The relation between tip clearance flow structure and axial compressor stall is interrogated via numerical simulations, to determine how casing treatment can result in improved flow range. Both geometry changes and flow field body forces are used as diagnostics to assess the hypothesis that the radial transport of momentum out of the tip region, and the consequent decrease in streamwise momentum in this region, is a key aspect of the flow. The radial velocity responsible for this transport is a result of the flow field set up by the tip clearance vortex. Altering the position of the tip clearance vortex can alter the amount of streamwise momentum lost due to radial transport and hence increase the compressor flow range. Circumferential grooves improve the flow range in the manner described above. In the presence of such a groove the radial velocity profile along the passage can be altered so that that the radial transport of streamwise momentum is decreased. The flow fields associated with grooves at different axial positions, and of different depths, are also examined, along with previous research on circumferential grooves, and it is shown that these are in accord with the hypothesis.
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