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Error in UDF for slip boundary

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Old   March 6, 2020, 06:31
Default Error in UDF for slip boundary
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Amit Varakhedkar
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I have written a UDF for providing slip conditions to the airfoil wall using the Navier slip equation. After compiling the code, when I initialize the solution, I get this error: received a fatal signal (Segmentation fault).
I am simulating in series and the simulation is running perfectly for no-slip condition (without UDF).
Thanks in advance.

The UDF is as follows:
#include "udf.h"
#define del 0.0004 // wall slip layer thickness [m]

DEFINE_PROFILE(slip_velocity_x,t,i)
{
cell_t c;
real x[ND_ND], gamma, c_t, c_x, u, v;
face_t f;

begin_f_loop(f,t)
{
F_CENTROID(x,f,t);
u = C_U(f,t);
v = C_V(f,t);
gamma = C_STRAIN_RATE_MAG(f,t); // tangential velocity component
c_t = gamma*del;
c_x = c_t*u/sqrt(u*u+v*v); // taking x component
F_PROFILE(f,t,i) = c_x;
}
end_f_loop(f,t)
}

DEFINE_PROFILE(slip_velocity_y,t,i)
{
cell_t c;
real x[ND_ND], gamma, c_t, c_y, u, v;
face_t f;

begin_f_loop(f,t)
{
F_CENTROID(x,f,t);
u = C_U(f,t);
v = C_V(f,t);
gamma = C_STRAIN_RATE_MAG(f,t);
c_t = gamma*del;
c_y = c_t*v/sqrt(u*u+v*v);
F_PROFILE(f,t,i) = c_y;
}
end_f_loop(f,t)
}
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Old   March 6, 2020, 07:26
Default Slip Velocity UDF
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Vinerm
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There are a few issues with the code. Any macro starting with C_ is meant for cell center. And it requires cell index and cell thread, i.e., the arguments of C_ macros have to be of type cell_t and Thread *t where t belongs to a cell zone and not a boundary zone. You have to define a thread variable outside the loop, i.e.,

Thread *tc;
tc = THREAD_T0(t);

and incorporate following changes within the loop

c = F_C0(f, t);
C_STRAIN_RATE_MAG(c, tc);

Furthermore, you don't need the command F_CENTROID as well as its corresponding variable x because these are not required by the equation.

Last comment is about its applicability. It is not recommended to apply the velocity for a slip or partial slip condition. Better approach is to determine shear stress due to this slip and then apply that.
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