Learning Forum

[jdch232]

Hello,

I'm attempting to model a flexible flat plate using using a UDF with the DEFINE_GRID_MOTION macro in Fluent 2025 R1 student through Workbench. I'm new to using UDFs, though I have gotten some working. I'm defining a flexible plate as one that alternates between flat and bulging outwards in several peaks over time.

I'm using the following equation for y-velocity: Vy = (5pi/33) * (sin{2pi * t - 1.5} + 1) * sin{10000pi * x}

This results in a plate alternating between flat at 0s, 1s, 2s, ... and a flexed at 0.5s, 1.5, 2.5s, ..., as shown below.

I'm using the following c code:

#include "udf.h"

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

{

  Thread *t;

  face_t f;

  Node *v;

  real NV_VEC(velocity);

  thread_loop_c(t, domain)

  {

      // Loop across faces in wall

      begin_f_loop(f, t)

      {

        // Loop across nodes of face

        f_node_loop(f, t, v)

        {

          // Current node's coordinates

          real x_coord = NODE_X(v);

          real y_coord = NODE_Y(v);

          // Calculate velocity based on x_coord and time

          velocity[0] = 0;

          velocity[1] = (5 * 3.14159 / 33) * (sin(2 * 3.14159 * time - 1.5) + 1) * (sin(1000 * 3.14159 * x_coord));

          // Update node's position from calculated velocity

          NODE_X(v) += velocity[0] * dtime;

          NODE_Y(v) += velocity[1] * dtime;

        }

      }

      end_f_loop(f, t)

    }

  }

When I compile the code in Fluent I get this warning: 

"warning: ordered comparison between pointer and integer ('Node *' (aka 'struct _tg_node_struct *') and 'int')

            f_node_loop(f, t, v)

            ^~~~~~~~~~~~~~~~~~~~"

I've looked into this and found it's saying I'm comparing a pointer variable and an integer. Though I'm having trouble seeing where.

Then when I create the dynamic mesh on the plate, I get this warning:

"Warning: incorrect cg motion UDF flexible_wall_velocity::libudf on zone 10 (assuming no motion)"

This confuses me as I'm not using the CG Motion macro. Does anyone have any advice or direction I could take?

[Mark O]

Please see the example for DEFINE_GRID_MOTION in the Fluent Customization manual. It is not f_node_loop(f, t, v). It is

int n;

f_node_loop(f,t,n)

{

  v = F_NODE(f,t,n);

}

 

[jdch232]

Yes I was able to get it working. For anyone who might find it helpful, this is the code that ended up working:

 

#include "udf.h"

#include "math.h"

#define PI 3.14159

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

{

    Thread *thread = DT_THREAD(dt);

    face_t f;

    Node *v;

    int n;

    real x, y, dy;

    begin_f_loop(f, thread)

    {

        f_node_loop(f, thread, n)

        {

            v = F_NODE(f, thread, n);

            if (NODE_POS_NEED_UPDATE(v))

            {

                x = NODE_X(v);

                y = NODE_Y(v);

                dy = (1.0 / 80000.0) * (sin((20000000.0 / 3.0) * PI * time - 1.5) + 1) * -cos(10000.0 * PI * x);

                NODE_Y(v) = dy;

                NODE_POS_UPDATED(v);

            }

        }

    }

    end_f_loop(f, thread)

}

Just plug that into the dynamic mesh and apply it to whatever boundary you're moving