[Syaida21]

I'm currently working on a 2D simulation in Fluent that requires me to simulate the radial motion of a pump geometry. To achieve this, I've implemented a UDF (User-Defined Function) for grid motion, which aims to change the radius of the pump geometry over time and return it to its original radius after a specified duration (to replicate inflation and deflation of the balloon pump).

I've encountered an issue where the pump geometry shifts laterally (to the right or left) instead of the intended radial motion. Below is the UDF code I'm using for the grid motion:

#include "udf.h"
#include "math.h"

DEFINE_GRID_MOTION(pump_wall_motion, domain, dt, time, dtime)
{
Domain *d;
Thread *t;
face_t f;
Node *node_p;
real current_time = CURRENT_TIME;
real final_radius = 154.94e-3;
real original_radius = 230.74e-3;
real transition_duration = 2.0; // Duration of the transition in seconds
real transition_speed = (final_radius - original_radius) / transition_duration; // Speed of transition
real center_x = 141.05; // X-coordinate of the pump center
real center_y = 5.0; // Y-coordinate of the pump center
int n;

// Calculate the new radius based on the linear transition
real new_radius;
if (current_time < transition_duration) {
new_radius = original_radius + transition_speed * current_time;
} else {
new_radius = final_radius;
}

// Get the domain and thread
d = Get_Domain(1);
t = Lookup_Thread(d, 9);

// Iterate over all faces in the thread
begin_f_loop(f, t)
{
f_node_loop(f, t, n)
{
node_p = F_NODE(f, t, n);
if (NODE_POS_NEED_UPDATE(node_p))
{
NODE_POS_UPDATED(node_p);

// Get current coordinates relative to the center
real x = NODE_X(node_p) - center_x;
real y = NODE_Y(node_p) - center_y;

// Calculate the distance from the center
real distance = sqrt(x * x + y * y);

// Prevent division by zero for nodes exactly at the center
if (distance > 1e-6)
{
// Calculate the scale factor based on the new radius
real scale_factor = new_radius / original_radius;

// Calculate the new position
real new_x = center_x + x * scale_factor;
real new_y = center_y + y * scale_factor;

// Update node positions relative to the center
NODE_X(node_p) = new_x;
NODE_Y(node_p) = new_y;
}
}
}
}
end_f_loop(f, t);
}

I've already tried verifying the centre coordinates, thread selection, boundary conditions, and mesh quality, but the lateral motion issue persists. Any insights or suggestions on resolving this issue would be greatly appreciated. Thank you!