Experimental study of a tip leakage flow

Abstract

Experimental acoustic characterization of a tip leakage flow developing downstream of a single blade test rig has been performed via wavelet-based post-processing techniques. Hot-Wire Anemometer, far-field and wall-pressure signals were processed which have been measured on an instrumented airfoil installed within the anechoic wind tunnel available at the Laboratory of Fluid Mechanics and Acoustics of the Ecole Centrale de Lyon. The adopted airfoil is a NACA 5510 airfoil and the test rig has adjustable tip gap, which allows for tuning of the gap width: two flow configurations, with and without gap at the airfoil tip, are investigated. The Reynolds number, based on the airfoil chord, was Rec 9.3105. In the experimental campaign, conducted by researchers of the Ecole Centrale de Lyon, measurements of the pressure both on the airfoil surface and in the acoustic field have been coupled with HWA and PIV acquisitions. The outcoming database provided a suitable characterization of the (strong unsteady) flow dynamics in the tip-gap region as well as the impact of the tip-gap turbulent interaction on to the measured acoustic emission. In the present research activity, advanced post-processing procedures based on the wavelet transform are applied to the data measured by ECL in order to extract the most energetic non-periodic contributions, localized in time and in space, and to detect the fluid dynamic structures which may act as noise sources. The events tracking method is based on the computation of the time-frequency maps from which it is possible to select events, determine their time of appearance, and perform conditional averages. The conditioning procedure has shown that the gap width plays a fundamental role in the noise generation mechanisms: the amplitude of the oscillations in the averaged wall-pressure signatures becomes larger for increasing width of the gap, probably as an effect of a roll–up phenomenon occurring at the tip edge of the airfoil. This phenomenon is known to produce complex unsteady interactions within the flow. Arising turbulent mechanisms dominate the flow behaviour not only in the gap region. Vortex structures shed by the tip leakage flow convect downstream, interact with the trailing edge flow and strongly modify the flow dynamics and the scattering mechanisms. Wavelet–conditioned HWA signals confirmed this features, as the averaged velocity fluctuations in the trailing edge region seem to be significantly affected by the gap width and statistically related to strong pressure energy fluctuations detected over the airfoil suction side close to the gap. The pressure–velocity cross–analysis here presented evidenced the effectiveness of the tip leakage flow as a noise source and provided useful information about the turbulent mechanisms excited by its formation. Pressure/velocity wavelet– based correlation obtained by processing HWA/far–field pressure and PIV/far– field pressure measurements yielded, in a statistic sense, the location of the major fluid dynamic structure which may be related to the largest pressure fluctuations at the wall and in the far field. A further step in the present research activity, has been the implementation of a boundary–to–field transfer function approach, as an attempt to model the acoustic phenomenon under investigation. The approach is based upon a theoretical formulation which uses the well-known concept of transpiration velocity and is devoted to the vorticity generated sound problem. Preliminary results were obtained by using this approach in a simple test–case, but the application to the configuration here analyzed is one of the future steps in the ongoing research activity.