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## Re: [ff3d-users] Navier Stokes equation

**From**: |
Stephane Del Pino |

**Subject**: |
Re: [ff3d-users] Navier Stokes equation |

**Date**: |
Sat, 8 Jan 2005 14:48:40 +0100 |

**User-agent**: |
KMail/1.7.1 |

Le Vendredi 7 Janvier 2005 22:08, Bharat Rangan a écrit :
>* Hi,*
>* While solving the NS equations, I used the following equation for pressure*
>
>* -div(grad(p)) + dx(u1)*dx(u1) + dy(u2)*dy(u2) + 2*dx(u2)*dy(u1) = 0;*
>
>* I got the following error. unexpected '*'. Expected +,- or =. Is this some*
>* parser error or am I making a mistake ??*
I think you did a mistake ... There are a few things that confuse me here.
If I am right, you want to solve something like
-div(grad(p))+div(U) = 0
where U=(u1,u2,u3). Then just write
-div(grad(p)) = -dx(u1)-dy(u2)-dz(u3);
If you are solving an equation on 'p', it is the only unknown, so only 'p'
may be on the left hand side of the equation. If it was a system, you should
of course write
-div(grad(p))+dx(u1)+dy(u2)+dz(u3)=0;
in ff3d, PDEs unknowns have always to be given at the left hand side and
data at the right hand side.
I really do not understand the terms dx(u1)*dx(u1) + ...
They are non-linear terms. Could you explain them to me?
Finally, as I see, you are not using 'dz' or 'u3'. Are you trying to solve a
2D problem? In that case, ff3d may not be the right answer, it only solves 3D
problems up to now ...
Best regards,
Stephane.