{"id":362336,"date":"2024-04-15T21:44:14","date_gmt":"2024-04-15T21:44:14","guid":{"rendered":"\/forum\/forums\/topic\/finding-lift-force-in-cg-motion-udf\/"},"modified":"2024-04-15T22:40:55","modified_gmt":"2024-04-15T22:40:55","slug":"finding-lift-force-in-cg-motion-udf","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/finding-lift-force-in-cg-motion-udf\/","title":{"rendered":"Finding Lift force in CG motion UDF"},"content":{"rendered":"

Is there any way to find the lift force in CG motion udf? I need to to use in the vertical velocity. Here is the code I have written so far. The 2D airfoil is flapping, so I described a flapping motion. The airfoil has boundary ID of 7. now I use Compute_Force_and_Moment macro to find lift force and use it in the vertical velocity. 
The problem is, it says floating point error after 2 or 3 time steps ( residuals goes super high and simulation stops), but if i dont use the compute force and moment macro, the flapping only works well. I need to find the lift force anyhow to get the transational velocity. can anyone help me please?<\/p>\n

#include “udf.h”<\/div>\n
DEFINE_CG_MOTION(paperfreq,dt,vel,omega,time,dtime)<\/div>\n
{<\/div>\n
Thread *t; <\/div>\n
face_t f;<\/div>\n
\/* get moment or force *\/<\/div>\n
Domain *d = Get_Domain(1); \/* for single phase flow*\/<\/div>\n
 <\/div>\n
Thread *t_object = Lookup_Thread(d, 7); \/* you get Boundary_ID from the boundary condition panel in Fluent*\/<\/div>\n
real force[ND_ND], moment[ND_ND], cg[ND_ND]; \/*initialise*\/<\/div>\n
 <\/div>\n
Compute_Force_And_Moment (d,t_object,cg,force,moment,TRUE);<\/div>\n
 <\/div>\n
 \/* force components of surface “Boundary_ID” real force_y = force[1];*\/<\/div>\n
 <\/div>\n
real w,a, pi; <\/div>\n
pi = 3.14159265; <\/div>\n
a = (pi*15)\/180; \/*pitch amplitude*\/ <\/div>\n
w = 0.62832; \/*angular velocity*\/  <\/div>\n
\/*defining the flapping motion*\/ <\/div>\n
omega[2]=-a*2*pi*2.97*cos(2*pi*2.97*time); \/*pitching motion*\/<\/div>\n
vel[1]= force[1]*dtime\/0.0116;<\/div>\n
 <\/div>\n
}<\/p>\n

The code above shows the error with very high residuals. But the bottom code works just fine,<\/p>\n

#include “udf.h”<\/div>\n
DEFINE_CG_MOTION(paperfreq,dt,vel,omega,time,dtime)<\/div>\n
{<\/div>\n
Thread *t; <\/div>\n
face_t f;<\/div>\n
real w,a, pi; <\/div>\n
pi = 3.14159265; <\/div>\n
a = (pi*15)\/180; \/*pitch amplitude*\/ <\/div>\n
w = 0.62832; \/*angular velocity*\/  <\/div>\n
\/*defining the flapping motion*\/ <\/div>\n
omega[2]=-a*2*pi*2.97*cos(2*pi*2.97*time); \/*pitching motion*\/<\/div>\n
 <\/div>\n
}<\/p>\n

UPDATE:
I found the possible reason of the error (not sure yet)<\/p>\n

Thread *t_object = Lookup_Thread(d, 7); \/* Boundary_ID from the boundary condition panel in Fluent*\/<\/p>\n

In this line the ID of my airfoil is 7. It is a wall. If I use this, the error occurs. but if I use other ID, the problem does not occur. But I need to use the airfoil in order to find the lift force on it.<\/p>\n<\/div>\n<\/div>\n","protected":false},"template":"","class_list":["post-362336","topic","type-topic","status-closed","hentry","topic-tag-2d-airfoil","topic-tag-ansys-fluent","topic-tag-dynamic-mesh","topic-tag-flappingwing-1","topic-tag-lift-force-1","topic-tag-udf"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t