| Author | Message | |||
     andreachan Member Username: andreachan Post Number: 6 Registered: 12-2009 |
The situation is a simple 2D-Channel. Viscous fluid flows in it. When I set the boundary conditions as below, the calculation in FlexPDE does not work. Exactly the error message is 'Floating point stack check' BCs: at the upper & botton walls: value(u)=0 value(v)=0 load(p)=0 at inlet: value(u)=u0 value(v)=0 at outlet: load(u)=0 load(v)=0 where u and v are the velocities in x- and y-directions. p is the pressure. u0 is a velocity-profile. the main equations are just the same as they are in the example lowvisc.pde u: visc*div(grad(u)) - dx(p) = dens*(u*dx(u) + v*dy(u)) v: visc*div(grad(v)) - dy(p) = dens*(u*dx(v) + v*dy(v)) p: div(grad(p)) = penalty*(dx(u)+dy(v)) Can anyone tell me the reason, why it doesn't work? | |||
| mgnelson Moderator Username: mgnelson Post Number: 185 Registered: 07-2007 |
If you attach your script, we can take a look at it. It is too difficult to guess based on the information listed here. We need to be able to run the script. | |||
| andreachan Member Username: andreachan Post Number: 7 Registered: 12-2009 |
here is the script. | |||
| rgnelson Moderator Username: rgnelson Post Number: 1338 Registered: 06-2003 |
1) your boundary conditions on P are all derivatives, which makes the pressure underspecified and free to float to arbitrary values. This is probably where the stack check is coming from. You need to put some kind of value constraint on P. Either a CONSTRAINT equation to force the integral, or (as I have done in the attached) simply set the outlet pressure to zero. 2) Uniform velocity on the input plane with zero velocity on the sides creates a non-physical condition (you can't implement such a thing in the real world). It requires an infinite pressure spike at the corners and infinite regridding to resolve the spike. You could put a short stub of Load(u)=0 on the corners to allow a smooth transition to zero, or you could input a shape that drops to zero at the walls. Or you can limit the mesh size (as the student version will do) and accept an approximate answer. 3) Forcing v=0 at the exit seems to be mildly inconsistent with something else in the problem. It seems to work a bit better if you use LOAD(v)=0 on the outlet. 4) If you input the quadratic channel distribution for u at the inlet, then v is nominally zero everywhere, and FLexPDE will continually readjust the scaling of v trying to find the range of activity. An appropriate THRESHOLD on V will prevent this from becoming an infinite search. See attached.
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