Negative volumes when using UDF in VIV in a cylinder
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Hello all,
I am trying to solve the VIV case in a cylinder when Re=100 in Fluent 6.3. I have the solution when the cylinder is at rest, and my results are accurate. However, my problems appear when I try to use an UDF. When it is iterating, it stops because negative volumes appear. I do not know what I am doing wrong. My udf that I am using is one that I found on Internet: #include "udf.h" static real v_prev = 0.0; DEFINE_CG_MOTION(osc,dt,vel,omega,time,dtime) { Thread *t; Domain *d = Get_Domain(1); real x_cg[3], force[3], moment[3]; real accl, dv; real mass = 0.2; real wn = 12.0; real k= mass * wn*wn; real c = 2*mass*wn*0.00; int i; NV_S(vel, =, 0.0); NV_S(omega, =, 0.0); t = DT_THREAD(dt); for(i=0;i<3;i++) x_cg[i]=DT_CG(dt)[i]; Compute_Force_And_Moment(d, t, x_cg, force, moment, TRUE); force[1] += k*x_cg[1]  c*vel[1]; accl = force[1]/mass; dv = accl*dtime; v_prev+=dv; vel[1] = v_prev; printf("Computed force: %g \n", force[1]); printf("Velocity: %g\n", vel[1]); } DEFINE_CG_MOTION(osc_inert,dt,vel,omega,time,dtime ) { vel[0] = 0.0; vel[1] = v_prev; } My parameters are based on smoothing and remeshing, and I am hooking osc to my cylinder. Here is attached my mesh. Please I need your help because I do not know if I need to change the mesh or I need to change some of the parameters. Thanks, Willy 
Hi:
Have u solve your problem regarding the negative grid? Could u tell me? Tks. 
You need to define another fluid zone which surrounds your body and moves with it...
Can you send me your case & mesh (jessica.simberg@gmail.com)? 
WCOCHE...have you fixed your problem?

Hi
Is the above udf correct? how and where you are calculating the pressure and viscous forces? I guess there should be a loop for calculating pressure and viscous forces acting on the cylinder. If someone have an answer to this please reply 
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