An Investigation of the Effects of Incidence on the Flow Around a Square Section Cylinder

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Abstract
A study has been made of the changes that take place in the flow around a square section cylinder as the angle of incidence is increased from 0° to 45°. Measurements of the Strouhal number, S, and the vortex longitudinal spacing, a/d, are presented and used to estimate the vortex strength, , and vortex street spacing ratio, b/a. is found to vary between about 1.2 and 1.7 depending on incidence, and is given approximately by 0.52(1 - Cpb)/2πS, where Cpb is the mean base pressure coefficient. As the incidence is increased from 0°, S at first decreases slightly and then rises sharply to a maximum at 13.5° incidence, which is the incidence where reattachment of the shear layer, in some mean sense, is expected to commence. The spectra of pressure and velocity fluctuations were measured and subharmonic peaks were found in both spectra at 5° and 10° incidence. It is suggested that they may have been caused by an interaction between a vortex and a trailing edge corner. The degree of organisation of the vortex shedding process was estimated by calculating the sharpness factor, Q, of the spectral peaks at the vortex shedding frequency. In general Q fluctuated with changes in incidence. High values of Q occurred at angles of incidence where the rate of change of the mean base pressure coefficient with incidence is very small whereas low values occurred where the flow is changing to a different state.

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