You should use UDF (User defined function) to interchange the boundary condition between inlet and outlet after certain time step. To make the UDF ,you should study the ANSYS UDF guide.

Like this ,

" /*********************************************************************** vprofile.c UDF for specifying steady-state velocity profile boundary condition ************************************************************************/ #include "udf.h" DEFINE_PROFILE(inlet_x_velocity, thread, position) { real x[ND_ND]; /* this will hold the position vector */ real y; face_t f; begin_f_loop(f, thread) { F_CENTROID(x,f,thread); y = x[1]; F_PROFILE(f, thread, position) = 20. - y*y/(.0745*.0745)*20.; } end_f_loop(f, thread) }"

The above one is just a simple udf. Try to connect the boundary condition value with the time step.

Thanks again Rashed. I'm familiar with DEFINE_PROFILE udf, and how we can use to apply in-direct BCs over the model. Since my problem is velocity inlet and pressure outlet, how a DEFINE_PROFILE can help to interchange these boundaries?

Please look at the question now, I have modified that with additional explanations.

It seems, previously i didn't understand your question properly. Now i get it. Though it looks difficult but not impossible. Every boundaries in Fluent have an individual ID number. Try to interchange this ID by using "DEFINE_ADJUST" or "DEFINE_EXECUTE_AT_END" at specific time interval. It's just a suggestion , not sure though.

is it possible to write a UDF to interchange the boundary condition as parameters ?

What I have used was based on scheme programing, look at this link:

https://www.cfd-online.com/Forums/fluent/151083-udf-change-boundary-condition-velocity-inlet-pressure-inlet-time-t.html

But, another way is to use UDF, don't know how!

could u edit Jpina code for me so that when the pressure exceeds a certain value the top becomes a velocity inlet and other wise its a wall? or am i asking for too much?