[ANSYS Meshing] negative cell volume detected while running ANSYS Fluent UDF

khushbu.bhavsar92 · March 29, 2016, 23:52

HI
I am trying to simulate vortex induced vibrations in a cylinder (2D). I am trying to achieve this using udf and six degree of freedom solver with smoothing and remeshing in Fluent. I keep getting an error -negative cell volume detected while running the simulation. I tried reducing the time step and increasing the stiffness constant. It still shows the same issue. Could someone please help me solve this problem.

thank you

vasava · April 12, 2016, 05:45

Can you post details of the 'Dynamic Mesh Zones' setup?

khushbu.bhavsar92 · April 12, 2016, 17:38

Hi,
The geometry includes a circle in a rectangular domain
I have kept a velocity inlet (0.4m/s) and pressure outlet.
The fluid body mesh is deforming.
The cylinder (0.27m diameter) is rigid with 6DOF activated and the following UDF code,
#include "udf.h"

DEFINE_SDOF_PROPERTIES(udfcyl, prop, dt, time, dtime)
{
real cgx;
real cgy;
real k = 718.88;
cgx = DT_CG(dt)[0];
cgy = DT_CG(dt)[1];
prop[SDOF_MASS] = 6146.18;
prop[SDOF_ZERO_ROT_X] = TRUE;
prop[SDOF_ZERO_ROT_Y] = TRUE;
prop[SDOF_ZERO_ROT_Z] = TRUE;
prop[SDOF_IXX] = 208.72;
prop[SDOF_IYY] = 208.72;
prop[SDOF_LOAD_F_X] = -k*cgx;
prop[SDOF_LOAD_F_Y] = -k*cgy;
}

vasava · April 13, 2016, 03:18

Could you post the numbers you have used for 'smoothing' and 'remeshing' in the 'Mesh Methods Settings.'?

khushbu.bhavsar92 · April 14, 2016, 00:45

in smoothing, I've taken the spring constant as 0.1 and rest as default values,
in remeshing min length=2.4cm, max length =7.9cm, max skewness=0.7, remesh interval=1
these are basically the default values.

vasava · April 14, 2016, 01:40

I recommend that you get the 'Mesh Scale Info' and modify the re-meshing numbers according to the Mesh scale.

Also check the mesh size at the surface of the cylinder and reduce your time-step so that the distance traveled by cylinder per time step is less than the smallest mesh elements. This will help you avoid negative cell volumes in case your mesh is not modifying properly.

I am sue you know this but I write anyways, use 'preview mesh motion' before you start the actual simulation.

khushbu.bhavsar92 · April 14, 2016, 15:50

I will keep that in mind. Thank you for the help !

March 29, 2016, 23:52	negative cell volume detected while running ANSYS Fluent UDF	#1
khushbu.bhavsar92 New Member Join Date: Mar 2016 Posts: 8 Rep Power: 10	HI I am trying to simulate vortex induced vibrations in a cylinder (2D). I am trying to achieve this using udf and six degree of freedom solver with smoothing and remeshing in Fluent. I keep getting an error -negative cell volume detected while running the simulation. I tried reducing the time step and increasing the stiffness constant. It still shows the same issue. Could someone please help me solve this problem. thank you

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April 12, 2016, 05:45		#2
vasava Senior Member Paritosh Vasava Join Date: Oct 2012 Location: Lappeenranta, Finland Posts: 732 Rep Power: 22	Can you post details of the 'Dynamic Mesh Zones' setup?

April 12, 2016, 17:38		#3
khushbu.bhavsar92 New Member Join Date: Mar 2016 Posts: 8 Rep Power: 10	Hi, The geometry includes a circle in a rectangular domain I have kept a velocity inlet (0.4m/s) and pressure outlet. The fluid body mesh is deforming. The cylinder (0.27m diameter) is rigid with 6DOF activated and the following UDF code, #include "udf.h" DEFINE_SDOF_PROPERTIES(udfcyl, prop, dt, time, dtime) { real cgx; real cgy; real k = 718.88; cgx = DT_CG(dt)[0]; cgy = DT_CG(dt)[1]; prop[SDOF_MASS] = 6146.18; prop[SDOF_ZERO_ROT_X] = TRUE; prop[SDOF_ZERO_ROT_Y] = TRUE; prop[SDOF_ZERO_ROT_Z] = TRUE; prop[SDOF_IXX] = 208.72; prop[SDOF_IYY] = 208.72; prop[SDOF_LOAD_F_X] = -kcgx; prop[SDOF_LOAD_F_Y] = -kcgy; }

April 13, 2016, 03:18		#4
vasava Senior Member Paritosh Vasava Join Date: Oct 2012 Location: Lappeenranta, Finland Posts: 732 Rep Power: 22	Could you post the numbers you have used for 'smoothing' and 'remeshing' in the 'Mesh Methods Settings.'?

April 14, 2016, 00:45		#5
khushbu.bhavsar92 New Member Join Date: Mar 2016 Posts: 8 Rep Power: 10	in smoothing, I've taken the spring constant as 0.1 and rest as default values, in remeshing min length=2.4cm, max length =7.9cm, max skewness=0.7, remesh interval=1 these are basically the default values.

April 14, 2016, 01:40		#6
vasava Senior Member Paritosh Vasava Join Date: Oct 2012 Location: Lappeenranta, Finland Posts: 732 Rep Power: 22	I recommend that you get the 'Mesh Scale Info' and modify the re-meshing numbers according to the Mesh scale. Also check the mesh size at the surface of the cylinder and reduce your time-step so that the distance traveled by cylinder per time step is less than the smallest mesh elements. This will help you avoid negative cell volumes in case your mesh is not modifying properly. I am sue you know this but I write anyways, use 'preview mesh motion' before you start the actual simulation.

April 14, 2016, 15:50		#7
khushbu.bhavsar92 New Member Join Date: Mar 2016 Posts: 8 Rep Power: 10	I will keep that in mind. Thank you for the help !