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National Advisory Committee for Aeronautics, Report - A Visualization Study of Secondary Flows in Cascades

Flow-visualization techniques are employed to ascertain the
streamline patterns of the nonpotential secondary flows in the
boundary layers of cascades, and thereby to provide a basis for
more extended analyses in turbomachines. The three—dimen-
sional deflection of the endv/wall boundary layer results in the
formation of a vortex within each cascade passage. The size
and tightness of the vortex generated depend upon the mainzflow
turning in the cascade passage. Once formed, a vortex resists
turning in subsequent blade rows, with consequent unfavorable
angles of attack and possible flow disturbances on the pressure
surfaces of subsequent blade rows when the cortices impinge on
these surfaces.

Two major tip-clearance ejects are observed, the formation of a
tip-clearance vortex and the scraping effect of a blade with relative
motion past the wall boundary layer. Theflow patterns indicate
methods for improving the blade tip-loading characteristics of
compressors and of low- and high-speed turbines.

An important problem arising in the design of turboma-
chines and in the analysis of their performance is an under—
standing of the nature and influence of so-called secondary
flows, which are here defined as any motions of boundaly-
layer fluid having components Of motionnormal to the through—
flow directions. These secondary flows, which are directly
responsible for boundary-layer accumulations and boundary—
layer flows within the passages of the turbomachine, often
lead to significant aerodynamic losses. The ideal answer
to this problem would be a practical design procedure based
on the complete solution of the three—dimensional viscous-
flow equations in the actual turbomachine configuration. At
the present time, such solutions have not been obtained because
of the mathematical complexity of the problem involved.