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[ff3d-users] Variational form of potential flow with free surface on mes

From: Thomas Ward
Subject: [ff3d-users] Variational form of potential flow with free surface on mesh
Date: Thu, 15 May 2008 21:03:07 +0100
User-agent: Thunderbird (X11/20080505)

First of all, thanks to the ff3d team for making ff3d available, I found the approach very natural and the documentation excellent.

I am using ff3d to solve a potential flow problem for forced periodic oscillation of fluid in a tank with gravity free surface waves.
The solution using "strong" form appears to work, see below.

vertex a = (0,0,0);
vertex b = (10,10,10);
vector n = (25,50,10);

mesh M = structured(n,a,b);

double f=1.4;
function alpha=f*f/9.81;
solve(phi) in  M
   dnu(phi)=f on M xmin; //forced velocity = - omega cos(wt)
   dnu(phi)=-f on M xmax;
   dnu(phi)=0 on M ymin;// normal velocity zero on y walls
   dnu(phi)=0 on M ymax;
   dnu(phi)=0 on M zmin; //normal velocity zero on bottom
   alpha*phi +dnu(phi) = 0 on M zmax; //free surface condition

I then tried to solve the same problem using the "weak" form

solve(phi) in  M
     int (dx(w)*dx(phi)+dy(w)*dy(phi)+dz(w)*dz(phi))
//    int(grad(phi)*grad(w))
    +int[M xmin](w*dx(phi)  +w*f)
    +int[M xmax](w*dx(phi)-w*f)
    +int[M ymin](w*dy(phi))
    +int[M ymax](w*dy(phi))
    +int[M zmin](w*dz(phi))
    +int[M zmax](w*dz(phi)+w*alpha*phi)

but I get the following error

== Text mode execution ==
FreeFEM3D computing thread: started
Parsing data

unexpected operator type

UNEXPECTED ERROR: this should not occure, please report it

BUG REPORT: Please send bug reports to:
  address@hidden or address@hidden
or better, use the Bug Tracking System:

any ideas?


One small documentation suggestion; the method of defining Robin type boundary conditions be added to the documentation or README file.


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