time varying inlet velocity
 

Hi

I am simulating the time varying inlet velocity profile, which is called buffeting in the bridge, i generate the velocity file, and use the user fortran to read it, here i simulate a square cylinder external flow, but when i watch the drag force, the drag force has negative value, and oscillating up and down, here the bluff body is horizontal to the inlet velocity direction and there is no attack angle, in fact when i did not consider the time varying inlet condition, only the mean velocity value is applied on the inlet, the drag force is the positive, i think the simulation is convergent, so anyone can tell me the reason why the pressure osicillating and negative value happens.