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Old   July 18, 2006, 16:23
Default Slip boundary Condition
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I'm trying to model a microchannel (pipe) flow that is still governed by NS equations with slip BCs (Knudsen Number between 10^-3 and 10^-1). The simple slip equation for the wall that I'm trying to model is:

U_slip = Kn*du_dy I think I'm able to get the du_dy function into the UDF. However, I have no clue how to incorporate the Knudsen number into the UDF. I know that Knudsen number isn't constant throughout the flow, as it increases along the length of the pipe.

Even assuming that Knudsen number is constant (Kn=0.01), I tried simulating the flow with just U_slip=0.1*du_dy, I get reversed flow and eventually "Floating point error: overflow". Here's my UDF code for the wall:

#include "udf.h"

DEFINE_PROFILE(wall_velocity, thread, index) { face_t f; cell_t c; Thread *cell_thread;

begin_f_loop(f,thread) { c=F_C0(f,thread); cell_thread=THREAD_T0(thread); F_PROFILE(f,thread,index)=fabs((1e-7)*C_U_G(c,cell_thread)[1]); } end_f_loop(f,thread)


Can you see what I'm doing wrong? Thanks!
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Old   July 28, 2006, 05:53
Default A related question
mahdi haddad
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Hi.The field of your project is similar to mine.I am modeling a micro nozzle.But i couldn't initialize my case file although i could compile my udf . the errors were : (access violation) 1.Not exact events leading to error. your case file and restart the fluent. to your distributer.

my fluent version is Fluent 6.0 If you had the similar problem or know any thing about it, i will appreciate you if you send me some guides.

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Old   July 29, 2006, 03:37
Default Re: A related question
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Hi. How can you activate your udf in the environment of fluent? how can you return a new value for the velocity of the flow in a particular face specially on the wall boundary? please help me . we can choose the specified shear instead of no slip boundary condition in wall boundary condition but we can't change or adjust the velocity on the wall . please help me . this is very vital for me.

thanks in advance.

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