Sarah A. Parrish

Harmonics of Jet Screech Tones

It is known that imperfectly expanded supersonic jets undergoing strong screech emit not only the fundamental screech tone but also multiple tones at the harmonics of the fundamental frequency. The dominant direction of radiation of the fundamental screech tone is in the upstream direction toward the nozzle exit. This is necessary to complete the feedback loop responsible for the generation of the screech tone. The harmonic tones, however, do not generally radiate in the same direction as the fundamental. For the mth harmonic screech tone, there are m distinct beams of radiation. Many of these beams are radiated in the sideline and downstream directions. The purpose of this study is to investigate the generation mechanism of the screech harmonics. It is proposed that the screech tone harmonics are produced by Mach wave radiation from supersonic traveling waves in the jet flow. These supersonic traveling waves are created by the interaction of the nonlinear instability wave of the jet flow and the shock-ce...

On the generation of indirect combustion noise

It is known that a uniform compressible flow supports three types of disturbances. They are the entropy waves, which consist of only temperature and density fluctuations, the acoustic waves, and the vorticity waves. When entropy waves are convected into a non-uniform flow region, they are no longer independent waves. Their presence leads to pressure fluctuations, which results in the noise radiation commonly referred to as indirect combustion noise. In the past, because entropy waves and acoustic waves are not independent waves in a nonuniform flow region, the mechanism responsible for the generation of indirect combustion noise has been attributed to mode coupling, that is, the coupling of entropy and acoustic modes. The principal objective of the present investigation is to seek a more physical explanation of how indirect combustion noise is generated. In addition, a numerical simulation is carried out to demonstrate the generation of indirect combustion noise arising from the passage of a line entropy wave pulse through the non-uniform flow field created by a turbine blade. This simulation serves as a simple example illustrating the complex processes involved in sound generation and radiation that take place when entropy waves from a combustor pass through a turbine stage.

Application of PML Absorbing Boundary Condition to Aeroacoustics Problems with an Oblique Mean Flow

For the case of uniform mean flow in an arbitrary direction, Perfectly Matched Layer (PML) absorbing boundary conditions are presented for both the linearized and nonlinear Euler equations. Perfectly matched side layers and stable corner layers are proposed. Stability issues are investigated by examining the dispersion relations of linear waves. For increased efficiency, a pseudo mean flow is included in the derivation of the PML equations for the nonlinear case. Numerical examples are given to support the validity of the proposed equations. Specifically, the linear PML formulation is tested for the case of entropy and vorticity waves traveling with oblique mean flow. The nonlinear formulation is tested with an isentropic vortex moving diagonally with constant velocity.

Physics of Acoustic Radiation from Jet Engine Inlets

Numerical simulations of acoustic radiation from a jet engine inlet are performed using advanced computational aeroacoustics (CAA) algorithms and high-quality numerical boundary treatments. As a model of modern commercial jet engine inlets, the inlet geometry of the NASA Source Diagnostic Test (SDT) is used. Fan noise consists of tones and broadband sound. This investigation considers the radiation of tones associated with upstream propagating duct modes. The primary objective is to identify the dominant physical processes that determine the directivity of the radiated sound. Two such processes have been identified. They are acoustic diffraction and refraction. Diffraction is the natural tendency for an acoustic wave to follow a curved solid surface as it propagates. Refraction is the turning of the direction of propagation of sound waves by mean flow gradients. Parametric studies on the changes in the directivity of radiated sound due to variations in forward flight Mach number and duct mode frequency, azimuthal mode number, and radial mode number are carried out. It is found there is a significant difference in directivity for the radiation of the same duct mode from an engine inlet when operating in static condition versus in the forward flight. It will be shown that the large change in directivity is the result of the combined effects of diffraction and refraction.

Investigation of noise radiation from a jet engine inlet by direct numerical simulation

In a jet engine, strong tones are produced by the fan and are radiated out of the inlet. Such fan noise is an important contributor to the total aircraft noise during take-offs and landings. Experimentally, it has been found that the sound radiation patterns from in-flight tests are quite different from those measured in static conditions. What accounts for this difference? In the current work, the radiation problem is studied computationally using direct numerical simulation based on the most advanced computational aeroacoustics methods. Both static conditions and flight conditions are reproduced. A thorough study of the computed results involving static and flight conditions leads to a physical explanation of the observed difference in the sound radiation patterns. (Invited for presentation in the Aeroacoustics Session)