AN AEROACOUSTICS INVESTIGATION OF AN IMPINGING JET USING 4D PARTICLE TRACKING VELOCIMETRY

Abstract

Turbulent impinging jests are applicable to many applications such as fighter aircraft which employ short take-off and landing. Impinging jets can produce high heat transfer rates and thus are widely used in cooling and heating applications for example, turbine blade cooling. From a fundamental perspective, an impinging jet presents interesting features, a free turbulent jet, a stagnation flow, and a wall jet. Of interest has been the noise emission due to the vortices impacting on the surface resulting in large surface-pressure fluctuations, which produce an upstream acoustic excitation (hence a feedback loop). There have only been limited studies investigating these fundamental flow phenomena with time-resolved volumetric measurement (4D) techniques. This study aims to provide further insight into the flow physics by using a recently developed 4D particle tracking velocimetry algorithm, namely Shake-The-Box (STB). The jet nozzle diameter, D = 0.11 m and the impact distance was 5D. Six high speed (PCO dimax S) cameras were used with acquisition rates up to 3.9 kHz. LED arrays provided light illumination of the tracer particles, Helium-filled Soap Bubbles (HSB), generated by a La Vision HSB generator. The HSB nominal diameter was 300 µm. Three microphones were flush mounted on the impinging plate and positioned at 1D, 2D and 3D from the jet centre. The microphone signals were acquired at 250 kHz for 60 s. The LEDs, cameras and microphones were synchronised by a La Vision timing unit (PTU X). The measured volume was relatively large at 0.5 x 0.5 x 0.2 m3. The STB technique provided accurate particle tracking from which the acceleration of particles was determined. From the acceleration fields the unsteady pressure distributions were calculated. The microphone frequency spectrum showed two peaks which corresponded to the impingement of large coherent structures and the fan noise. Synchronisation of the flow measurements with the surface pressure allowed phase averaging and a causality correlation technique to be applied to identify the sources of sound. Cross-correlations of near-field measured quantities and the microphone pressure were used to determine the acoustic propagation paths.