Reply To: Finding Lift force in CG motion UDF

[nahian.masud]

Hi, thanks for replying. this is the latest UDF I am using,

#include "udf.h"

#define MASS 0.0143

 

/* Declare a global variable to store the lift force */

static real lift = 0.0;

 

/* Function to compute the CG motion, including flapping motion */

DEFINE_CG_MOTION(paperfreq, dt, vel, omega, time, dtime)

{

    Domain *d = Get_Domain(1); // Assuming single phase flow

    Thread *t_object = Lookup_Thread(d, 7); // Airfoil's thread ID

    real force[2], moment[2], cg[2]; // Initialize arrays

    real x=0;

    NV_S(vel, =, 0.0);

    /* Compute force and moment at the boundary */

    Compute_Force_And_Moment(d, t_object, cg, force, moment, FALSE);

 

    /* Update global variable for lift force */

    lift = force[1];

 

    real w, a, pi;

    pi = 3.14159265;

    a = (pi * 15) / 180; // Pitch amplitude in radians

    w = 0.62832; // Angular velocity in rad/s

 

    /* Define the flapping motion */

    omega[2] = -a * 2 * pi * 2.97 * cos(2 * pi * 2.97 * time); // Pitching motion

    vel[1]=lift*dtime/MASS;

    x=vel[1];

    

Message("lift:%f x:%f\n",lift,x);

}

The problem is, if i use the vel[1], the lift force goes very high, so does the velocity. This makes the airfoil go outside domain and it creates floating point exception error. If I dont use the vel[1], and I print the lift force using Message, it shows correct lift value. 

What I need is, change the location of the airfoil based on the lift force. If there is any other way to give it a y-displacement it is also ok. I am giving it a flapping motion, which generates various lift force based on time and I need the latest lift force to update the airfoil location before going to next time step.