[Albino]

Hello!

I am trying to simulate displacements of a cylinder caused by unsteady flow over a cylinder in front of it. My geometry is very simple - basicly a circular domain (tried rectangular as well) with two circles created with a boolean operation (64x40m domain, circles 1m diameter).

The rest is very similair to the tutorial here; https://confluence.cornell.edu/displ...+Specification

I have tried different meshes - the latest attempt was with triangels (60.000 elements). In fluent I enable dynamic mesh (and 6DOF), compile/load an UDF (will post below), add this motion to the second cylinder, and to the "interior-solid body" (fluid). The first cylinder I choose as "stationary".

My problem is that after some time steps (usually just a few time steps after the second cylinder starts moving) fluent stops with the error message;

"Error [client] [time 10/25/13 18:37:20] Update-Dynamic-Mesh failed. Negative cell volume detected."

The UDF applied to second cylinder and fluid;
#include "udf.h"
DEFINE_SDOF_PROPERTIES(sdof_props, prop, dt, time, dtime)
{
real cg;
real k = 1;
cg = DT_CG(dt)[1];
prop[SDOF_MASS] = 1;
prop[SDOF_LOAD_F_Y] = -k*cg;
prop[SDOF_ZERO_TRANS_X] = TRUE;
prop[SDOF_ZERO_ROT_X] = TRUE;
prop[SDOF_ZERO_ROT_Y] = TRUE;
}

I have tried lowering the time step, and it still fails just after some movement on the second cylinder. Also tried enabling "re-meshing" under dynamic mesh - didn't help.

Any advice? I believe the mesh is failing here since its after the second cylinder starts moving that the error pops up but I am not sure how to fix it. I'm pretty new to fluent so it could very well be something very obvious that is the issue.

Thanks for your time

[Albino]

I still haven't figured this one out Here comes more info (2D problem):



Blue line is velocity inlet, red is pressure outlet. I want to simulate the flow over these two circles in the "box" - i want the left circle (cylinder1) to be in a fixed position, while the right circle (cylinder2) is free to move up/down (y-axis).

My mesh is unstructered (just loading default mesh with fluent, and changing max size to like 0.5 to get around 100.000 elements.

To do this I am enabling the dynamic mesh setting, with smoothing and remeshing enabled. Also, I enable the 6DOF setting.

Under dynamic mesh zones I have tried the many combinations (which all fail..) but basicly;

Cylinder1 - stationary.
Cylinder2 - rigid body with 6DOF enabled and UDF (see first post) loaded.
surface_body - tried both deforming and same as cylinder2. This is supposed to be the fluid.
interior-surface_body - usually i set this the same as surface_body.
I leave farfields and walls on default.

After 6 iterations I always get "Update-Dynamic-Mesh failed. Negative cell volume detected." (tried smaller time steps and still get the same error after 6 iterations).

I can upload case/mesh files if it helps.

Thanks!

[DCUser]

Hi Albino,

I'm currently trying to simulate an oscillating 2D cylinder using the dynamic mesh tool, but I can't find any tutorial except the one about the valve (included in Fluent). Would you have any tutorial or hints for me please ?

Thanks

[Albino]

Originally Posted by DCUser

Hi Albino,

I'm currently trying to simulate an oscillating 2D cylinder using the dynamic mesh tool, but I can't find any tutorial except the one about the valve (included in Fluent). Would you have any tutorial or hints for me please ?

Thanks

Sorry for the late response;

As for tutorial you can just use any "flow over cylinder" tutorial that is available (just google it). Adding oscillation is done via the use of a user defined function (UDF).

Basicly you have to create (or use exsisting) an UDF that describes the oscillating motion and then add it to the cylinder under the "dynamic mesh" menu in fluent.

When you have a UDF (.c file) you need to compile it via fluent as per link below. However this only works if you launch fluent via windows SDK. You can google on how to do this (you need to install visual studio etc - there is a guide for this!)

This should be helpful:
http://www.cfd-online.com/Forums/flu...r-problem.html

Good luck!

[Giulio.Biscarini]

Hi Albino,
I'm trying to simulate an oscillating 2D cylinder using the dynamic mesh tool but i had the same problem as you:
fluent, after several steps, stops with the following error message:

"Error [client] [time 10/25/13 18:37:20] Update-Dynamic-Mesh failed. Negative cell volume detected."

Did you find any solution ??

Thanks