3D Lagrangian particle tracking using 4-pulse Shake-The-Box synchronised with microphone measurements on a subsonic jet at Mach 0.9

Abstract

Synchronized 3D Lagrangian particle tracking (LPT) using the multi-pulse implementation of Shake-The- Box (STB) algorithm and microphone measurements were performed on a subsonic jet flow with ideally expanded flow of Mach = 0.9 generated by a round nozzle and a chevron nozzle, respectively. The study of subsonic jets has a long history both in the fields of fluid mechanics and aeroacoustics. To date there has only been a limited number of studies investigating these flow phenomena with volumetric measurement (3D) techniques. This study aims to provide further insight into the flow physics by using a recently developed 3D particle tracking algorithm, namely Shake-The-Box (STB) to provide highly resolved accurate velocity and acceleration data suitable for instantaneous 3D pressure field extraction. The application of STB in aeroacoustics provides a means of establishing a direct connection between the flow dynamics in the jet near field and the acoustics pressure fluctuations. STB for time resolved applications is described in Schanz et al (2016) and allows for accurate particle tracking of densely seeded (>0.05ppp) flows. Novara et al (2016a and 2016b) showed that a short time sequence (or multi-pulse) implementation of STB was able to retrieve most particle tracks with values of velocity and acceleration comparable to those obtained with STB in the time-resolved domain, and they stipulated the possibilities of measuring the material acceleration (and pressure) for high speed flows by means of multi-pulse acquisition systems. Following Novara et al (2016b) successful validation of the STB processing techniques on synthetic data at Mach = 0.7, this experimental investigation will serve to provide further validation and assessment in real experimental conditions. The experimental measurements were performed on a round nozzle and a chevron nozzle, respectively. The nozzle diameter, D = 15 mm at the exit. The nozzles including the inner contour which follows a polynomial of 7th order is described in Miguel and Henning (2013) and Henning et al (2014). Measurements were conducted for ideally expanded jet flow of Mach = 0.9. The sound pressure fluctuations are recorded by a polar microphone array consisting of 13 condenser microphones arranged at angles 90˚ to 30˚ in 5˚ increments to the jet axial axis. A further 4 microphones were positioned at 180D at angles 90˚, 60˚, 45˚ and 30˚ to the jet axis. The microphone sampling frequency was 250 kHz and a high pass analog filter was used at 500 Hz. Both, the velocity and the pressure measurements are synchronized by the master clock of the microphone data-acquisition system (Viper HDR). The measurements were performed in an anechoic-chamber at the DLR Göttingen. The STB experimental setup consisted of two 4 camera tomographic particle tracking systems separated by polarization as demonstrated by Schröder et al 2013 and Novara et al 2016a. All eight cameras were sCMOS (PCO-edge type) with resolution 2560 x 2160 pixels. The digital resolution for the experiment was approximately 28 μm/pixel. The measurement volume encompasses approximately 70x90x10 mm3 along the jet axial (X), radial (Y) and out-of-plane (Z) directions, respectively. The first results using four-pulse STB on a subsonic jet flow of Mach 0.9 are presented. As only preliminary results are available at this time, a discussion of the future outlook is presented in the abstract.