Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A29: Acoustics I: Aeroacoustics |
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Chair: Daniel Bodony, University of Illinois at Urbana-Champaign Room: 2014 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A29.00001: Examining PIV windowing effects on high-speed jet flow physics with POD Matthew Berry, Andrew Magstadt, Zachary Berger, Patrick Shea, Christopher Ruscher, Sivaram Gogineni, Mark Glauser The current investigation examines a 2 inch, high-speed, axisymmetric jet with two different PIV setups. Each PIV configuration is simultaneously sampled with far-field pressure. A time-resolved, 10 kHz, PIV system captures a high resolution 1.5 diameter sized window at several downstream locations. A standard, 4 Hz, PIV system utilizes 3 simultaneously captured cameras combined to view a single large interrogation window. Velocity measurements are taken at Mach 0.6, 1.0, and 1.1, along the centerline of the jet in the streamwise (r-z) direction. The low-dimensional modeling technique, proper orthogonal decomposition (POD), is implemented to help resolve the large scale, energetic events, within the flow field. Previous work used these modes to understand how certain flow structures correlated to the far-field acoustics. Due to the different interrogation regions of the PIV systems, windowing effects can yield different results between the setups. We can use this information to determine how windowing effects play a role in the POD convergence rates as well as the velocity to acoustic correlations. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A29.00002: Near-field Nonlinear Interactions Leading to Jet Crackle David Buchta, Jonathan Freund The noise from high-specific thrust jet exhausts, such as on military jets, is not just particularly intense but also exhibits a peculiar raspy crackling sound. The near acoustic field of this peculiar sound has weak shock-like waves that radiate at a distinct angle with a steepened ``footprint'' having higher peaks than valleys and thus a positive pressure skewness. We use large-scale direct numerical simulations of free-shear-flow turbulence with Mach numbers ranging from $M=0.9$ to $3.5$ to study the very near acoustic field and the nonlinear turbulence interactions that lead to this sound. Our simulations reveal that for $M \ge 2.5$ sharp, Mach-like waves radiate at a distribution of angles in the near acoustic field. As they propagate, these waves interact nonlinearly. Wave merging increases the length of the correlated waves and decreases the wave density with distance from the turbulence. Locally, the merging waves generate intense pressure fluctuations with elevated pressure skewness, $S_k(p') > 0.4$, which correlates with the perception of ``crackle.'' These very-near-field nonlinear interactions may explain the peculiar positive $S_k(p')$---the ``footprint'' of which has been experimentally observed to propagate approximately linearly to larger distances from the shear layer. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A29.00003: Synchronized LES for acoustic near-field analysis of a supersonic jet Unnikrishnan S, Datta Gaitonde We develop a novel method using simultaneous, synchronized Large Eddy Simulations (LES) to examine the manner in which the plume of a supersonic jet generates the near acoustic field. Starting from a statistically stationary state, at each time-step, the first LES (Baseline) is used to obtain native perturbations, which are then localized in space, scaled to small values and injected into the second LES (Twin). At any subsequent time, the difference between the two simulations can be processed to discern how disturbances from any particular zone in the jet are modulated and filtered by the non-linear core to form the combined hydrodynamic and acoustic near field and the fully acoustic farfield. Unlike inverse techniques that use correlations between jet turbulence and far-field signals to infer causality, the current forward analysis effectively tags and tracks native perturbations as they are processed by the jet. Results are presented for a Mach 1.3 cold jet. Statistical analysis of the baseline and perturbation boost provides insight into different mechanisms of disturbance propagation, amplification, directivity, generation of intermittent wave-packet like events and the direct and indirect effect of different parts of the jet on the acoustic field. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A29.00004: Acoustic source analysis of a rectangular supersonic jet Jordan Kreitzman, Joseph W. Nichols We apply Goldstein's generalized acoustic analogy to identify acoustic sources in two high-fidelity unstructured large eddy simulation databases of a Mach 1.4 rectangular jet with and without chevrons. Two-point, two-time correlations of the acoustic source terms are evaluated at different positions in the three dimensional flow that develops downstream of the complex nozzle. Two-point statistics are compared to single-point statistics to test the quasi-normality hypothesis and other noise source models for a non-axisymmetric jet. In particular, we assess the predictive capability of a Gaussian model, a fixed-frame model and a modified-distance model. The nozzle geometries used for the simulations exactly match an experimental configuration tested at the NASA Glenn Research Center, allowing for validation in terms of both farfield noise as well as turbulence statistics. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A29.00005: Sound amplification by jittering wavepackets in subsonic turbulent jets Mengqi Zhang, Aaron Towne, Peter Jordan, Tim Colonius, Guillaume Br\`es, Sanjiva Lele Recent research confirms that coherent structures in turbulent jets can be understood as hydrodynamic instabilities (wavepackets) of the turbulent mean that amplify and decay as they convect downstream. Linear models used to compute such wavepackets obtain compelling agreement with experiment in terms of both wavepacket structure and phase speed. But the radiated sound can have errors of several orders of magnitude. Data analysis suggests that this is because individual wavepackets evolve, not on the long-time mean of the turbulence, but on a slowly varying mean, which may be described statistically via an ensemble of short-time averages. We use data from a Large Eddy Simulation to explore this idea. The simulation has been carefully validated by an accompanying experiment and found, in particular, to reproduce loud intermittent events observed in the measurements. Slowly varying and short-time-averaged mean flows are extracted from the LES. The Linearised Euler Equations are solved using the slowly varying mean--obtained by low-pass filtering the LES data--as a base flow. The Parabolised Stability and One-Way Euler equations are solved using the short-time ensemble. The solutions comprise jittering wavepackets whose sound radiation is enhanced by several orders of magnitude. