[asking]

Hi,

I am trying to move two mesh zones at the same time per time step. I used the DEFINE_CG_MOTION, which moves a zone as a rigid body (no internal deformation), and I prescribed a sinusoidal velocity to move both mesh zones.

Here is my code:

#include "udf.h"
#include "mem.h"

static real vy = 0.0;

DEFINE_CG_MOTION(rigid,dt,vel,omega,time,dtime)
{

/*Define variables*/
real cg[3], vcg[3];
int i;
NV_S(vel, =, 0.0);
NV_S(omega, =, 0.0);
Thread *thread;
Domain *d = Get_Domain(1);
face_t f;
thread = Lookup_Thread(d,zoneID);

real ctime = RP_Get_Real("flow-time");

for (i=0;i<3;i++)
{
cg[i]=DT_CG(dt)[i];
vcg[i] = DT_VEL_CG(dt)[i];
}

vy = 0.1*sin(31.41*ctime);

vel[0] = 0.0;
vel[1] = vy;
}

DEFINE_CG_MOTION(follower,fdt,fvel,fomega,time,dtime)
{

/*Define variables*/
NV_S(fvel, =, 0.0);
NV_S(fomega, =, 0.0);
real fcg[3], fvcg[3];
real fctime = RP_Get_Real("flow-time");
int i;

Thread *thread;
Domain *d = Get_Domain(1);
thread = DT_THREAD(fdt);

for (i=0;i<3;i++)
{
fcg[i]=DT_CG(fdt)[i];
fvcg[i] = DT_VEL_CG(fdt)[i];
}

fvel[0] = 0.0;
fvel[1] = vy;
}

And here are the results of the position of the centre of gravity in both mesh zones. Notice that they both start at the same position (0,0).

time - vel_rigid - vel_follower
0.0000 - 0.00000 - 0.00000
0.0010 - 0.00314 - 0.00000
0.0020 - 0.00628 - 0.00314
0.0020 - 0.01253 - 0.00628
... etc

As you can see, there is a delay in the "follower" velocity compared to the "rigid" one. Thus, both zones are not moving as a block.

Any ideas / suggestions on how to move both zones at the same time?

Thank you,

[blackmask]

You use the static variable to transport information between functions but how could you be sure that "follower" is actually called after "rigid"? You need to assign the variable upon first call at each time step. A conceptual code looks like:

int cur_time_step = -1;

func_rigid() {
if (cur_time_step < N_TIME) { cur_time_step = N_TIME;vy = ...}

vel[1] = vy;
}

func_follower() {
if (cur_time_step < N_TIME) { cur_time_step = N_TIME;vy = ...}

vel[1] = vy;
}

[asking]

Originally Posted by blackmask

You use the static variable to transport information between functions but how could you be sure that "follower" is actually called after "rigid"? You need to assign the variable upon first call at each time step. A conceptual code looks like:

int cur_time_step = -1;

func_rigid() {
if (cur_time_step < N_TIME) { cur_time_step = N_TIME;vy = ...}

vel[1] = vy;
}

func_follower() {
if (cur_time_step < N_TIME) { cur_time_step = N_TIME;vy = ...}

vel[1] = vy;
}

Thank you very much. You were absolutely right. Your code also prevents the calculation of the velocity when the UDF its called multiple times within one timestep, which fixed another UDF I am implementing.