A Laboratory Framework for Synchronous Near/far-field Acoustics and Mhz Piv in High-temperature, Shock-containing, Jets

This paper describes the experimental study of the noise generating characteristics of high-temperature, shock-containing jets emanating from conic-section, converging-diverging (C-D) nozzles. Conic C-D nozzles consist of two conic sections, one contracting and the other expanding, joined to form a supersonic nozzle with a very sharp radius of curvature at the nozzle throat. An experiment is conducted in which temporally resolved flowfield measurements are acquired simultaneously with near-field and far-field acoustics to allow investigation of the turbulence associated with noise generation. The MHz rate PIV system and its synchronization with acoustic measurements is described along with methods for data analysis. General acoustic results are presented to characterize the spectral content present, and preliminary results on the measured turbulence structures are discussed. ∗Address all correspondence to this author. NOMENCLATURE Ae Nozzle exit area. At Nozzle throat area. AR Exit-to-throat area ratio, AR = Ae/At . AJL Anechoic Jet Laboratory. D j Nozzle exit diameter. Ma Acoustic Mach number. Md Nozzle design Mach number based on AR. M j Jet Mach number computed from the NPR. U j Jet exit velocity. NPR Nozzle pressure ratio, NPR = P0/Pa. P0 Jet stagnation pressure. Pa Ambient pressure measured inside the AJL. x Axial coordinate, x = 0 at the nozzle exit, positive in the flow direction. r Radial coordinate such that r/D j = 0.5 at the nozzle lip. θ Polar angle measured from the positive x-axis. PBL Pulse Burst Laser System 1 Copyright c © 2012 by ASME FIGURE 1. The mean velocity field (CFD computation) for the Centerbody configuration at fully-expanded conditions illustrating the shock structure in the plume. INTRODUCTION Jet noise generated by supersonic flows from convergingdiverging nozzles that generate internal shocks possess an added level of complexity in that they include an additional shock system in the jet plume and can involve flow separation inside the diverging section of the nozzle. Typical variable area nozzles found on modern high-performance military aircraft are representative of this situation. A renewed interest in studying the related jet noise has been prompted by the US Navy to specifically address the noise-induced hearing loss and degraded operational awareness resulting from the ever increasing noise levels of higher thrust engines [1, 2]. The present work focuses specifically on the noise generating characteristics of high-temperature, shock-containing jets emanating from conic-section, converging-diverging (C-D) nozzles. A conic C-D nozzle here describes the joining of two conic sections, one contracting and the other expanding, to form a supersonic nozzle with a very sharp radius of curvature at the nozzle throat. The near discontinuity in the slope of the nozzle contour at the throat generates a shock even when the nozzle is operated at its design Mach number, Md . This is illustrated in Fig. 1 which shows the mean streamwise velocity field computed using a Hybrid RANS/LES approach. To study the noise generating features of these jets with sufficient temporal fidelity, a synchronized system is here developed that includes a MHz rate PIV system, and near-field pressure transducer array, and a far-field microphone array. The MHz rate flow measurement system offers the opportunity to obtain time-resolved, high-dynamic range velocity measurements. Synchronizing the PIV with the near-field and far-field arrays provides the means for evaluating the noise generation and propagation.This laboratory framework shares similarities to previous recent works utilizing either high-frame-rate PIV [3, 4] and/or near-field/far-field correlation measurements [5–7]. While no results from the fully synchronous system have yet been obtained, the various components and the synchronized data acquisition system are here described in detail. A discussion of the high-dynamic range PIV analysis methodology is presented along with preliminary flow-visualization results obtained with the pulse burst laser system. To characterize the jet itself, the bulk of the paper focuses on the results of mean flow probe FIGURE 2. The NCPA Anechoic Jet Laboratory setup for the simultaneous near-field/far-field/MHz-PIV measurements. measurements and the near-field and far-field acoustic data. DETAILS OF THE EXPERIMENTAL SETUP To date, three main experimental entries have been completed. The first measured the mean total pressure distribution for each jet configuration. The second measured the fluctuating pressure field using a near-field line array of pressure transducers simultaneously with a far-field arc array of microphones for each jet configuration. The third entry involved the synchronous measurement of the near-field and far-field pressure along with particle image velocimetry (PIV) measurements of the velocity field use a MHz rate system. A description of the experimental facility and setup for each of these components is discussed below. Anechoic Jet Laboratory The Anechoic Jet Laboratory (AJL) at the University of Mississippi’s National Center for Physical Acoustics is a small facility purpose built for the study of high-temperature, supersonic jet noise [8, 9]. The facility was designed with upstream and downstream “stagnation” chambers through which ambient air is pulled by a 10,000 SCFM fan. The air is allowed to percolate into the 19-by-20-by-8 foot chamber (wedge tip to wedge tip) through 50% porosity sliding panels achieving approximately 1 ft/s in the anechoic section (without jet flow). The openings in the upstream wedge wall allowing the aspiration of the chamber can be seen in Fig. 2. The aspiration of the chamber results in a very even temperature distribution throughout the room and allows the jet entrainment to be less effected by the enclosed space. The jet rig utilizes a propane burner system as shown in 2 Copyright c © 2012 by ASME FIGURE 3. Illustration of the propane burner system and nozzle as-