Separation Bubbles Under Steady and Periodic-Unsteady Main Flow Conditions

Abstract

Based on an experimental investigation carried out in a low-speed test facility at the Berlin University of Technology, this paper describes the formation of separation bubbles under steady and periodic-unsteady main flow conditions. The aim of the investigation was to understand the mechanism of separation, transition, and reattachment, and the effect of main flow unsteadiness on it. Separation bubbles for various main flow conditions were generated over a large flat plate, which experienced a similar pressure distribution to that on the suction surface of blades in turbomachines. The pressure distribution was generated by a contoured wall opposite the plate. Aimed at separating the effect of the velocity and the turbulence wake, this paper considers only the influence of the velocity wake. To this effect, a rotating flap was mounted downstream of the test section to produce periodic oscillations of the main flow. The overall flow field under steady main flow conditions was obtained by hot-wire measurements. Pressure taps were used to measure the pressure distribution over the plate. The Reynolds number effects were determined and compared to the measurement results in the literature. Results for periodic-unsteady separation bubbles are shown using different Strouhal numbers, oscillation amplitudes, and Reynolds numbers. Ensemble-averaged mean velocity profiles and the ensemble-averaged rms velocity profiles are used to demonstrate the development of the periodic boundary layer. Time–space diagrams are plotted to show the development of the periodic-unsteady boundary layers. The characteristic instability frequencies in the free shear layer are identified. The impact of the major parameters, Strouhal number and amplitude, on the bubble formation are discussed. [S0889-504X(00)01204-6]

A Bypass Transition Model Based on the Intermittency Function

“…This test case was firstly studied experimentally by Lou and Hourmouziadis (2000), and numerically by Wissink and Rodi (2006b) based on DNS and by Lardeau et al (2012) based on LES. The current model is able to predict locations of transition and reattachment with decent accuracy compared to the LES data of Lardeau et al (2012).…”

An assessment of the laminar kinetic energy concept for the prediction of high-lift, low-Reynolds number cascade flows

“…The formation of a long bubble arises predominantly at low Reynolds numbers, and this tends to influence the pressure distribution along the entire blade surface. The switching of the bubble structure from the short-to-long configuration usually occurs in high lift airfoils when decreasing the Reynolds number, and it is usually referred to as bursting [9,10]. Such a phenomenon occurs in quite a sudden manner when the Reynolds number goes below a certain critical value.…”

“…Separation bubbles are commonly classified as 'short' and 'long' [9,10]. Short bubbles occur at higher Reynolds numbers, and are expected to have only a local effect on the pressure distribution.…”

Dynamic pitching effect on a laminar separation bubble

“…Direct numerical simulations (DNS) revealed that the application of volume forcing and low-frequency main flow disturbances to the LSB triggers the K-H instabilities more upstream, reducing bubble size and therefore improving aerodynamic performance (Lou and Hourmouziadis 2000;Wissink and Rodi 2004;Jones et al 2008). Burgmann and Schröder (2008) demonstrated experimentally a decreasing bubble size as well as an upstream shift of the separation, transition and reattachment locations, when increasing Reynolds number (2 × 10 4 -6 × 10 4 ), angle of attack (4 • -8 • ) or free stream turbulence intensity level.…”