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A29.00006: Jet noise models using one-way Euler equations Aaron Towne, Tim Colonius Experimental and numerical investigations have correlated large-scale coherent structures in turbulent jets with acoustic radiation to downstream angles, where sound is most intense. These structures take the form of wavepackets and can be modeled as linear instability modes of the turbulent mean flow. The parabolized stability equations have been successfully used to estimate the near-field evolution of these wavepackets, but are unable to properly capture the acoustic field. We have recently developed an efficient method for calculating linear instability modes that properly capture both the near-field wavepacket and the associated acoustic field. The linearized Euler equations are modified such that all upstream propagating acoustic modes are removed from the operator. The resulting equations, called one-way Euler equations, can be stably and efficiently solved in the frequency domain as a spatial initial value problem. In this work, we use the one-way Euler equations to model sound generation and propagation in subsonic and supersonic jets. The mean flows are obtained from high resolution large-eddy-simulation (LES) data, and the one-way Euler solutions are validated against direct solution of the linearized Euler equations and compared to the LES data. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A29.00007: Correlations Between Large-scale Flow Structures and Acoustic Signatures in an Axisymmetric Jet Andrew Magstadt, Matthew Berry, Zachary Berger, Patrick Shea, Mark Glauser In a test campaign studying jet noise, simultaneous far-field acoustic measurements and near-field particle imaging velocimetry (PIV) data were sampled from a supersonic underexpanded axisymmetric jet operating at a Reynolds number of 1.3x10\textasciicircum 6. Using overlapping snapshots from three adjacent cameras, separate images of the velocity field were stitched together to form an uninterrupted window. Centered about the axis of the jet, the effective field of view spanned two jet diameters in the cross-stream direction ($r)$ and seven diameters in the streamwise direction ($z)$. This area proved to be sufficiently large to capture important scales of supersonic flow relevant to noise generation. Specifically, Proper Orthogonal Decomposition (POD) has extracted particular energy modes thought to be associated with the large-scale instability wave, shock cells, and turbulent mixing characteristic of supersonic noise. As example, time-dependent modal correlations present evidence linking the existence of shock cells to screech tones. From the data gathered, these experimental and analytical techniques are believed to be valuable tools in isolating energy-based flow structures relevant to noise generation. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A29.00008: Simulation and stability analysis of supersonic impinging jet noise with microjet control Nathaniel Hildebrand, Joseph W. Nichols A model for an ideally expanded 1.5 Mach turbulent jet impinging on a flat plate using unstructured high-fidelity large eddy simulations (LES) and hydrodynamic stability analysis is presented. Note the LES configuration conforms exactly to experiments performed at the STOVL supersonic jet facility of the Florida Center for Advanced Aero-Propulsion allowing validation against experimental measurements. The LES are repeated for different nozzle-wall separation distances as well as with and without the addition of sixteen microjets positioned uniformly around the nozzle lip. For some nozzle-wall distances, but not all, the microjets result in substantial noise reduction. Observations of substantial noise reduction are associated with a relative absence of large-scale coherent vortices in the jet shear layer. To better understand and predict the effectiveness of microjet noise control, the application of global stability analysis about LES mean fields is used to extract axisymmetric and helical instability modes connected to the complex interplay between the coherent vortices, shocks, and acoustic feedback. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A29.00009: A Method for Estimating Far-Field Acoustics Generated by a Turbulent Wall Jet Adam Nickels, Lawrence Ukeiley, Robert Reger, Louis Cattafesta Noise generated via flow interactions within a turbulent wall jet is investigated using a multi-stage estimation method. Two component Particle Image Velocimetry (PIV) and surface pressure measurements are acquired at a Reynolds number based on nozzle height of 25,500. The PIV snapshots and surface pressure measurements were acquired in a synchronous manner to allow for correlation between the quantities. A Proper Orthogonal Decomposition (POD) and Linear Stochastic Estimation (LSE) based technique is used to estimate a set of time-resolved velocity fields at a higher time resolution than was measured. To improve the estimates, the Kalman filter is employed to leverage measurements against a dynamical model established by employing Dynamic Mode Decomposition (DMD) on an independent set of time resolved PIV data. With the estimated time resolved velocity fields and surface pressure measurements, Poisson's equation for fluctuating pressure is solved to find the pressure acting along the entire surface under the wall jet. The far-field acoustics generated by the wall jet are then estimated by solving Curle's acoustic analogy, using the integrated surface pressure as the source term. [Preview Abstract] |
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