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An axial skewed slot casing treatment has been tested over the tips of an isolated low-speed rotor with a hub-tip ratio of 0.4. An improvement in stall margin of 22 percent at nominal clearance was obtained. Detailed measurements of the loss pattern downstream of the rotor were made with a hot wire. Measurements were also made in the blade passage with a traversing gear moving with the rotor and in the treatment slots themselves using hot wires. The tentative conclusion is that unsteady effects in the slot are of secondary importance. Of primary importance is the selective removal of high absolute swirl, high loss fluid from the endwall near the trailing edge of the pressure surface of the blade, and reintroduction of this, with absolute swirl direction reversed, near the blade leading edge.
Smith et al. (Sun,) studied this question.