CFD Online Discussion Forums - using DEFINE_GEOM to change 2D geometry- shear constant area

CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)

- FLUENT (https://www.cfd-online.com/Forums/fluent/)

- - using DEFINE_GEOM to change 2D geometry- shear constant area (https://www.cfd-online.com/Forums/fluent/226672-using-define_geom-change-2d-geometry-shear-constant-area.html)

using DEFINE_GEOM to change 2D geometry- shear constant area

Hi All,

I have been trying to deform a square into a rhombic shape such as the attached pure strain image and have been testing out a few UDFs to deform the boundaries.
Attachment 77223

I start out with a simple mesh geometry like this:
Attachment 77225
and use DEFINE_GEOM to deform all four walls (labeling is wall12 top right wall and then going clockwise).
This is my dynamic mesh setup page:
Attachment 77229

Current issues with my UDF:
(1) Something that I am worried about is CURRENT_TIME, so I tried to output it as a message, but it always shows like 21 times? Should i put it outside the function? such as in my TUI:

"Updating mesh to time 3.00000e-02 (step = 00003)... time = 0.020000
time = 0.020000
time = 0.020000
time = 0.020000
time = 0.020000
time = 0.020000
time = 0.020000
time = 0.020000
time = 0.020000
"

(2) my mesh is only moving in the y direction, but I know I need to be stretching all four "corners" so it should update x and y.
Attachment 77230

Any help is appreciated. I have also tried to use DEFINE_GRID_MOTION to move the walls at an angle, but to no avail.

I have attached my full UDF, but a snippet of it is as follows:

#include "udf.h"
#include "unsteady.h"

#define W 64 /* side wall of trough, mm */
#define gamma 0.2 /* desired strain */
#define vel_exp 5e-2 /* expansion velocity mm/s */
#define vel_comp 4.52267e-2 /* compression velocity mm/s */

DEFINE_GEOM(wall_12geom, domain, dt, position)
{ Message("time = %f\n ", CURRENT_TIME , '\n');
real D,R,L,l;
real hD,hL,hl;
real dlength_L, dlength_l;
real update_length_L, update_length_l;

/*constants */
D = sqrt(2)*W; /*diagnol of square, intial*/
R = sqrt((2+gamma)/(2-gamma));
L = R*D; /*expansion axis diagnol, final*/
l = pow(D,2)/L; /* compression axis diagnol, final*/
hD=D/2; /* use these variables to calculte angle*/

real t = CURRENT_TIME;
real slope12, intercept12; /* geometry line */

dlength_L = vel_exp*t; /*delta change in expansion axis from ORIGIN, lengthening x axis*/
dlength_l = -1*vel_comp*t; /*delta change in compression axis from ORIGIN, shortening y axis*/

update_length_L = hD + dlength_L; /* every time step length, L from ORIGIN to sharp corner*/
update_length_l = hD + dlength_l; /* every time step length,l from ORIGIN to sharp corner*/

/* set variables */
slope12 = dlength_l/dlength_L; /*neg */
intercept12 = update_length_l; /*pos*/

/* set y = mx + b dependent on time*/
position[0] = position[0] + dlength_L; /* START HERE */

position[1] = slope12*position[0] + intercept12;

}

Shape Deformation

Instead of using DEFINE_GEOM, use DEFINE_GRID_MOTION. This way you can control motion of each individual node instead of a boundary. If you want to use DEFINE_GEOM, then you have to define four different functions, one for each edge.

Quote:

Originally Posted by vinerm (Post 768790)

Hi Vinerm,

Thank you for your reply. I had originally used DEFINE_GRID_MOTION to control the edge boundaries ( one function for each edge), but did not get it to work properly. With DEFINE_GRID_MOTION, I moved each node of the boundaries based on the angle of the two corners that were moving. My code for one of the wall boundaries is below.

However, I think you are saying I should use DEFINE_GRID_MOTION on the interior surface body too? I'm a little confused how you would avoid cell skew using that? Could you expound on that? Thanks!

I wonder if this has to do with constraining a mesh that resists the movement of the boundary walls to turn " inward."

