Noise emission of free turbulent jets, a novel approach by multiple plane PIV

Abstract

According to the acoustic analogy of Lighthill the flow of a turbulent free jet is replaced by classical acoustic multipols. It is possible to obtain the Lighthill tensor by means of space-time-correlations of the fluctuation velocity components. The energy emitted in space deviates extremely from the energy dissipated by viscosity within the jet, so the signal-to-noise ratio plays an important role for modern measurement techniques. In order to achieve a good signal-to-noise ratio it is necessary to reach a high resolution in space and time. These measurement requirements can be fulfilled by Multi-plane Stereo PIV now. MSPIV allows to measure three velocity components in time and space for two independent planes. The request of time resolution cannot be achieved with contemporary systems (High-Speed-PIV at about 4 kHz). With MSPIV it is possible to decrease the time delay between two independent planes down to nano seconds. The reconstruction of emitted sound by a turbulent free jet is an aero acoustic standard problem since the invention of jet engines. For the experimental realisation two stereo PIV systems were used to configure a MSPIV setup, which allows single measurements separated by space (three velocity components in a plane) and time.