Session PM: Acoustics II

Investigation of the Near-Field Acoustic Properties of Supersonic Jets with Fluidic Injection using Large-Eddy Simulations

Numerical simulations of Imperfectly Expanded Supersonic Jets from a CD nozzle with fluidic injection have been carried out. A MILES (Monotonically Integrated Large Eddy Simulations) approach with a finite element version of Flux-Corrected Transport algorithm is used. It is found that the fluidic injection alters both shock-cell structures and the near-field noise spectra. It eliminates screech tones and reduces the noise intensities, especially in the region near the main nozzle exit. A wide range of injection conditions, such as the size and number of fluidic nozzles and the injection angle, are simulated and studied. Since both fluidic injection and chevron geometry introduce counter-rotating streamwise vortices that alter the shear layers and the shock-cell structures of the main jet, a detailed comparison of the mechanism of these two approaches for noise reduction are made. Furthermore, the effect of a combination of chevrons and fluidic injection on noise reduction is also investigated numerically. Work sponsored by SERDP.   

Investigation of the Acoustic Properties of Supersonic Jets with Fluidic Injection on Chevrons: Experimental Results

An experimental investigation of two jet noise reduction techniques is presented. These techniques are currently employed on commercial aircraft, and we now apply them to a convergent-divergent nozzle with geometry typical of military aircraft. The acoustic effects of chevrons and fluidic injection on chevrons are quantified by Near-Field and Far-Field acoustic measurements. Experimental tests are shown for overexpanded, underexpanded, and on design nozzle pressure ratios to simulate the entire flight envelope of a military aircraft. In nearly all cases chevrons are shown to reduce noise and eliminate screech tones. Adding fluidic injection to chevrons shows additional far-field noise reduction for underexpanded conditions. This presentation is the experimental portion of a joint numerical/experimental program.   

Global Modes of High Speed Jet Noise

We consider instability wave mechanisms for sound generation in supersonic jets from the perspective of global mode analysis. Using a shift-and-invert Arnoldi method, global modes are extracted from direct numerical simulations of perturbations governed by the fully compressible linearized Navier--Stokes equations. As fully 3D eigenfunctions of the linear stability problem, global modes capture directly effects of base flow non-parallelism. For example, we find a significant sound producing region just downstream of the supersonic core of a transonic jet. In addition, maximum transient growth is computed from an optimal superposition of non-normal global modes. While purely linear, we suggest that this transient growth, composed of several frequencies, may be key to predicting eventual nonlinear mode interactions responsible for low frequency sound production. Finally, sensitivities of the global eigenvalues to base flow modifications are calculated from an overlap of direct and adjoint global modes, suggesting strategies for passive control of jet noise.   

Acoustic resonance in a supersonic ducted jet

We consider an overexpanded jet confined by a finite-length duct, which is a model for a particular jet-engine test cell flow in which high-amplitude resonances are observed. This resonance is studied with a two-dimensional model configuration, the flow in which is solved using high-order finite differences on a staggered mesh. A resonant regime was identified for a Mach 1.2 jet, with the amplitude of the acoustic fluctuations of the same order to those observed in test cells. The feedback mechanisms of the resonance will be discussed, and it will be shown that the resonance is suppressed when a damping term designed to remove a small amount of energy from the dominant acoustic mode is added to the equations. The observations from these simulations suggest a scaling that seems to collapse results for the most excited tones observed in several experiments over a broad range of parameters (jet Mach numbers, and duct and nozzle diameters), in both cylindrical and rectangular duct geometries.   

Flow and noise prediction of transonic turbulent jets including nozzle geometry using LES

An unstructured large eddy simulation (LES) method is employed to investigate a turbulent jet in transonic regime. The far-field noise is computed using the integral solution to the Ffowcs Williams-Hawkings equations. The approach has been validated by comparing the near field flow and the far-field sound with the experimental data of Brown and Bridges (AIAA 2006 {\&} 2008) for a jet with a Mach number 0.89 and a temperature ratio 0.84. Although some differences between power-spectra densities from simulation and the corresponding experimental measurements have been observed in regions near the nozzle exit, they are in excellent agreement with experimental data elsewhere. Along the centerline the mean velocity decay is well predicted and turbulent intensity profiles are to within 10-20{\%} of the experimental data. The predicted far-field noise spectra at different polar angles are all within 3dB of the measured experimental ones for Strouhal numbers ranging from 0.05 to 3. Comparisons of flow and sound fields of the heated and unheated jets will be presented.   