Code:

/*expansion*/

DEFINE_CG_MOTION(corner2cg, dt, cg_vel, cg_omega, time, dtime) 

{

  cg_vel[0] = vel_exp;

  cg_vel[1] = 0.0;

  cg_vel[2] = 0.0;





}



/*compression*/

DEFINE_CG_MOTION(corner3cg, dt, cg_vel, cg_omega, time, dtime) 

{

  cg_vel[0] = 0.0;

  cg_vel[1] = vel_comp;

  cg_vel[2] = 0.0;





}



/*expansion*/

DEFINE_CG_MOTION(corner4cg, dt, cg_vel, cg_omega, time, dtime) 

{

  cg_vel[0] = -1*vel_exp;

  cg_vel[1] = 0.0;

  cg_vel[2] = 0.0;





}



/* PROGRESS */

DEFINE_GRID_MOTION(wall_12grid, domain, dt, time, dtime)

{

  Thread *tf = DT_THREAD (dt);

  face_t f;

  Node *v;

  real D,R,L,l;

  real hD,hL,hl;

  real dlength_L, dlength_l;

  real update_length_L, update_length_l;

  real alpha_new,d_alpha;

  real NV_VEC(centroid), NV_VEC(rvec), NV_VEC(dvel), NV_VEC(dx);

  real NV_VEC(d_omega);

  int n;

  

  /* !!!! update here !!!!*/

  /* initialize the vectors */

  NV_D(centroid, =, 0, 0.0, 0.000); /* meters CONSTANTLY MOVING! */

  NV_S(d_omega,=,0.0); /* cycles per second */

  

  /*constants */

        D = sqrt(2)*W; /*diagnol of square, intial*/

        R = sqrt((2+gamma)/(2-gamma)); 

        L = R*D; /*expansion axis diagnol, final*/

        l = pow(D,2)/L; /* compression axis diagnol, final*/

        

        hD=D/2; /* use these variables to calculte angle*/

        hL=L/2;

        hl=l/2;



        

        dlength_L = vel_exp*time; /*delta change in expansion axis from ORIGIN, lengthening*/

        dlength_l = -1*vel_comp*time; /*delta change in compression axis from ORIGIN, shortening*/

        

        update_length_L = hD + dlength_L; /* every time step length, L from ORIGIN*/

        update_length_l = hD + dlength_l; /* every time step length,l from ORIGIN*/

        

        alpha_new = atan2(update_length_l,update_length_L); /* change in angle of wall,rad */

        d_alpha = alpha_prev - alpha_new; /* change in angle from prior timestep to now, rad */

        alpha_prev = alpha_new; /* store change in angle (rad) per time step */

        

        d_omega[2] = d_alpha/dtime/(2*M_PI); /* cycles per second [Hz] every time step update*/

        

        /* change the reference vector */ 

        centroid[0] = update_length_L;



        Message("centroid_x = %f,dlength_L = %f, dlength_l = %f, alpha_new = %f, alpha_prev = %f", centroid[0],dlength_L, dlength_l, alpha_new,alpha_prev );

  

/* Set/activate the deforming flag on adjacent cell zone, which      */

/* means that the cells adjacent to the deforming wall will also be  */

/* deformed, in order to avoid skewness.                             */

  SET_DEFORMING_THREAD_FLAG (THREAD_T0 (tf));



/* Loop over the deforming boundary zone's faces;                    */

/* inner loop loops over all nodes of a given face;                  */

/* Thus, since one node can belong to several faces, one must guard  */

/* against operating on a given node more than once:                 */



  begin_f_loop (f, tf)

    {

      f_node_loop (f, tf, n)

        {

          v = F_NODE(f, tf, n);



          /* Update the current node only if it has not been         */

                  /* previously visited:                                     */

          if (NODE_POS_NEED_UPDATE (v))

            {

              /* Set flag to indicate that the current node's        */

                          /* position has been updated, so that it will not be   */

              /* updated during a future pass through the loop:      */

              NODE_POS_UPDATED (v);

                          

                          NV_VV(rvec, =, NODE_COORD(v),-,centroid); /* setting vector distance*/

                          NV_CROSS(dx, d_omega, rvec); /* velocity vector, m/s = (1/s * m )*/

                          NV_S(dx,*=,dtime); /* distance vector*/

                          NV_V(NODE_COORD(v),+=,dx); /* updating current nodes to new position) */

                

            }

        }

    }

  end_f_loop (f, tf);

  

  b++;

}