Simultaneous multiplane PIV and microphone array measurements on a rod-airfoil configuration

Abstract

One major objective in aeroacoustic research is the identification of flow structures that are involved in the generation mechanisms of aeroacoustic noise sources. The simultaneous measurement of acoustic pressure fluctuations in the far-field together with some near-field quantity has been investigated by various authors to achieve this objective. The idea of this concept is to correlate the measured far-field pressure with the near-field quantity and in doing so identify regularities in the near field fluctuations that are related to the radiated sound field. Previous experiments have shown that synchronized PIV and microphone measurements can be used to obtain the cross-correlation function between a near-field quantity and the acoustic pressure in the far field. The authors have shown that in case of a relatively simple flow configuration (e.g. cylinder wake flow) a reasonable amount of 5000 PIV recordings are sufficient to obtain a significant correlation between the velocity fluctuations and the far field pressure in the whole region of interest measured via PIV. This has not been the case for a more complex configuration (e.g. a slat flow field of a swept wing), probably because of the more three dimensional character of this flow field compared to the cylinder wake flow field. The studies performed previously by the authors have been performed in a closed test section wind-tunnel which is not optimized for acoustic measurements. Besides, the sound field has been recorded by one in-flow microphone only. In the present study measurements are conducted in an aeroacoustic wind tunnel with an open test section in an anechoic chamber. The acoustic data where acquired using a microphone array consisting of 87 microphones. Furthermore, a multi plane PIV system is used, enabling the calculation of temporal derivatives. A rod-airfoil configuration is investigated by means of the described correlation method. In such a configuration the airfoil undergoes a broadband perturbation dominated by the turbulent periodic vortex shedding frequency separating from the rod. Compared to the flow field generated by a single cylinder the radiated noise is increased over a wide range of frequencies.