Coding suggestion needed. (code is attached)

I have written a simple C++ code for flow over cylinder (2D).

Parameters for coding are as follows;

Rep = 4
nx = 500
ny = 80
Dp = 10
Lx = 125 (x-direction particle position)
Ly = 40

@ inlet = uniform velocity corresponding to Rep = 4
@ outlet = extrapolation BC (zero gradient)
@ cylinder surface = Mei BC ([Mei_1999]_An_accurate_boundary_treatment_in_the_LBM)

all calculations were done using tau = 0.6

Code is running fine, till certain time step force goes on increasing and reaches to stable value but after that
the value of force starts dropping and continue till infinity.

Can anybody please have a look on code and suggest me some correction to remove above mentioned problem.

Dear Atul,
I am trying to simulate Poiseuille flow with various boundary conditions. I have managed to get results with the periodic boundary conditions But when I try with inlet velocity and zero-gradient at the outlet the results are very absurd.I am not able to find my mistake, could you post your code here. It would be really helpful. Thank you.