[asking]

Hi all,

I wrote an UDF using the DEFINE_CG_MOTION macro. The problem I have is that it executes at every iteration and I only need it to execute at the end of every time step. I found the macro DEFINE_EXECUTE_AT_END which does what I want to but I can't make it work with the other macro.

I tried:

DEFINE_EXECUTE_AT_END(exec_end)
{
DEFINE_CG_MOTION(...)
{
script
}
}

but didn't work.

I tried to follow the example of this document (http://willem.engen.nl/uni/fluent/do...ips-Tricks.pdf page 42) but in my script I use a lot of information from DEFINE_CG_MOTION which I cant apply if they are separated as the example:

DEFINE_EXECUTE_AT_END(exec_end)
{
script
}

DEFINE_CG_MOTION(...)
{
script
}

thanks!

[asking]

Forgot to add the script I am trying to implement:

#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 = 0.022;
real wn = 3.0;
real k = mass*wn*wn;
real c = 2*mass*wn*0.0; /*no damping*/
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;
}

[`e`]

What are you trying to model? Why would you only want to apply the motion to the very last iteration of a time step? The forces acting on the surfaces are coupled with the flow field. Consider a uniform and constant velocity inlet boundary condition with a speed of 1 m/s, if you solve the transport equations with the inlet speed at 0 m/s for all iterations except the final iteration then you would have (at least) convergence issues.

[eagle_001]

Hi, South Yorkshire! I have just asked you in your post:
Runge-kutta fails after some iterations
whether you have tried the EXECUTE_AT_END with CG_MOTION strategy, and now I find your this post.
Just as you did, I wanted to simulate the VIV of the single-dof cylinder. And I also tried the udf you mentioned (Page 42 of http://willem.engen.nl/uni/fluent/do...ips-Tricks.pdf).
But the generated displacement grew more and more large unrealistically. I hope you have solved this problem and I will greatly appreciate you for your suggestion. Thank you very much!

[Keith Love]

Did you solve this problem? My problem is also the displacement getting bigger and bigger.

[asking]

Originally Posted by Keith Love

Did you solve this problem? My problem is also the displacement getting bigger and bigger.

Yes, I posted an improved version of the UDF on:
Runge-kutta fails after some iterations

check it out.

Regards,