Mapping of laminar separation bubble and bubble-induced vibrations over a turbine blade at low Reynolds numbers

Abstract

A detailed experimental investigation on the mapping of the LSB formation over a turbine blade with different airfoils, and their vibration effects were considered first in this study. Wind tunnel experiments were performed on FX 84-W-150, SD7062, Clark-Y and WASP airfoils at Re = 3.5 x 10(4), Re = 7 x 10(4) and various angles of attack. Experimental arrangements consisted of detailed instantaneous and time-dependent flow visualizations, quasi-wall shear stress measurements by means of a hot-film sensor, aerodynamic force measurements with an external force balance system. It was observed that there was a strong correlation between voltage signals obtained from the hot-film sensor and the smoke-wire results. Fluctuation amplitude started to increase within the LSB especially at the aft portion due to the transition induced trapped roll-up vortices. Then, fluctuations peaked upstream of the reattachment point. It was observed that the airfoil with different thickness and camber caused the LSB (either short or long) to form, resulting in variation of aerodynamic forces with the time. This could trigger the stochastic vibrations such as flap-wise and edge-wise vibrations at the blade, causing the stochastic fatigue loads and limiting the lifespan of turbine blades.