From this research, we see that: at the same rpm, the rotor thrust decreases,the shroud thrust increases as tip clearance decreases. But why does the rotor thrust decrease at smaller tip clearance? You can see author said at page 11:
"Note that the increased inflow in the plane of the rotor at smaller tip clearances causes a small reduction in the effective angle of attack seen by the rotor blade, which results in a small decrease in the rotor thrust."
I understand that the inflow increases that will causes a small reduction in the effective angle of attack seen by the rotor blade. But final question: why do we have increased inflow at smaller tip clearance?