Session M18: Acoustics I: Scattering, Jets and Cavity Flows

Large-Eddy Simulations of Supersonic Heated Jets

Large-eddy simulations of imperfectly expanded jet flows from a convergent-divergent nozzle with a heated (600K) and a cold jet conditions have been carried out. Mach wave radiation is present outside the jet core and the heated jet shows a much larger density shear-layer spreading than the cold jet. The potential core length is shortened, but the shock-cell shape and shock-cell spacing remain similar to those shown in the cold jet. It is found that the Crocco-Busemann equation captures well the correlation between the temperature and the axial velocity in the cold jet, but it underestimates the nonlinear variations in the shock-containing region in the heated jet. Temperature effect has more impact on the high-frequency components of pressure fluctuations near the nozzle exit, but the impact moves to low- and mid-frequency ranges further downstream. The convection speed of the near-field pressure waves increases with the temperature. The heated jet is found to be more sensitive to the inflow pressure perturbations than the cold jet.   

Parabolized stability equation (PSE) models for the prediction of mixing noise in turbulent jets: nonlinearity and comparison with experiments

The wavepackets responsible for the generation of the dominant low-frequency noise of turbulent round jets are modeled via PSE. Previous experience shows that wavepackets delivered by linear PSE are in agreement with pressure fluctuations on a near-field microphone array. Nonlinear interactions between PSE modes is the subject of the present effort. Pressure fluctuations in the near-field and hot wire measurements at the jet centerline are used to determine initial amplitudes for the PSE computations, and then perturbed randomly to introduce some degree of nondeterminism. A projection technique is used to compare the noise radiated to the far-field by wavepackets with experiments. First results indicate that nonlinearity is not a determinant factor in the wavepackets evolution, especially for unheated, low Mach number jets, while small variations in the initial conditions might introduce $O(1)$ changes.   

Parabolized stability equation (PSE) models for the prediction of mixing noise in turbulent jets: Comparison with Large Eddy Simulation

The wavepacket structures responsible for the generation of the dominant low- frequency noise of turbulent round jets are modeled via PSE. To deliver meaningful results, PSE models require an accurate description of the disturbance conditions in the vicinity of the nozzle. The nature of the unforced turbulent flow avoids the existence of a single, deterministic initial condition. A high fidelity database obtained by Large Eddy Simulation is used to determine the initial conditions for the PSE modes: the total length of the simulation is split into several realizations, and Linear Stability Theory based biorthogonal decomposition is employed to initialize the PSE with the adequate wavepacket. The PSE solution corresponding to each realization is then compared with the wavepackets educed from the LES dataset. The ensemble of the realizations is considered then to study the effect of nondeterministic conditions on the wavepacket evolution.   

Modeling unsteady sound refraction by coherent structures in a high-speed jet

We construct a visual model for the unsteady refraction of sound waves from point sources in a Ma = 0.6 jet. The mass and inviscid momentum equations give an equation governing acoustic fluctuations, including anisotropic propagation, attenuation and sources; differences with Lighthill's equation will be discussed. On this basis, the theory of characteristics gives canonical equations for the acoustic paths from any source into the far field. We model a steady mean flow in the near-jet region including the potential core and the mixing region downstream of its collapse, and model the convection of coherent structures as traveling wave perturbations of this mean flow. For a regular distribution of point sources in this region, we present a visual rendition of fluctuating distortion, lensing and deaf spots from the viewpoint of a far-field observer.   

Noise source characterization from far-field data in high-speed jets

We analyze pressure data from 3 microphones in the far-field coherent noise cone of a Ma=0.6 jet. 3500 individual sources are identified in the time-frequency domain, and some of their properties are extracted from the wavelet coefficients: magnitude, frequency, time of arrival. The sources' signature therefore includes the lags between the sound arrival times at the microphones. Based on a modeled mean jet velocity field, we calculate the refracted acoustic paths and propagation times from a grid of source locations to the microphones. The excellent agreement between measured and calculated lags provides a mapping between the measured lags and the approximate source locations. For our catalog of sources, we report on some statistics of source properties as a function of their location in the near-jet's shear layer and developing region beyond the end of the potential core. The ability to narrow the search (time and location) for sources in near-field data (LES or PIV) may be established on this basis.   

