I'm using VOF in Fluent to simulate impact of a falling droplet onto a solid particle in mid-air (pls see the picture). Instead of using a constant contact angle (CA), I want to apply a dynamic CA which varies based on the liquid velocity on the triple line (Kistler method).
I don't know what to do with no slip which makes the velocity on the surface zero. Should I come a few cells up?
Could someone give me some hints please for how to write the UDF?
I haven't worked much with udfs.
Thanks!
Hi Sayed,
I have been trying to model similar problem in Fluent for a while. I’m going to give an expression for dynamic contact angle which is a function of contact line velocity. Did you write the UDF? If so, can you explain to me how did you solve?
I have written this UDF but it doesnt work
#include "udf.h"
int T_e = 90; //Equilibrium contact angle
int T_a = 60; //Advancing contact angle
int T_r = 110; //Receding contact angle
double U_t = 0.6; //Characteristic velocity
double contact_velocity=0.0; //Contact velocity
double dynamic_contact_angle=90.0; //Dynamic contact angle
double contact_line_position=0.0;
double time=0.0;
FILE *file;
double sum=0.0 ;
DEFINE_ADJUST(Contact_Angle_Update, domain)
{
Thread *thread = Lookup_Thread(domain, 5);
Thread **pt = THREAD_SUB_THREADS(thread);
cell_t cell;
face_t f;
real x[ND_ND];
double max_y=0, volume;
int n;
sum=0.0;
begin_c_loop_all (cell,pt[1])
{
if(C_VOF(cell,pt[1])!=0)
{
C_CENTROID(x,cell,pt[1]);
printf("## x=%f y=%f z=%f vof=%f\n",x[0],x[1],x[2],C_VOF(cell,pt[1]));
if(x[1]>max_y)
max_y=x[1];
}
sum+=C_VOF(cell,pt[1]);
}
end_c_loop_all (cell,pt[1])
contact_velocity= (max_y-contact_line_position)/(RP_Get_Real("physical-time-step"));
contact_line_position = max_y;
time+= RP_Get_Real("physical-time-step");
printf("max_y=%f contact_velocity=%f time=%f dynamic_contact_angle=%f sum=%f\n contact_line_position=%f",max_y,contact_velocity,time,dynamic_contact_angle,sum,contact_line_position);
dynamic_contact_angle=T_e+(T_a-T_r)*tanh(contact_velocity/U_t);
}
DEFINE_PROFILE(Contact_Angle_Set_Profile,t,i)
{
face_t f;
begin_f_loop(f,t)
{
F_PROFILE(f,t,i) = dynamic_contact_angle;
}
end_f_loop(f,t)
}
I hope you can help me on this issue. Thanks for your valuable time and help
We tried three models. Kistler was found to be the best to simulate liquid-liquid flow in a microchannel. DOI: 10.1002/ceat.201800497
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