Separated-Flow Transition: Part 3 — Primary Modes and Vortex Dynamics

Abstract

This paper clearly identifies the possible modes of transition in the separated boundary layers and their specific characteristics. This study distinguishes between the short and long bubbles primarily based on the separated flow structure. A hypothetical description of the vortex structure and evolution for each separated-flow transition mode is provided. The present approach in analyzing separated-flow transition is based on the assumption that the transition to turbulence in separated boundary layers is a result of the superposition of the effects of two different types of instability. The first type of instability is the Kelvin-Helmholtz (KH) instability. It occurs and develops in the shear layer at a specific location downstream of the separation point. The concentration of spanwise vorticity grows in time and remains in place through the vortex sheet roll-up mechanism. The roll-up vortex interacts with the wall and induces periodic ejection of near-wall fluid into the separated shear layer. The ejection process takes place at a location identifiable by the maximum displacement of shear layer, xMD. The second type of instability is the (convective) Tollmien-Schlichting (TS) instability. It originates in the boundary layer prior to the separation point and continues to evolve in the separated shear layer. The mechanism for the TS instability also leads to roll-ups, but it involves viscous tuning of the instability waves. Thus, the separated-flow transition is the result of spatially developing, often competing instabilities. The ejection induces the onset of transition for laminar short and long bubble modes of transition and controls the mid-transition point of transitional separation mode. The ejection may be accompanied by vortex shedding. Shedding occurs in the laminar separation - short bubble mode and occasionally in the transitional separation mode; however, it is not present in the laminar separation - long bubble mode of transition.