# Problem with velocity results of oscillatory flow

Dear all,

I intend to simulate an oscillatory Couette flow between two parallel walls. The upper wall moves as a sinusoidal function of time. I used “fix move/wiggle” command to apply such motion to the wall. The simulation consists of periods of time till the velocity reaches a time-dependent behavior of time.

My goal is to compare velocity profiles in different specific times of a period (T). I want to make an averaging of velocity for the last periods of time, specifically t=0, T/8, T/4,… 3T/4. But, my problem is that when I set ave/spatial parameters to these specific times, the velocity is always very small and just oscillates around zero, even when I suppose a high amplitude for oscillation of upper wall.

Could you help me what magnitudes should I consider for “fix ave/spatial” command (Nevery=T, Nfreq=?)? And why the fluid particles do not have greater velocity similar to velocity of wall?

The command which I set for applying oscillation and velocity averaging:

velocity lower set 0.0 0.0 0.0 units box
fix 4 upper move wiggle 3.0 0.0 0.0 628 units box
fix 3 wall setforce 0.0 0.0 0.0

fix vel1 flow ave/spatial 125600 10 1256000 z upper 0.2 vx ave one file vel1.profile units box ##wt=0

run 2512000

I appreciate anyone’s guide in this regards and thanks in advance for your help.

Kind Regards,
Hamed Rahmatipour