Far-field radiation of large-scale turbulent structures using wave-packet models

Our study concerns sound generation from large-scale turbulent structures of both heated and cold round jets. We obtain predictions for the far-field sound based on wave-packet models that are, in turn, motivated by pressure fluctuations measured experimentally using a microphone array. A Kirchhoff-surface-based projection method has been developed to predict the far-field sound from an equivalent source, defined using the two-point space-time correlations of hydrodynamic pressure measured near a conical surface surrounding the jet plume. The predictions for the aft angles, particularly at lower frequencies are generally good. However, the sensitivity of this projection method to various model and physical parameters is not well understood. Techniques like near-field filtering of the microphone data, which can separate out the acoustic and hydrodynamic components of the pressure signal and also proper orthogonal decomposition (POD), which tends to filter out the uncorrelated fluctuations, are examined to provide better understanding of how these large-scale structures and their wave-packet representations radiate to the far field.   

Acoustic Measurement of Body Forces Created by Dielectric Barrier Discharge Plasma Actuator and Comparison with Time-Dependent Empirical Model

Single Dielectric Barrier Discharge (SDBD) plasma actuators have been proven to an effective means of flow control in a variety of applications. As new applications for these actuators emerge, it has become increasingly important to develop a computationally efficient, yet accurate, numercal model to be used in CFD simulations. In previous work, a time-dependent empirical model of a SDBD plasma actuator was developed and validated using time-averaged body force vector results and experimental observations. However, the model is capable of predicting the body force field at the applied a.c. time-scale (on the order of 1 kHz). In order to validate the time-dependent behavior of the model, measurements of the body forces produced by the actuators at the a.c. time-scales are needed. In the present work, we present acoustic measurements of a SDBD plasma actuator in a hemi-anechoic environment. The measurements were taken in 5 degree increments in an arc oriented in the direction of the induced flow. Both the magnitude and phase information were considered and then compared to the results of the lumped circuit element model previously developed.   

On the reflectivity of sponge regions in compressible flow simulations

In finite-domain compressible flow simulations, one remedy to address lack of boundary information is to gradually relax the flow near the external boundary to a known consistent far-field solution of the Navier-Stokes equations. This treatment, called the sponge treatment, is adopted in many calculations owing to its simplicity, generality and robustness. In practical calculations however, interactions of the sponges with flow features can reflect unphysical signatures into the CFD domain. If the sponge is not carefully designed these reflections can overwhelm the physics of interest particularly when acoustics are concerned. In this work we examine the physics of sponge/flow interactions through analytical and semi-analytical approaches. The reflectivity due to non-linear terms, oblique waves intersecting, and sponge/vortex interactions are separately analyzed. The optimal sponge profiles and the reflection coefficients for asymptotically small or large sponges (compared to flow features) are investigated. These analyses provide estimates of the sponge requirements for CFD calculations in a relatively general framework. In steady state problems the effect of sponge zones on the solution will be discussed.   

Application of Stochastic Galerkin Methods (SGM) to Uncertainty Analysis in Computational Aeroacoustics (CAA)

In problems of CAA, uncertainties in the flow field data inherent in any turbulent flow calculation, e.g. those due to discretization errors, must be incorporated into predictions of the far-field sound. This is particularly relevant for hybrid CAA methods, which require the flow field data as input and then propagate the effects to the far-field by means of an acoustic analogy. We illustrate the application of a SGM to a canonical problem involving the acoustic far-field generated by a monopole source radiating in the vicinity of a rigid, infinite cylinder in an otherwise quiescent medium, in which the location of the source is assumed to be uncertain. The accuracy and efficiency of this method, in this context, are evaluated by comparison with a local sensitivity analysis and an equivalent Monte-Carlo type simulation, respectively. The present method is transparent to the way in which the flow field data and corresponding uncertainties are obtained, implying that further application to complex flow calculations may be feasible.