutta 4/30

Unipas vs. multipas Unipas vs. multipas Formule de integrare numerica Newton-Cotes Metode unipas - folosesc informaţii din intervalul [t j, t j+1 ] Metode θ x j + x j+1 = h [ θf(x j, t j )+(1 θ)f(x j+1, t j+1 ) ] Metode Runge-Kutta x j + x j+1 = I unde I t j+1 t j f(x, t) dt I = h ν b i f(x(t j + c i h), t j + c i h) i=1 unde x(t j + c i h) nu sunt cunoscute şi sunt aproximate cu formule liniare ale unor valori calculate succesiv. 5/30

Unipas vs. multipas Unipas vs. multipas Formule de integrare numerica Newton-Cotes Metode unipas - folosesc informaţii din intervalul [t j, t j+1 ] Metode θ x j + x j+1 = h [ θf(x j, t j )+(1 θ)f(x j+1, t j+1 ) ] Metode Runge-Kutta - explicite x j+1 = x j + hφ, j = 0,...,n 1 (1) ν Φ = b i K i (2) i=1 K 1 = f(x j, t j ) (3) K i i 1 = f(x j + h a ip K p, t j + c i h) i = 2,...,ν (4) p=1 6/30

Unipas vs. multipas Unipas vs. multipas Formule de integrare numerica Newton-Cotes Metode unipas - folosesc informaţii din intervalul [t j, t j+1 ] Metode θ x j + x j+1 = h [ θf(x j, t j )+(1 θ)f(x j+1, t j+1 ) ] Metode Runge-Kutta - implicite x j+1 = x j + hφ, j = 0,...,n 1 (5) ν Φ = b i K i (6) i=1 K 1 = f(x j, t j ) (7) ν = f(x j + h a ip K p, t j + c i h) i = 2,...,ν (8) K i p=1 7/30

Unipas vs. multipas Unipas vs. multipas Formule de integrare numerica Newton-Cotes Metode multipas - folosesc informaţii din intervalul [t j, t j+m ] Metode liniare multipas 1 Calculul unui pas nou x j+m se face folosind relaţia a 0 x j +a 1 x j+1 + a m x j+m = h[b 0 f(x j, t j )+b 1 f(x j+1, t j+1 )+ b m f(x j+m, t j+m )] a i şi b i sunt aleşi convenabil (convergenţă); a m = 1; dacă b m = 0 metoda este explicită; dacă b m 0 metoda este implicită; cazul m = 1, a 0 = 1 (din motive de convergenţă), b 0 = θ, b 1 = 1 θ metode unipas de tip θ. 1 Există şi metode neliniare multipas. 8/30

Unipas vs. multipas Formule de integrare numerica Newton-Cotes Formule de integrare numerică Newton-Cotes Algoritmii multipas pentru rezolvarea ODE se bazează pe formulele de cuadratură Newton-Cotes (formule de integrare numerică scrise pentru reţele de discretizare uniformă). 1 Formule NC "închise" - folosesc inclusiv valorile în capete. Se folosesc în calculul integralelor definite şi în rezolvarea ODE cu metodele multipas implicite. 2 Formule NC "deschise" - nu folosesc valorile în capete. Se folosesc în rezolvare ODE cu metodele multipas explicite. 9/30

Formule Newton-Cotes închise Unipas vs. multipas Formule de integrare numerica Newton-Cotes Grid uniform x 0, x 1,...,x n, pas h f i = f(x i ) Formulele conţin f 0 şi f n. n (gradul Pasul Numele uzual Formula Eroarea polinomului) h al formulei locală 1 x 1 x 0 trapezului h 2 (f 0 + f 1 ) O(h 3 ) 2 x 1 x 0 = x 2 x 0 2 Simpson 1/3 h 3 (f 0 + 4f 1 + f 2 ) O(h 5 ) 3 x 1 x 0 = x 3 x 0 3 Simpson 3/8 3h 8 (f 0 + 3f 1 + 3f 2 + f 3 ) O(h 5 ) 10/30

Formule Newton-Cotes deschise Unipas vs. multipas Formule de integrare numerica Newton-Cotes Grid uniform x 0, x 1,...,x n, pas h f i = f(x i ) Formulele nu conţin f 0 şi f n. Gradul Pasul Numele uzual Formula Eroarea polinomului h al formulei locală 0 x 1 x 0 = x 2 x 0 2 1 x 1 x 0 = x 3 x 0 3 regula dreptunghiului 2hf 1 O(h 3 ) sau punctului din mijloc trapezului 3h 2 (f 1 + f 2 ) O(h 3 ) 2 x 1 x 0 = x 4 x 0 4 regula lui Milne 4h 3 (2f 1 f 2 + 2f 3 ) O(h 5 ) 11/30

Metode Milne explicite Metode Milne explicite Metode Adams-Bashforth x j+m = x j + I I tj+m t j f(x, t) dt j = 0, n m, x 0 este cunoscut; Integrala I se aproximează cu formule Newton-Cotes deschise. m = 2 NC cu 1 punct interioar x j+2 = x j + 2hf(x j+1, t j+1 ) 12/30

