How to explain this picture?

http://www.lbmethod.org/doku.php?id=numerics:matlab_samples
Lid-driven cavity
There are a minimum velocity region in the center.How did it occur?

Nash Wrote:

http://www.lbmethod.org/doku.php?id=numerics:matlab_samples
Lid-driven cavity
There are a minimum velocity region in the
center.How did it occur?

I think the reason is that the fluid is rotating about this point. The center of a rotational flow has zero velocity (if there is no additional translation).

Timm

I agree that.And I think of whether this can happen in the temperature field?That will be interesting.