An Example of Self Acceleration for Incompressible Flows. Key Words: incompressible flows, Euler equation, Navier-Stokes equation, self-acceleration

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1 Bol. Soc. Paran. Mat. (3s.) v (213): c SPM ISSN on line ISSN in press SPM: doi:1.5269/bspm.v31i An Example of Self Acceleration for Incompressible Flows Jens Lorenz and Randy Ott abstract: In this paper we consider the Cauchy problem for the unforced Euler and Navier Stokes equations for incompressible flows. We give an example of a smooth initial velocity field of finite energy which is self accelerating, i.e., the maximal speed increases for some time. The self acceleration is due to the non Bernoulli part of the pressure generated by the velocity field. Key Words: incompressible flows, Euler equation, Navier-Stokes equation, self-acceleration Contents 1 Introduction The Acceleration Term 15 3 Example of a Self Accelerating Velocity Field Remarks on the Pressure Equation Appendix: Some Elementary Integrals Introduction Consider the Cauchy problem for the Euler or the Navier Stokes 1 equations: v t +v v + p = ν v, v =, (1.1) v(x,) = u(x), x R 3. (1.2) Here ν = for the Euler equation and ν > for the Navier Stokes equation. We assume that the initial vector field, v(x, ) = u(x), is given by a smooth, bounded, divergence free function u(x) of finite energy. Then a smooth solution v(x, t) is known to exist in some finite time interval t < T. It is an open problem, for both the Euler and the Navier Stokes equations, if blow up can occur in finite time or if a smooth solution v(x,t) always exists for t <. 2 Mathematics Subject Classification: 35Q3, 35Q31, 35Q35, 76D5 1 We consider the equations in nondimensional form with density ρ = Typeset by B S P M style. c Soc. Paran. de Mat.

2 15 Jens Lorenz and Randy Ott Denote the spatial maximum norm by 2 v(,t) = sup v(x,t). x We will use the following terminology: Definition 1.1 The initial velocity field u = u(x) in (1.2) is self accelerating for equation (1.1) if there is a time t > so that v(,t) > u for < t t. (1.3) We use the term self accelerating since no forcing terms or inhomogeneous boundary conditions are present in the problem. As we will see in the next section, self acceleration at time t =, if it occurs, is due to the pressure term p in (1.1). Here p = p(x) = p(x,) is determined in terms of u by the Poisson equation 3 p = 3 D i D j (u i u j ), p(x) as x. (1.4) i,j=1 Interest in self accelerating flows is motivated by the (open) blow up problem for the Navier Stokes equation. 4 For the Navier Stokes equations, it is known (see, for example, [1] or [2]) that if the maximal interval of existence of a smooth solution v(x,t) is t < t then lim v(,t) t t =. Therefore, if blow up occurs, self acceleration must occur at some time t less than t. What properties do self accelerating flow fields have? In Section 2 we give some simple properties. We could not find any example of a self accelerating flow field with finite energy in the literature. In Section 3 we give an explicit example for the Euler equation. A simple scaling argument then shows how one can obtain an example for the Navier Stokes equation. 2. The Acceleration Term Without loss of generality, we will assume that the maximal speed at time t = occurs at the point x = : 2 With a,b = 3 j=1 a jb j and a = a,a 1/2 we denote the Euclidean inner product and norm on R 3. 3 We denote D j = / x j. 4 This blow up problem is one of the Millennium Problems chosen by the Clay Mathematics Institute. The official description by Charles Fefferman can be found at

3 An Example of Self Acceleration for Incompressible Flows 151 u() = u. Then, since the function φ(x) = 1 2 u(x) 2 is maximal at x =, we have and = D j φ() = 3 i=1 (u i D j u i ) () (2.5) D 2 jφ() = 3 i=1 (u i D 2 ju i ) ()+ 3 (D j u i ) 2 () (2.6) i=1 for j = 1,2,3. The vector field u(x) is self accelerating if We have, at x =,t = : A := 1 d t= 2dt v(,t) 2 >. (2.7) A = v,v t = v,v v v, p +ν v, v =: C +P +νv (2.8) Here the convection term is (at x =, t = ): C = i,j v j v i D j v i = because of (2.5). The viscous term is (at x =, t = ): V = i,j = j v i D 2 jv i D 2 jφ() i,j (D j u i ) 2 () because of (2.6). Therefore, to obtain a vectorfield u(x) with A > (see (2.7)) it is necessary that the pressure term P in (2.8) satisfies < P = u(), p(). (2.9) The condition (2.9) is also sufficient for self acceleration if ν =, i.e., for the Euler equation. In the next section we will give an example of a velocity field u(x) which has maximal speed at the origin and satisfies (2.9). Thus, this vector field is self accelerating for the Euler equation.