Metode Milne explicite Metode Milne explicite Metode Adams-Bashforth x j+m = x j + I I tj+m t j f(x, t) dt j = 0, n m, x 0 este cunoscut; Integrala I se aproximează cu formule Newton-Cotes deschise. m = 3 NC cu 2 puncte interioar x j+3 = x j + 3h 2 (f(x j+1, t j+1 + f(x j+2, t j+2 ) 12/30

Metode Milne explicite Metode Milne explicite Metode Adams-Bashforth x j+m = x j + I I tj+m t j f(x, t) dt j = 0, n m, x 0 este cunoscut; Integrala I se aproximează cu formule Newton-Cotes deschise. m = 4 NC cu 3 puncte interioare x j+4 = x j + 4h 3 (2f(x j+1, t j+1 ) f(x j+2, t j+2 )+2f(x j+3, t j+3 )+O(h 5 ) 12/30

Adams-Bashforth Metode Milne explicite Metode Adams-Bashforth Formula generală a metodelor liniare multipas: a 0 x j + a 1 x j+1 + a mx j+m = h[b 0 f(x j, t j )+b 1 f(x j+1, t j+1 )+ b mf(x j+m, t j+m )] x j+m unde a m = 1 Familia Adams-Bashfort: a m 1 = 1 a i = 0, i < m 1 b m = 0 (metodă explicită) x j+m = x j+m 1 + h[b 0 f(x j, t j )+b 1 f(x j+1, t j+1 )+ b m 1 f(x j+m 1, t j+m 1 )] m 1 x j+m = x j+m 1 + b i f(x j+i, t j+i ) i=0 13/30

Adams-Bashforth Metode Milne explicite Metode Adams-Bashforth Familia Adams-Bashfort: m 1 x j+m = x j+m 1 + b i f(x j+i, t j+i ) i=0 b i se determină astfel încât metoda are ordinul m Metodă, ordin b 0 b 1 b 2 b 3 AB cu 1 pas, ordin 1 1 AB cu 2 paşi, ordin 2 3/2-1/2 AB cu 3 paşi, ordin 3 23/12-16/12 5/12 AB cu 4 paşi, ordin 4 55/24-59/24 37/24-9/24 AB cu un pas este Euler explicit. 14/30

Metode Milne implicite Metode Milne implicite Metode Adams-Moulton BDF (metoda lui Gear) x j+m = x j + I I tj+m t j f(x, t) dt j = 0, n m, x 0 este cunoscut; Integrala I se aproximează cu formule Newton-Cotes închise. Exemplu: m = 2 NC cu 3 puncte x j+2 = x j + h 3 (f(x j, t j + 4f(x j+1, t j+1 + f(x j+2, t j+2 ) Ecuaţie neliniară are nevoie de o estimare iniţială pentru x j+2. Se poate face cu o formulă Milne explicită. Evaluarea primelor puncte se face cu metode unipas. 15/30

Adams-Moulton Metode Milne implicite Metode Adams-Moulton BDF (metoda lui Gear) Formula generală a metodelor liniare multipas: a 0 x j + a 1 x j+1 + a mx j+m = h[b 0 f(x j, t j )+b 1 f(x j+1, t j+1 )+ b mf(x j+m, t j+m )] x j+m unde a m = 1 Familia Adams-Moulton: a m 1 = 1 a i = 0, i < m 1 item b m 0 (metodă implicită) x j+m = x j+m 1 + h[b 0 f(x j, t j )+b 1 f(x j+1, t j+1 )+ b mf(x j+m, t j+m )] x j+m = x j+m 1 + m b i f(x j+i, t j+i ) i=0 16/30

Adams-Moulton Metode Milne implicite Metode Adams-Moulton BDF (metoda lui Gear) Prin eliminarea restricţiei b m = 0 de la AB, metodele devin implicite, şi o metodă cu m paşi poate ajunge la ordinul m+1. Metodă, ordin b 0 b 1 b 2 b 3 AM cu 1 pas, ordin 1 0 1 AM cu 1 pas, ordin 2 1/2 1/2 AM cu 2 paşi, ordin 3 15/12 8/12-1/12 AM cu 3 paşi, ordin 4 9/24 19/24-5/24 1/24 AM cu un pas este Euler implicit (ordinul 1) sau metoda trapezelor (ordinul 2). 17/30

BDF (Gear) Introducere Metode Milne implicite Metode Adams-Moulton BDF (metoda lui Gear) Formula generală a metodelor liniare multipas: a 0 x j + a 1 x j+1 + a mx j+m = h[b 0 f(x j, t j )+b 1 f(x j+1, t j+1 )+ b mf(x j+m, t j+m )] x j+m Dacă b i = 0 i < m şi notând b m = β - metodele se numesc BDF 2 m 1 a i x j+i + x j+m = hβf(x j+m, t j+m ) x j+m i=0 unde h = t i+m t i+m 1 2 Backward Differentiation Formula 18/30

