LAMINAR SEPARATION BUBBLES

Abstract

The phenomenon of leading edge stall is associated with the “bursting” of leading edge separation bubbles from a short form, where the length is roughly 100 momentum thicknesses, to a long form that maybe 1000 or more momentum thicknesses long. The paper reports experiments and theoretical discussions of work carried out by the author 50 years ago during his PhD study on bubbles. Detailed measurements of the flow within bubbles are shown together with the oscillogram traces of the velocity fluctuations present. A linear model of the stability of separated shear layers was developed that suggested that the disturbances were spatially evolving waves described by modes with complex wavenumbers and not the temporal modes usually used in stability studies. It was noted that some modes appeared to have a very small group velocity. Although at the time the full implications of this were not properly understood, the conjecture was put forward that a true instability (or absolute instability as it is now called) could therefore exist. A change in the sign of the group velocity could dramatically change the transition process and thus explain the bursting phenomenon.