Near field of a transient, acoustically forced transitional and turbulent jets

Acoustic liners are widely used to reduce aircraft engine noise. They work by converting acoustic-bound energy into vorticity-bound energy, in the form of a transient jet, at an orifice that is very small relative to the incident sound wavelength. At low sound amplitudes ($<$ 130 dB) the forced jet is laminar. At higher amplitudes ($>=$ 150 dB) vortical instabilities appear and the jet becomes turbulent. In this work the behavior of transitional and fully turbulent transient jets are studied using direct numerical simulations of the compressible Navier-Stokes equations. We focus on the near-aperture dynamics of the acoustically-forced fluid by quantifying the jets' phase-averaged properties and linking these to a reduced order dynamical model with the objective of understanding the motion of transient turbulent jets. Results indicate that boundary layer separation from the orifice walls is critical to seeding instabilities within the jets as they develop while at later times disturbances from the previous acoustic cycle reinforce the jets' unsteadiness.   

Analysis of the near-field of an adjoint-based noise controlled Mach 1.3 turbulent jet

A noise-reduced Mach 1.3 turbulent jet is investigated to identify its sound-generating mechanisms. The adjoint of the perturbed and linearized Navier--Stokes equations is used in an inverse fashion to provide the control sensitivity to directly reduce sound radiation. The accuracy of this gradient in control parameter space is assessed by examining the variation of the cost functional for small-amplitude controls. The control found by the optimization algorithm reduces noise mainly via suppressing intermittent, large-amplitude acoustic radiation at lower angles and at jet preferred mode frequencies. This observation is similar to the adjoint-based optimization of sound radiated by two-dimensional non-turbulent mixing layers and suggests the subtle modification of large-scale vortical motions for noise reduction. The proper orthogonal decomposition is applied to the near-field pressure fluctuations and the changes in the optimal, orthogonal basis due to the control are analyzed. When controlled, the downstream evolution of large-scale structures shows less abrupt changes, which may be associated with the suppression of the intermittent acoustic radiation. The noise-reducing control is broadbanded in both space and time and demonstrates the characteristics of modulated instability wave packets. Optimizations are ongoing.   

Large-eddy simulation for the prediction of supersonic rectangular jet noise

We investigate the noise from isothermal and heated under-expanded supersonic turbulent jets issuing from a rectangular nozzle of aspect ratio 4:1 using high-fidelity unstructured large-eddy simulation (LES) and acoustic projection based on the Ffowcs-Williams Hawkings (FWH) equations. The nozzle/flow interaction is directly included by simulating the flow in and around the nozzle in addition to the jet plume downstream. A grid resolution study is performed and results are shown for unstructured meshes containing up to 300 million control volumes, generated by a massively parallel code scaled to as many as 65,536 processors. Validated against laboratory measurements using a nozzle of precisely the same geometry, we find that mesh isotropy is a key factor in determining the quality of the far-field aeroacoustic predictions. The full flow fields produced by the simulation, in conjunction with particle image velocimetry (PIV) data measured from experiment, allow for a detailed examination of the interaction of large-scale coherent flow features and the resultant far-field noise, and its subsequent modification in the presence of heating.   

High Speed Shadowgraph Images around a Mach 1.5 Cavity Flow Field

An examination of a rectangular cavity with an L/D of 5.67 was tested at Mach 0.7 and 1.5 with corresponding Reynolds numbers of 2x106/ft and 2.3x106/ft, respectively. High speed shadowgraph movies were simultaneously sampled with the dynamic pressure sensors at 75 kHz. Fourier analysis was performed on the high speed movies as well as the dynamic pressure data which resulted in determining the locations of dominant cavity frequencies in the flow field. From the high speed shadowgraph movies, observations of the in the cavity flow physics are discussed. Several cavity related issues are examined e.g. How do vortices form in the shear layer? What is the actual starting mechanism for these cavity acoustic tones? How do the cavity acoustic tones affect the shear layer?   

Cavity noise control using a small obstacle on cavity floor

An experimental and numerical study to control the cavity noise using a small two-dimensional rectangular cylinder set on a cavity floor has been carried out. In the experiment, it was found that the cavity noise could be reduced to a background noise level by placing a two-dimensional rectangular cylinder at a proper location of the cavity floor. PIV measurement of the flow inside the cavity indicated that flow was forced to separate by the obstacle, changing the recirculating flow inside the cavity. The numerical simulation of the same flow field revealed that the small bump on the cavity floor introduced a large number of longitudinal vortices which interacted with the main shear layer of the cavity flow. The interaction deformed the spanwise vortices, and hence suppressed the cavity noise.