Qrthogonal Quality of rotating sliding mesh keeps decreasing
 

In fluent 14.0,I find the Qrthogonal Quality in the rotating area decreases slowly when I use sliding mesh.And Negative volumes appear near the rotating surface after about 1900 time steps.

Attached is a picture of mesh near the cylinder surface (after 1300 time steps).

The rotating speed is 1250 rad/s , one time step is 2.5132e-5 s , the mesh is imported from Gridgen1518.

Why rotating mesh becomes worse and worse? Could anyone give me some help please?:)