Analysis of the effect of broadband acoustic excitation on a laminar separation bubble

Authors

Vasyl Sokolenko - Department of Thermal Machinery, Czestochowa University of Technology,Armii Krajowej 21 42-200, Czestochowa, Poland
Witold Elsner - Department of Thermal Machinery, Czestochowa University of Technology,Armii Krajowej 21 42-200, Czestochowa, Poland
Artur Dróżdż - Department of Thermal Machinery, Czestochowa University of Technology,Armii Krajowej 21 42-200, Czestochowa, Poland
Renata Gnatowska - Department of Thermal Machinery, Czestochowa University of Technology,Armii Krajowej 21 42-200, Czestochowa, Poland
Rarata Zbigniew - Faculty of Power and Aeronautical Engineering, Institute of Aeronautics and Applied Mechanics, Nowowiejska 24, Warsaw 00-665, Poland
Slawomir Kubacki - Faculty of Power and Aeronautical Engineering, Institute of Aeronautics and Applied Mechanics, Nowowiejska 24, Warsaw 00-665, Poland

Abstract

In the modern turbofan engine, the boundary layers developing on surfaces of turbine blades are exposed to strong perturbations caused by acoustic waves. A high concentration of acoustic energy inside such an engine can have a major impact on the stability of the attached and separated laminar boundary layer and can trigger an earlier laminar-turbulent transition.The paper presents the results of an experimental investigation of the effect of broadband on separated shear layer developing on the flat plate subjected to an adverse pressure gradient. The study includes the effect of sound power level (SPL) as well as the Reynolds number determined based on the distance from the leading edge of the flat plate to the separation point. The concept of the work was to investigate how the boundary layer reacts under conditions similar to those found in an aircraft engine. The inherent complexity of the problem is simplified by providing acoustic forcing from a controlled source, acting on the boundary layer developing on the flat plate of a channel with the assumed distribution of pressure gradient corresponding to the conditions encountered in the axial compressor blading. In addition to the naturally developing flow (NE-no excitation), the flow was exposed to the pink noise characterized by the sound pressure levels: SPL = 125 dB and 135 dB at the frequency range 100 – 650 Hz. Two values of Reynolds number i.e. Rex = 150 000 and 300 000, were considered. The velocity measurements were performed with the use of hot-wire anemometry (HWA) Dantec Dynamics Streamline Pro apparatus, while Sound field measurements were performed with the use of advanced GRAS microphones system.It has been shown that the acoustic excitation generated in the frequency range from 100 Hz to 650 Hz can lead to a more rapid increase in flow instability followed by an earlier l-t transition. It can be the result of the coupling of the subharmonic forcing generated by acoustics to the natural frequency of an inviscid Kelvin-Helmholtz instability occurring in the flow. It is interesting to note that the effect on the onset of separation is hardly noticeable as a function of the sound pressure level. On the other hand, the earlier reattachment point and a decrease in the height of the bubble are observed.