stationary wall and moving wall
 

for wall B.C., what is the difference between stationary wall and moving wall with absolute zero velocity?

There should not be any difference as far as what the BC actually accomplishes (both types will create a non-moving wall). However, the equations that are solved by Fluent in actually implementing the BCs may be slightly different (this can vary with different CFD programs). Some shortcuts and code optimizations are likely implemented in the stationary wall BC as compared to the moving wall with zero absolute velocity. I would recommend using the most simple BC possible to help with solution speed and stability, so use the Stationary wall option if this is your intention.

Kevin Erhart, PhD
Research Vice President
Central Technological Corporation
www.centecorp.com