Analysis of boundary layer separation phenomena by infrared thermography Use of acoustic and/or mechanical devices to avoid or reduce the laminar separation bubble effects

Abstract

Aerodynamic bodies operating at low Reynolds numbers are normally subjected to local boundary layer separation phenomena; these strongly modify airfoils behavior. The body geometry and the incoming flow angle of attack define the pressure pattern over the surface in a way that there will be sections with a favorable pressure gradient and others with a negative one, the latter causing the flow to slow down and sometimes to separate. A laminar separation, followed by a turbulent transition in the separated shear layer and a subsequent turbulent reattachment, causes the Laminar Separation Bubble (LSB) phenomenon. The LSB presence induces an aerodynamic drag increase and an efficiency decrease; this problem is of interest in many application fields including wind turbine energy production, where a LSB may induce less energy output and occasionally mechanical problems due to pulsating pressure variations (bubble bursting phenomena). This paper illustrates an infrared measurement approach, useful for the instantaneous observation of the boundary layer pattern on an airfoil surface, as well as an analysis of the effects of both an acoustic and a mechanical system on the behavior of the LSB. These latter induce disturbances in the developing boundary layer in order to promote the turbulent transition and contrasting the LSB presence. The first one involves the use of a subwoofer and the analysis is performed on an Eppler 205 airfoil; the second one makes use a Micro Electro Mechanical System (MEMS) and the IR observation is carried out on a WT01 airfoil developed for small wind turbines (about 1 kW). The results show a pronounced effectiveness for both the methodologies particularly when using MEMS.