Learning Forum

[Matthew Campbell]

Hello,

 

I am using a UDF for a simple store separation case, where external forces should be applied to the CG while it is less than a certain Z displacement (0.1m). The reference frame is oriented at the CG, with axes consistent with the body frame orientation (Z down, X Fwd). From what I can tell, the UDF I am using  (which is from the udf guide) is not applying these external forces but is applying the physical properties (Ixx, Iyy, Izz) to the body which then experiences a gravity drop. I was hoping someone could look at this UDF and provide insight into why the external forces are not being applied. My time step is 0.01. It appears that the Z displacement that I want the external forces to be acting before is not met until appx. 9 time steps. Below is a copy of the UDF I am trying to use:

/*******************************************************

SDOF property compiled UDF with external forces/moments

*******************************************************/

#include "udf.h"

DEFINE_SDOF_PROPERTIES(delta_missile, prop, dt, time, dtime)

{

   prop[SDOF_MASS]       = 907.185;

   prop[SDOF_IXX]        = 27.116;

   prop[SDOF_IYY]        = 488.094;

   prop[SDOF_IZZ]        = 488.094;

   /* add injector forces, moments */

   {

     register real dfront = fabs (DT_CG (dt)[2] -

                            (0.179832*DT_THETA (dt)[1]));

     register real dback  = fabs (DT_CG (dt)[2] +

                            (0.329184*DT_THETA (dt)[1]));

     if (dfront <= 0.100584)

       {

         prop[SDOF_LOAD_F_Z] = 10676.0;

         prop[SDOF_LOAD_M_Y] = -1920.0;

       }

     if (dback <= 0.100584)

       {

         prop[SDOF_LOAD_F_Z] += 42703.0;

         prop[SDOF_LOAD_M_Y] += 14057.0;

       }

   }

   printf ("\ndelta_missile: updated 6DOF properties");

}

[SRP]

Hi,

You mentioned that the desired Z displacement is not met until approximately 9 time steps. Ensure that the time step used in the simulation is appropriate for capturing the desired displacement within a reasonable number of time steps.

To debug the issue further, you can add additional printf statements within the conditional blocks to check whether the conditions for applying external forces are being met during the simulation.