4 152 Jens Lorenz and Randy Ott 3. Example of a Self Accelerating Velocity Field In this section we use the notation x = (x,y,z) for the space variable. Define the velocity field u := ψ where ψ(x,y,z) = e (x2 +y 2 +z 2 ) 4y x. xy It is clear that u is divergence free and u as well as all its derivatives lie in L 2. Elementary calculations show that x 2xy 2 2xz u(x,y,z) = e (x2 +y 2 +z 2 ) y 8yz +2x 2 y (3.1) 5+2x 2 +8y 2 which yields u() = 5. Furthermore, it is elementary to check that the function φ(x,y,z) = 1 2 u(x,y,z) 2 has a gradient which vanishes at the origin and has a negative definite Hessian at the origin: H = φ () = This implies that the speed u(x,y,z) has a local maximum at the origin. A field plot of the velocity is shown in figure 1.

5 An Example of Self Acceleration for Incompressible Flows 153 Figure 1: A Self-Accelerating Velocity Field In fact, the local maximum is a global maximum as can be verified by numerical computations. See Figure 2.

6 154 Jens Lorenz and Randy Ott (1/2)max x =r u(x) r Figure 2: The function max x =r 1 2 u(x) 2 For the above example, the pressure term P (see (2.9)) becomes P = 5 z p(x,y,z) x=y=z= where the pressure p is determined by the Poisson equation (1.4). Let Q(x,y,z) denote the right hand side of (1.4). Then we have P = 5 z p(x,y,z) x=y=z= 5 z = Q(x,y,z) 4π R 3 (x 2 +y 2 +z 2 ) 1/2 = 5 zq(x,y,z) 4π R 3 (x 2 +y 2 +z 2 ) 3/2 =: J Elementary computations, for which we applied Maple, show that the right hand side Q = D i D j (u i u j ) for the above example reads Q(x,y,z) = q(x,y,z)e 2(x2 +y 2 +z 2 )

7 An Example of Self Acceleration for Incompressible Flows 155 where q is the polynomial ( q(x,y,z) = 2 48x 2 y 2 z +6z +24y 2 z 36x 2 z 32x 2 212y x 2 z 2 +44x 2 y y 2 z 2 +12x y 4 +84z 2 +8x 2 y 4 +8x 4 y 2) The first four terms in the formula for q(x,y,z) are antisymmetric in z whereas the remaining terms are symmetric in z. Therefore, noting the factor z in the integrand of the integral J, only the first four terms of q(x,y,z) lead to a non zero contribution in J. One obtains where (see the Appendix) J = J 1 +J 2 +J 3 +J 4 Similarly, J 1 = π = π = 4 7 R3 x 2 y 2 z 2 e 2(x 2 +y 2 +z 2 ) (x 2 +y 2 +z 2 ) 3/2 r 5 e 2r2 dr π cos 2 θsin 5 θdθ 2π sin 2 φcos 2 φdφ Therefore, J 2 = π = 5 J 3 = π = 2 J 4 = π = 3 J = 4 7 R3 z 2 e 2(x 2 +y 2 +z 2 ) (x 2 +y 2 +z 2 ) 3/2 R3 y 2 z 2 e 2(x 2 +y 2 +z 2 ) (x 2 +y 2 +z 2 ) 3/2 R3 x 2 z 2 e 2(x 2 +y 2 +z 2 ) (x 2 +y 2 +z 2 ) 3/ = 24 7 >. Lemma 3.1. Let u denote the velocity field (3.1). Then its speed u(x) is maximal at x = and if p is the pressure corresponding to u (see (1.4)) then