BDF Introducere Metode Milne implicite Metode Adams-Moulton BDF (metoda lui Gear) Coeficienţii undei metode BDF de ordin m se determină pornind de la polinomul Lagrange de ordin m, notat p i,m (t) care trece prin punctele (t i, x i ),...,(t i+m, x i+m ). x (t i+m ) p (t i+m ) şi x (t i+m ) = f(x(t i+m ), t i+m ) 19/30

BDF Introducere Metode Milne implicite Metode Adams-Moulton BDF (metoda lui Gear) Metodă, ordin β a 0 a 1 a 2 a 3 BDF cu 1 pas, ordin 1 1-1 1 BDF cu 2 paşi, ordin 2 2/3 1/3-4/3 1 BDF cu 3 paşi, ordin 3 6/11-2/11 9/11-18/11 1 BDF cu un pas este Euler implicit (ordinul 1). 20/30

COMSOL Matlab COMSOL 21/30

COMSOL Matlab COMSOL 22/30

COMSOL Matlab COMSOL 23/30

COMSOL Matlab Matlab (non-stiff) https://ch.mathworks.com/help/matlab/math/choose-an-ode-solver.html 24/30

COMSOL Matlab Matlab (non-stiff) https://ch.mathworks.com/help/matlab/math/choose-an-ode-solver.html Single-step ode45 is based on an explicit Runge-Kutta (4,5) formula, the Dormand-Prince pair. ode23 is an implementation of an explicit Runge-Kutta (2,3) pair of Bogacki and Shampine. It may be more efficient than ode45 at crude tolerances and in the presence of moderate stiffness. Multi-step ode113 is a variable-step, variable-order Adams-Bashforth-Moulton PECE solver of orders 1 to 13. The highest order used appears to be 12, however, a formula of order 13 is used to form the error estimate and the function does local extrapolation to advance the integration at order 13. It may be more efficient than ode45 at stringent tolerances or if the ODE function is particularly expensive to evaluate. 25/30

COMSOL Matlab Matlab (stiff) https://ch.mathworks.com/help/matlab/math/choose-an-ode-solver.html 26/30

COMSOL Matlab Matlab (stiff) https://ch.mathworks.com/help/matlab/math/choose-an-ode-solver.html Single-step ode23s is based on a modified Rosenbrock formula of order 2. Because it is a single-step solver, it may be more efficient than ode15s at solving problems that permit crude tolerances or problems with solutions that change rapidly. It can solve some kinds of stiff problems for which ode15s is not effective. The ode23s solver evaluates the Jacobian during each step of the integration, so supplying it with the Jacobian matrix is critical to its reliability and efficiency. ode23t is an implementation of the trapezoidal rule using a "free" interpolant. This solver is preferred over ode15s if the problem is only moderately stiff and you need a solution without numerical damping. ode23t also can solve differential algebraic equations (DAEs) Multi-step ode15s, ode23tb 27/30

COMSOL Matlab Matlab (stiff) https://ch.mathworks.com/help/matlab/math/choose-an-ode-solver.html Single-step ode23s, ode23t Multi-step ode15s is a variable-step, variable-order (VSVO) solver based on the numerical differentiation formulas (NDFs) of orders 1 to 5. Optionally, it can use the backward differentiation formulas (BDFs, also known as Gearş method) that are usually less efficient. Like ode113, ode15s is a multistep solver. Use ode15s if ode45 fails or is very inefficient and you suspect that the problem is stiff, or when solving a differential-algebraic equation (DAE). ode23tb is an implementation of TR-BDF2, an implicit Runge-Kutta formula with a trapezoidal rule step as its first stage and a backward differentiation formula of order two as its second stage. By construction, the same iteration matrix is used in evaluating both stages. Like ode23s and ode23t, this solver may be more efficient than ode15s for problems with crude tolerances. 28/30

COMSOL Matlab Matlab (fully-implicit) https://ch.mathworks.com/help/matlab/math/choose-an-ode-solver.html Multi-step ode15i is a variable-step, variable-order (VSVO) solver based on the backward differentiation formulas (BDFs) of orders 1 to 5. ode15i is designed to be used with fully implicit differential equations and index-1 differential algebraic equations (DAEs). The helper function decic computes consistent initial conditions that are suitable to be used with ode15i. 29/30

Referinţe Introducere COMSOL Matlab [Cheney08] W.Cheney, D.Kincaid, Numerical Mathematics and Computing, Brooks/Cole publishing Company,2000. (Capitolele 10 şi 11) Disponibilă la http://www.physics.brocku.ca/courses/5p10/references/cheneykincaid.pdf [Press02] W.H.Press, S.A.Teukolsky, W.T. etterling, B.P. Flannery, Numerical Recipies in C, 2002. (Capitolul 16) Disponibilă la https://www2.units.it/ipl/students_area/imm2/files/numerical_recipes.pdf [Strang&Moler15] Introduction to Differential Equations and the MATLAB ODE Suite - Open Courseware at MIT Disponibil la http://ocw.mit.edu/res-18-009f15 sau https://www.youtube.com/watch?v=zvl88xqysak [Trefethen18] N.Trefethen, Exploring ODEs, SIAM 2018. Disponibil la https://people.maths.ox.ac.uk/trefethen/books.html 30/30

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