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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 2.0.0.14 (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
{
pde(phi)
-div(grad(phi))=0;
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
};
save(medit,"tank1-phi",M);
save(medit,"tank-phi",phi,M);
I then tried to solve the same problem using the "weak" form
solve(phi) in M
{
test(w)
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)
=0;
};
but I get the following error
=========================
== Text mode execution ==
=========================
FreeFEM3D computing thread: started
Parsing data
VariationalFormulaExpression.cpp:73:
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:
http://savannah.nongnu.org/bugs/?group=ff3d
any ideas?
PS

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

**[ff3d-users] Variational form of potential flow with free surface on mesh**,
*Thomas Ward* **<=**