Behavior of Separation Bubble and Reattached Boundary Layer Around a Circular Leading Edge

Abstract

The flow around a circular leading edge airfoil is investigated in an incompressible, low turbulence freestream. Hot-wire measurements are performed through the separation bubble, the reattachment and the recovery region till development of the fully turbulent boundary layer. The results of the experiments in the range of Reynolds numbers 1.7×103 to 11.8×103 are analysed and presented in this paper. A separation bubble is present near the leading edge at all Reynolds numbers. At the lowest Reynolds number investigated, the transition is preceded by strong low frequency oscillations. The correlation given by Mayle for prediction of transition of short separation bubbles is successful at the lower Reynolds number cases. The length of the separation bubble reduces considerably with increasing Reynolds number in the range investigated. The turbulence in the reattached flow persists even when the Reynolds number based on momentum thickness of the reattached boundary layer is small. The recovery length of the reattached layer is relatively short and the mean velocity profile follows logarithmic law within a short distance downstream of the reattachment point and the friction coefficient conforms to Prandtl-Schlichting skin-friction formula for a smooth flat plate at zero incidence.

Features of the turbulent flow around symmetric elongated bluff bodies

“…The reattachment length reported for the square-edged case by Djilali and Gartshore (1991) is also noticeably higher than that found currently and, as previously mentioned, by Roshko and Lau (1965). The reattachment length for the circular-edged body is small being 0.94t, which is similar to the reattachment length of 0.95t found by Hazarika and Hirsch (1994) in their experiments on an infinite plate with a circular leading edge. Their experiments were performed at a Reynolds number, based on thickness, of 2.4 Â 10 4 in smooth flow with a blockage ratio of 2.3%.…”

Studies on a Blade Leading Edge Separation Bubble Affected by Periodic Wakes: Its Transitional Behavior and Boundary Layer Loss Reduction

“…In contrast, fewer studies have been made on the investigation of periodic wakes affecting leading edge separation bubble on a compressor blade, while several studies have been made on steady-state separation bubbles on the blade leading edge [7], [8], [9]. Paxson and Mayle [10] measured wake-affected velocity profiles of the boundary layer on the leading edge of a blunt test model.…”

“…Lou and Hourmouziadis [5] documented transitional behaviors of separation bubbles on a flat plate subjected to main flow fluctuations, where a pressure gradient over the flat plate was imposed using a contoured wall. A similar study was made by Volino and Hultgren [6], focusing on the effect of free-stream turbulence upon the separation bubble.In contrast, fewer studies have been made on the investigation of periodic wakes affecting leading edge separation bubble on a compressor blade, while several studies have been made on steady-state separation bubbles on the blade leading edge [7], [8], [9]. Paxson and Mayle [10] measured wake-affected velocity profiles of the boundary layer on the leading edge of a blunt test model.…”

Transition Over C4 Leading Edge and Measurement of Intermittency Factor Using PDF of Hot-Wire Signal

“…10) show the typical profiles of a thick separation bubble close to the nose. In the early part of the bubble (x < 20 mm) a smooth velocity profile could be obtained through the reverse flow region by using the method given in Hazarika and Hirsch (1994). Down-stream of 20 mm it is not possible to obtain a smooth velocity profile, thus all velocities are left as positive.…”