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

Combined near and far field aeroacoustic measurements have been carried out during an original laboratory scale low Mach number (0–0.3) experiment about the tip leakage flow past a single non-rotating airfoil. Such measurements were made possible by the use of a single airfoil located in the potential core of an open jet flow in the anechoic wind tunnel of the Ecole Centrale de Lyon. The airfoil was mounted between two flat plates. A strong tip clearance flow was achieved without rotation by paying a special attention to the choice of the airfoil which was a 5% camber, 10% thickness NACA5510 airfoil that provided a high lift at a 15° angle of attack. The experiment gave rise to an extensive data set obtained with several flow velocity measurement techniques (HWA, LDA, PIV), steady and unsteady pressure measurements on the airfoil and the casing plate as well as far field pressure measurements. Further, cross-analyses of various velocity and pressure signals allowed to locating sources and identifying their mechanisms. Results showed evidence of two components of tip leakage broadband self noise.

Time-resolved PIV and time-resolved stereo PIV measurements are carried out in the tip leakage vortex of a single non-rotating airfoil placed in the potential core of a flanged rectangular jet in free field conditions. The experiment is based on the improvement of an existing rig: a cambered airfoil (NACA5510) now mounted with a $16.5^{\circ }\pm 0.5^{\circ }$ angle of attack between two horizontal plates, a $10\text{mm}$ gap being maintained between the airfoil tip and the lower (casing) plate. The mean flow velocity is $70\text{m}/\text{s}$ , which corresponds to a $0.2$ Mach number and a chord-based Reynolds number of $933,000$ . Unlike in the former experiment carried out with this rig, the boundary layer thickness is now smaller than the gap, which significantly reduces the interaction between the upstream turbulence and the airfoil leading edge as well as the resulting interaction noise. The measurements described here include the far field. The upstream flow is characterised with hotwire anemometry. LDV profiles are also obtained in the tip leakage region and compared to the PIV measurements. The experiment is also designed to provide validation data for unsteady CFD computations of the same configuration as shown in a companion paper.