Experimental study of a tip leakage flow: wavelet analysis of pressure fluctuations

Abstract

A wavelet-based conditional analysis of unsteady flow and sound signals highlights\ud the role of intermittent perturbations both in the sound generation and the unsteady\ud field of an aerofoil tip leakage flow experiment. It is shown how the most probable\ud flow perturbations generated at the pressure side tip edge are convected through\ud the gap and swept downstream along the suction side past the trailing edge tip\ud corner, where they radiate sound. The nascent sound sources are identified and\ud localized in the clearance between 40% and 60% of the chord. It is also found\ud that the time dependence of the averaged intermittent structures scales with the\ud inverse of the square root of the mean velocity and a physical interpretation based\ud on a simple potential vortex model is proposed. The data are retrieved from an\ud experiment that has been carried out at low Mach number (Ma<0.3) in an anechoic\ud test facility. A single motionless instrumented NACA 5510 aerofoil was mounted into\ud the potential core of an open rectangular jet between two plates with an adjustable\ud clearance. The tip leakage flow was ensured by the 5% camber and a 15◦ angle\ud of attack. A large database obtained by a variety of measurement techniques is\ud thus available for the present analysis. More specifically, the conditional approach\ud is applied to joint far field, wall pressure and particle image velocimetry (PIV)\ud measurements. The wall pressure probes are located along the suction side tip edge\ud and on the tip inside the gap, whereas the PIV plane is parallel to the mid-gap\ud plane. Additional joint wall pressure and single hot-wire anemometry (HWA)\ud measurements are also analysed with a hot-wire probe located near the trailing edge tip\ud corner. The conditional averaging is triggered by high-energy wavelet events selected\ud in a reference signal by setting a threshold to the so-called local intermittency measure

Wavelet Analysis of a Blade Tip-Leakage Flow

“…Since these events are unevenly spaced and show variations in their duration, wavelet analysis appears to be promising if not more appropriate than classical Fourier analysis to identify such intermittent events. The detection of flow events with wavelets is a well-known topic of research and has already been applied to the present configuration by Camussi et al [16] on the experimental results of the first campaign. The present paper is not focused on the mathematics of the wavelet technique but merely relies on an elementary wavelet decomposition, with the objective to identify the unsteady flow phenomena in the tip region.…”

Experimental approaches for the diagnostics of hydroacoustic problems in naval propulsion

“…A key issue for the success of conditional averaging is obviously the proper choice of the trigger condition. The most simple trigger condition is a threshold applied to the original signal or to a derived quantity, such as, for example, the Local Intermittency Function (LIM) of its wavelet transform as in Camussi et al (2010).…”

“…In particular, following former works in the aeroacoustic field (see e.g. Camussi et al, 2008Camussi et al, , 2010, the event-tracking method used to perform the conditional analysis is based on the computation of the so-called Local Intermittency Measure or LIM (Camussi and Guj, 1997;Farge, 1992). Without going into the details of the implementation, an overview of the conditional procedure is schematically represented in the block diagram of Fig.…”

Pressure-density gradient correlations in the near-field of a transonic jet

“…The conditioning is dealing with the detection of energetic events, as examples, the wavelet-based studies of wall pressure fluctuations induced by attached or separated turbulent boundary layers [40][41][42] and the wavelet-based investigation of the wall and far field pressure induced by an aerofoil in a tip-leakage configuration 13 . For applications related to jet-noise see Camussi and Guj 31 and Onorato et al 28 .…”

“…However, due to the local nature of the fluid structures responsible for the noise emission [10][11][12] , this approach can be unreliable. In order to overcome the limit of the Fourier transform in representing localized events, the wavelet analysis can be adopted [13][14][15][16] . In a recent paper of Grizzi and Camussi 17 , following the approach developed by Farge 16 and Ruppert-Felsot et al 18 , was proposed a novel wavelet based technique to decompose a pressure signal into its acoustic and hydrodynamic components.…”