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Mathematica and Euler's equation

Posted 10 years ago

Hi,

I've been trying to solve Euler's equation for fluid motion with the simplest possible boundary conditions but from the way I'm writing I'm getting nowhere. Mathematica just outputs

NDSolve::femnonlinear: Nonlinear coefficients are not supported in this version of NDSolve

I've tried everything I found on the internet even some voodoo about telling Mathematica the equations are time-dependent fails. Is this really it? Mathematica can't numerically solve a coupled pde with non-constant coefficients? If Mathematica can't who can?

I have to admit I'm not symplifing anything, I've just copied Marsden's (A Mathematical Introduction to Fluid Mechanics) equations and hoped for the best. I'm doing this to get an idea about solutions and how they behave but this has proven to be a fruitless path.

Thanks in advance,

E.

4 Replies
Posted 2 years ago

Hello, in case anyone still needs assistance, this link may help: https://mathworld.wolfram.com/EulerEquation.html .

POSTED BY: Farial Mahmod

Hi,

so here's the code:

b = {0, -10*y}

NDSolve[ {[Rho][x, y, t](D[ux[x, y, t], t] + ux[x, y, t]D[ux[x, y, t], x] + uy[x, y, t]D[ux[x, y, t], y]) == -D[p[x, y, t], x] + [Rho][x, y, t]b[[1]], [Rho][x, y, t](D[uy[x, y, t], t] + ux[x, y, t]D[uy[x, y, t], x] + uy[x, y, t]D[uy[x, y, t], y]) == -D[p[x, y, t], y] + [Rho][x, y, t]b[[2]],

D[[Rho][x, y, t], t] + ux[x, y, t]*D[[Rho][x, y, t], x] + uy[x, y, t]*D[[Rho][x, y, t], y] == 0, D[ux[x, y, t], x] + D[uy[x, y, t], y] == 0, ux[0, y, t] == 0, ux[10, y, t] == 0, uy[x, 0, t] == 0, uy[x, 10, t] == 0, [Rho][x, y, 0] == 1, p[x, 10, t] == 10}, {ux, uy, [Rho], p}, {x, 0, 10}, {y, 0, 10}, {t, 0, 10}]H

Boundary conditions translate to n dot u == 0 at the boundary. I wish I had a better way to post this code. I hope it's readable. [Rho] is the density, p is the pressure, u is the velocity and b is the body force, I just added gravity. The problem happens even in one dimension when you multiply de density times the velocity field.

Anyway as you sad, they are hard equations to solve. What's the procedure people usually do to solve them?

Thanks,

E.

P.S.: I'm attaching the notebook hoping it helps to read the stuff.

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Posted 10 years ago
POSTED BY: David Keith

Hi, You might want to show your code. euler's equation is nonlinear and with some exceptions extremely difficult to solve!

POSTED BY: Kay Herbert
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