[Acharya_Arbind]

I'm performing the simulation for flow around free transversely oscillating cylinder for a Reynold's Number of 200 using Standard K-e model. I'm currently facing negative cell volume detected error because of weird mesh deformation, ig I have made few mistakes in defining the parameters in dynamic mesh setting but I'm unable to pin point my mistake. Also the simulation occurs for a few time steps(50-100) with too high values of lift and drag coefficients and then negative volume error occurs. I'm attaching below the pictures of deformed mesh, lift, drag coefficient curves, my smoothing and remeshing parameters too. Also I'm using a udf to give SDOF property to the cylinder body. I had created a total thickness inflation mesh around my cylinder which is of 5 layers, growth rate of 1.2 and maximum thickness of 0.025. Hence, I first separate the cylinder along with the 5 layered inflation from the interior surface body, then I hook the udf to both the cylinder and the 5 inflation layers that I had separated. The SDOF is ON for Cylinder and SDOF is ON and Passive for the zone separated with the cylinder. Other than that I just keep the smoothing and remeshing parameters turned on. That's all the dynamic meshing I do. I feel like there's an issue with the dynamic mesh and I kindly request you all to help me regarding this issue as I've been trying to solve this issue since past 4 weeks, and the situation is kinda demotivating for me. I took reference of changing the time step size from various articles but still I get the same error. Also kindly check the udf too.

No. of Time Steps=1500, Time Step Size= 0.001, Iterations/timestep=30. Below is the udf that I'm using for simulation:-

#include "udf.h"
#include "dynamesh_tools.h"
#include "unsteady.h"
#include <stdio.h>

FILE *fout; /* Write the output file */
static real v_prev; /* Velocity in the previous time step */

DEFINE_SDOF_PROPERTIES(stage, prop, dt, time, dtime)
{
Thread *t;
Domain *d = Get_Domain(1);
real x_cg[3], force[3], moment[3];
real cg; /* Centre of gravity position */
real vel; /* Cylinder velocity */
real Fy; /* Lift force */
real mass = 0.03578; /* Cylinder mass */
real k = 69480.0; /* System stiffness */
real c = 0.0039; /* System damping */

/* Get the thread pointer for which this motion is defined */
t = DT_THREAD(dt);

prop[SDOF_MASS] = 0.03578; /* System mass of the 6 DOF */

/* Limit 5 DOFs from 6 DOF Macro */
prop[SDOF_ZERO_TRANS_X] = TRUE;
prop[SDOF_ZERO_TRANS_Z] = TRUE;
prop[SDOF_ZERO_ROT_X] = TRUE;
prop[SDOF_ZERO_ROT_Y] = TRUE;
prop[SDOF_ZERO_ROT_Z] = TRUE;

cg = DT_CG(dt)[1];
vel = DT_VEL_CG(dt)[1];
prop[SDOF_LOAD_F_Y] = -k * cg - vel * c;

fout = fopen("results.txt", "a"); /* Open file */

/* Write data into file */
fprintf(fout, "%g %g %g\n", time, DT_CG(dt)[1], DT_VEL_CG(dt)[1]);
fclose(fout);
}