8 156 Jens Lorenz and Randy Ott P = u(), p() = 24 7 >. Therefore, u is self accelerating for the Euler equation. Scaling argument for Navier Stokes. Let λ > denote a scaling parameter and let u denote the velocity field (3.1). We set u λ (x) = λu(x). Then the pressure determined by u λ is p λ = λ 2 p and the term corresponding to P (see (2.9)) is P λ = λ 3 P. If V λ = u λ (), u λ () denotes the viscous term corresponding to u λ (see (2.8)) then V λ = λ 2 V. Therefore, the acceleration term (see (2.8)) corresponding to u λ is A λ = λ 3 P +νλ 2 V. Since P > it follows that A λ > for sufficiently large λ. Theorem 3.1. Let u denote the velocity field (3.1) and let u λ (x) = λu(x). If λ > is large enough, then u λ is self accelerating for the Navier Stokes equation. 4. Remarks on the Pressure Equation If p is the pressure in the Navier Stokes equation then p = (u u) = (D i u j )(D j u i ) = D i D j (u i u j ) =: Q Using the vector identity (F G) = F G+G F +F ( G)+G ( F) with F = G = u and taking the divergence yields (u u) = 1 2 ( u 2 )+ (ω u). Therefore, p = Q = 1 2 ( u 2 )+ (ω u),

9 An Example of Self Acceleration for Incompressible Flows 157 i.e., (p+ 1 2 u 2 ) = (ω u). If one defines q(x) by q = (ω u), q(x) as x, then p = 1 2 u 2 +q. One may call the Bernoulli part of p and p B = 1 2 u 2 q = p nb the non Bernoulli part of p. (Note that we have nondimensionalized the Navier Stokes equations and assumed the density to be one. In dimensional variables one has p B = ρ 2 u 2 where ρ is density.) The Bernoulli part p B = 1 2 u 2 of p is irrelevant for determining the crucial quantity P (see (2.9)): If the speed u(x) has a maximum at any point x = x, then the gradient of p B is zero at x = x. For the non Bernoulli part q = p nb we have q = (ω u) = u ( ω) ω 2 showing the importance of vorticity for self acceleration. 5. Appendix: Some Elementary Integrals The evaluation of p() leads to integrals of the form J klm = x 2k y 2l z 2m r 3 e 2r 2 R 3 where r 2 = x 2 +y 2 +z 2 and where k, l, m are nonnegative integers with N := k +l+m 1.

10 158 Jens Lorenz and Randy Ott In spherical coordinates: x = rsinθcosφ y = rsinθsinφ z = rcosθ dv = r 2 sinθdrdθdφ Therefore, J klm = Here r 2N 1 e r2 dr π sin 2k+2l+1 θcos 2m θdθ 2π cos 2k φsin 2l φdφ. In particular, J r (N) = = 1 4 = 1 4 = Γ(N) 2 N+1 r 2N 1 e r2 dr r 2N 2 (4re 2r2 )dr (2r 2 = s,4rdr = ds) (s/2) N 1 e s ds J r (1) = 1 4, J r(2) = J r (3) = 1 8. The angular integrals lead to Euler s Beta function, One obtains B(α,β) = 2 π/2 = Γ(α)Γ(β) Γ(α+β) sin 2α 1 scos 2β 1 sds J θ (k,l,m) = = 2 π sin 2k+2l+1 θcos 2m θdθ π/2 sin 2k+2l+1 θcos 2m θdθ = B(k +l+1,m+ 1 2 )

11 An Example of Self Acceleration for Incompressible Flows 159 and J φ (k,l) = = 4 2π π/2 sin 2l φcos 2k φdφ sin 2l φcos 2k φdφ = 2B(l+ 1 2,k ) Therefore, J klm = Γ(N) 2 N Γ(k )Γ(l+ 1 2 )Γ(m+ 1 2 ) Γ(N ) where N = k +l+m. In particular, J 1 = 1 π πγ( ) Γ( ) = π = π 3 J 11 = 1 4 = π 3 πγ( 3 2 )Γ(3 2 ) Γ( ) J 111 = 2 8 Γ(3 2 )Γ(3 2 )Γ(3 2 ) Γ( ) π = 21 References 1. A. Bertozzi and A. Majda, Vortocity and incompressible flows, Cambridge U. Press, Cambridge, H. O. Kreiss and J. Lorenz, Initial boundary value problems and the Navier Stokes equations, Classics in Applied Mathematics 47, SIAM, 24. Jens Lorenz and Randy Ott Department of Mathematics and Statistics, UNM, Albuquerque, NM 87131