UDF to automatically change timestep size
There might be a need to change timestep size based on number of iterations taken for a case to converge within each timestep. This could be accomplished through an UDF which is attached at the end of this notes. The UDF needs to be hooked in the iterate panel under "UserDefined TimeStep" which is available after switching on Adaptive method. /********************************************************************** UDF that changes the time step value for a timedependent solution.This UDF is linked in iterate panel after switching on adaptive timestepping method. Note that UDF is called at the beginning of every timestep.This implies, if a time step size of 100 is given in Fluent panel,then 200 is taken as initial timestep size. Presently, if the number of iterations is less than 10, the timestep is doubled. If the number of iterations is more than 20, the timestep is halved. **********************************************************************/ #include "udf.h" static int last_total_niter = 0; DEFINE_DELTAT(mydeltat, domain) { real time_step; int iter = N_ITER  last_total_niter; if (iter < 10) time_step = CURRENT_TIMESTEP*2; else { if (iter > 20) time_step = CURRENT_TIMESTEP*0.5; else time_step = CURRENT_TIMESTEP; } printf("TIME_STEP_SIZE= %f n",time_step); last_total_niter = N_ITER; return time_step; } 

