Grid independent problem for lid-driven flow
 

Hi, I have calculated a lid-driven flow at Re=100. I used the pre-conditioned compressible RANS to simulate such imcompressible preoblem.
the u/U on the vertical line x/L = 0.5 match the results of Ghia well using a uniform grid 41*41. But when I cluster the grid twice into 82*82, the result get worse. I do not know what may cause this problem. Thanks!