Aeroacoustics and Flow Dynamics of an airfoil with a Gurney flap using TR-PIV

Abstract

Aircraft noise has come under severe scrutiny due to growing number of airports and airline operations. The noise generated by high lift devices is now comparable to the turbofan engines due to increasing bypass ratios. A Gurney flap is a suitable starting point to study trailing edge flap aeroacoustics due to additional lift characteristics and a simple geometry. The thesis aims in using Time-Resolved Particle Image Velocimetry to understand the dynamics of flow as well as identifying structures responsible for the tonal noise in a Gurney flap. TR-PIV was used to obtain, at a high spatial and temporal resolution, the flow dynamics associated with a Gurney flap at low Mach and high Reynolds number. A part of the thesis focussed on validating the two modes of shedding as an additional lift increment mechanism in a Gurney flap in case of a fully developed turbulent boundary layer. Flow statistics showed an upstream recirculation region, upper and lower separating shear layers and a main recirculation region whose size varied with Gurney flap size and angle of attack downstream of the flap. The instantaneous velocity and vorticity fields showed the flapping motion of the von Karman wake and corresponding vortex roll-up and shedding process. The power spectral density of flow fluctuations as well as the acoustic spectra, however, did not indicate any second mode of shedding. The Strouhal numbers of the vortex shedding for various Gurney flaps were close to that of a bluff body in a flow. Stereoscopic PIV measurements showed the periodic vortex shedding to be highly correlated in the spanwise direction (correlation length of 4cm). The large spanwise correlation length indicates the use of serrations can be useful in breaking-down such coherent structures. The tonal peaks were clearly audible and correspond to the vortex shedding frequency. Causality correlation between pressure fluctuations generated in the far-eld and the near-field fluctuations indicate a high correlation with unsteady vertical velocity in the wake (associated with unsteady lift). It was found that the dynamics in the upstream recirculation region (cavity) was not responsible in the production of noise. Further, the results indicate the location of the actual source of noise to be the flap. The high values of correlation further downstream are due to the presence of an upstream source (the flap) and are not sources of sound themselves. Thus, TR-PIV in a way clears the ambiguity expressed by other researchers in the past in interpreting the high values of correlation to be sources of sound.