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National Advisory Committee for Aeronautics, Technical Notes - Stall Propagation in a Cascade of Airfoils

An experimental investigation of stall propagation in a stationary
circular cascade in which high-speed schlieren and interferometer photog-
raphy is used is described. This investigation suggests an analytical
approach to the study of stall propagation which is valid only for an
isolated blade row in an infinite flow field but which is not restricted
to small unsteady perturbations or to an assumed simplified cascade
geometry. Conditions necessary for the existence of the assumed type of
stall cells are described and equations are derived for the velocity of
stall-cell propagation.

The propagation velocities predicted for the theoretical potential—
flow model correlate with all the experimental values measured in an
isolated rotor within 15 percent.

Analysis of the flow model leads to the prediction of a tendency
for the assumed type of stall cell to split with increasing incidence
of the mean flow through the blade row. This tendency appears to corre-
late with the experimental Observation of a trend for increasing numbers
of cells in the rotor.

The Objective of the analytical and experimental work presented
herein is the development of a theory which will enable the prediction
of the flow through a cascade of rigid airfoils, or an isolated blade
row of an axial compressor, when the incidence of the fluid on the air-
foils is high. It was discovered in the early days of British jet-
engine development that the flow can be unstable under these conditions
and that self—induced periodic disturbances on the flow can develop.

The disturbances are caused by the propagation along the cascade, at
approximatehy the relative tangential component of main-stream velocity,
of regions where the flow is badly separated from the airfoils. These
regions where the blades are severely stalled are generally called stall
cells.