[Daban]

HELLO WORLD
i am doing a simulation on effect of vortex induced vibration on cylinders, i wrote a udf file like this:
#include "udf.h"
DEFINE_SDOF_PROPERTIES(sdof_props, prop, dt, time, dtime)
{
real cgy;
real k = 170;
cgy = DT_CG(dt)[1];
prop[SDOF_MASS] = 170;
prop[SDOF_ZERO_TRANS_Y] = TRUE;
prop[SDOF_IXX] = 122.4;
prop[SDOF_IYY] = 122.4;
prop[SDOF_IZZ] = 40.8;
prop[SDOF_LOAD_F_Y] = -k*cgy;
printf("\n updated 6DOF properties");
}
what i expect is a movement (vibration) in y direction but when i do the analysis my cylinder moves in x direction. can anyone tell me if i wrote the correct udf file !!? von karman effect.PNG

[pakk]

You say you expect a movement in y-direction. But you type:

prop[SDOF_ZERO_TRANS_Y] = TRUE;

This tells Fluent that there should be zero translation in y-direction...

Maybe you want it to be frozen in x-direction?

prop[SDOF_ZERO_TRANS_X] = TRUE;

[Daban]

thank you sir for ur reply, i am new to ansys fluent, so if i want a spring based oscillation in y direction, how should i write a udf for it?

[pakk]

Euh... Didn't I already answer that in my first reply???

[rahilvalani]

Hi there,

I am also trying to simulate Vortex induced vibrations of a cylinder. My question is that once you hook this udf to the cylinder, does ansys automatically calculated the velocity of the cylinder or do you have to write another udf for this?

Also I tried using this by using dynamic mesh in fluent but its giving me weird Coefficient of lift of about 100-200 and also the cylinder just moves in one direction and goes off the domain. Can someone please help?

[Sajid Sarfraz]

Hello

I am Sajid

I am doing vortex induced vibrations in ANSYS Fluent using following UDF.
#include "udf.h"
DEFINE_SDOF_PROPERTIES(sdof_props, prop, dt, time, dtime)
{
real cg;
real k = 1*1.26*1.26;
cg = DT_CG(dt)[8];
prop[SDOF_MASS] = 1;
prop[SDOF_ZERO_TRANS_X] = TRUE;
prop[SDOF_ZERO_ROT_X] = TRUE;
prop[SDOF_ZERO_ROT_Y] = TRUE;
prop[SDOF_LOAD_F_Y] = -k*cg;
}

However i am not getting movement of cylinder in Y dirn. Can anybody guide me

Thx

[obscureed]

Hi Sajid,

"DT_CG(dt)" is the position vector of the centre of gravity of the dynamic thread. So it does not make sense to use "DT_CG(dt)[8]" -- if you want the y-coordinate, then it is "DT_CG(dt)[1]". What was the basis for choosing 8?

The rest of the UDF looks OK (presuming this is a 2D simulation). It might help to send a message to screen before the end of the function, just to check that the UDF is hooked in correctly and to see if anything is strange:
Message("Exiting SDOF UDF with y = %g, force = %g\n",cg,-k*cg);

[nirajnisa]

I am trying to do VIV simulation of cylinder but new to ansys..can you provide me proper procedure to carry out my work