Separation of Turbulent Boundary Layer on a Lifting Cylinder

Abstract

for the higher Mach numbers, especially for M = 0.86. The jaggedness of this plot demonstrates a cellular spanwise separation pattern, as described qualitatively by the surface flow data of Ref. 1. A clearly separated boundary-layer profile was measured on the trailing edge upper surface at the center-span location (y/c = 0), where the CPte is negative. Figure 4 compares spanwise-displaced upper surface trailing edge boundary-layer profiles for both M = 0.60 and M = 0.80. A triple-tube pressure probe with a tip depth of 0.08 mm, developed for this series to yield velocity vector data, was employed. It is seen that the spanwise deviation is much greater at the higher Mach number, whether measured by speed or by angularity. (This was the case despite the fact that the lower Mach number case carries a higher d, which is manifest in a larger over-all pressure rise and a much thicker upper surface boundary layer.) The main conclusion from the data discussed herein is that significant three-dimensional effects occur in transonic airfoil tests, even for an aspect ratio of four. This is especially true at the supercritical Mach numbers, for which lateral propagation of disturbances is effective. Onset of trailing edge separation is cellular. These observations do not necessarily preclude the existence of approximately two-dimensional flow at any one spanwise section, for spanwise gradients may not be large; however, they indicate the importance of obtaining all measurements at a single spanwise location, e.g., chordwise surface pressure distributions (lift) and wake total pressure profiles (drag). In fact, C/ rather than a is the independent variable to be used, and the observed spanwise variations may be viewed as representing primarily spanwise differences in effective section angle of attack. References 1 Gregory, N, Quincy, V. G., O'Reilly, C. L., and Hall, D. J., Progress Report on Observations of Three-Dimensional Flow Patterns Obtained during Stall Development on Aerofoils, and on the Problem of Measuring Two-Dimensional Characteristics, Aero. Rept. 1309, 1970, National Physical Lab., England.