Garret C. Y. Lam

Aeroacoustics of T-junction merging flow

This paper reports a numerical study of the aeroacoustics of merging flow at T-junction. The primary focus is to elucidate the acoustic generation by the flow unsteadiness. The study is conducted by performing direct aeroacoustic simulation approach, which solves the unsteady compressible Navier-Stokes equations and the perfect gas equation of state simultaneously using the conservation element and solution element method. For practical flows, the Reynolds number based on duct width is usually quite high (>10(5)). In order to properly account for the effects of flow turbulence, a large eddy simulation methodology together with a wall modeling derived from the classical logarithm wall law is adopted. The numerical simulations are performed in two dimensions and the acoustic generation physics at different ratios of side-branch to main duct flow velocities VR (=0.5,0.67,1.0,2.0) are studied. Both the levels of unsteady interactions of merging flow structures and the efficiency of acoustic generation are observed to increase with VR. Based on Curles analogy, the major acoustic source is found to be the fluctuating wall pressure induced by the flow unsteadiness occurred in the downstream branch. A scaling between the wall fluctuating force and the efficiency of the acoustic generation is also derived.

Validation of CE/SE Scheme in Low Mach Number Direct Aeroacoustic Simulation

Abstract The space-time conservation element and solution element (CE/SE) scheme has caught many attention in aeroacoustic research community as an alternative numerical strategy for direct aeroacoustic simulation (DAS). As a result of its strict conversation of flow flux in both space and time, the low-order CE/SE scheme possesses excellent non-dissipative characteristics, expedient in calculating low Mach number DAS which requires uniform numerical accuracy to resolve the widely disparate flow and acoustic scales of the problem. In this paper, an attempt of validating a simplified Courant Number Insensitive CE/SE scheme using carefully selected aeroacoustic benchmark problems is reported. Excellent agreement with the benchmark results obtained firmly establishes that CE/SE scheme is a viable scheme for resolving the nonlinear physics of low Mach number aeroacoustic problems.

Numerical analysis of aeroacoustic-structural interaction of a flexible panel in uniform duct flow

Accurate prediction of the acoustics of fluid-structure interaction is important in devising quieting designs for engineering systems equipped with extensive flow duct networks where the thin duct wall panels are in contact with the flowing fluid. The flow unsteadiness generates acoustic waves that propagate back to the source region to modify the flow process generating them. Meanwhile the unsteady flow pressure excites the thin panels to vibrate, which in turn modifies the flow processes. Evidently a strong coupling between the fluid aeroacoustics and the panel structural dynamics exists. Such coupled physical processes have to be thoroughly understood; otherwise, effective quieting design is never achieved. This paper reports an analysis, using a time-domain numerical methodology the authors have recently developed, of the nonlinear aeroacoustic-structural interaction experienced by a flexible panel in a duct carrying a uniform mean flow. With no mean flow, the numerical results agree well with existing theories and reveal the physics of duct transmission loss. Four regimes of aeroacoustic-structural interaction are identified when the duct flow velocity increases from low subsonic to low supersonic values. Insight in the underlying physics of duct transmission loss at different velocities are highlighted and discussed.

A Numerical Methodology for Resolving Aeroacoustic-Structural Response of Flexible Panel

Fluid-structure interaction problem is relevant to the quieting design of flow ducts found in many aeronautic and automotive engineering systems where the thin duct wall panels are directly in contact with a flowing fluid. A change in the flow unsteadiness, and/or in the duct geometry, generates an acoustic wave which may propagate back to the source region and modifies the flow process generating it (i.e. an aeroacoustic process). The unsteady pressure arising from the aeroacoustic processes may excite the flexible panel to vibrate which may in turn modify the source aeroacoustic processes. Evidently there is a strong coupling between the aeroacoustics of the fluid and the structural dynamics of the panel in this scenario. It is necessary to get a thorough understanding of the nonlinear aeroacoustic-structural coupling in the design of effective flow duct noise control. Otherwise, an effective control developed with only one media (fluid or panel) in the consideration may be completely counteracted by the dynamics occurring in another media through the nonlinear coupling. The present paper reports an attempt in developing a time-domain numerical methodology which is able to calculate the nonlinear fluid-structure interaction experienced by a flexible panel in a flow duct and its aeroacoustic-structural response correctly. The developed methodology is firstly verified able to capture the acoustic-structural interaction in the absence of flow where the numerical results agree with theory very well. A uniform mean flow is then allowed to pass through the duct so as to impose an aeroacoustic-structural interaction on the flexible panel. As a result, the nonlinear coupling between the flow aeroacoustics and panel structural dynamics are found completely different from the case without mean flow. A discussion of the new physical behaviors found is given.

Numerical Study of Nonlinear Fluid–Structure Interaction of an Excited Panel in Viscous Flow

Vibration of flexible panel induced by flow and acoustic processes in a duct can be used for silencer design, but it may conversely generate noise if structural instability is induced. Therefore, a complete understanding of fluid–structure interaction is important for effective noise reduction. A new time-domain numerical methodology has been developed for the calculation of the nonlinear fluid–structure interaction of an excited panel in internal viscous flow. This paper reports its validation with two experiments. The first aims to validate that the methodology is able to capture flow-induced structural instability and its acoustic radiation. The second one is to show that the methodology captures the aeroacoustic–structural interaction in a low-frequency silencer and its response correctly. The importance of inclusion of viscous effect in both cases is also discussed.

A Time-Domain Analysis for Aeroacoustics-Structure Interaction of Flexible Panel

A time domain numerical method is proposed to investigate flow effect to acoustics in a duct with flexible panel in the present paper. When a mean flow carrying a plane wave is passing over a flexible panel in a duct, complex aeroacoustics-structure interaction (ASI) will occur. The acoustic wave and flow will induce vibration of the panel. The vibration will then re-radiate wave back to the fluid. The flow, acoustics and flexible panel dynamics are affecting each other. A numerical model is developed to correctly solve such kind of ASI problem. The numerical model introduced in the paper involves three parts: aeroacoustic model, structural dynamic model and numerical coupling. The aeroacoustic model resolves flow dynamic and acoustics; and the structural dynamic model resolves the panel vibration. A numerical methodology is developed to make sure two models are strongly coupled together. The ASI model is verified with theoretical analysis. For a single frequency incident wave propagating in duct, we calculated transmission loss for different panel length. Actually, the flexible panel in duct can effectively reduce transmission of acoustic wave. The mechanism of transmission loss due to flexible panel will be discussed. However, the duct system behaves different when a mean flow is present. Cases with Mach number = 0.1, 0.5 and 0.8 are calculated. The transmission loss is found generally much smaller than cases without flow. The flow affects propagation of flexural wave on the panel and effect transmission loss change. This effect of flow will be discussed.

Low-dimensional modelling of sound generation by a flow past a bluff body

Searching for a unified methodology for controlling aeroacoustics of common structural discontinuities (e.g. bluff body, open cavity, etc.) has been a major topic among aeroacoustics research community. However, constrained by the complexity and nonlinearity of the flow governing equations, the process of reduced order modeling is usually required in which a low-dimensional, reduced-order model is created for approximating the full high-dimensional dynamics of the flow unsteadiness for control implementation purpose. The present study aims to extend two model reduction approaches, namely Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) via Galerkin projection method, to develop the reduced-order models for full compressible flows. The versatility of these methods are evaluated and compared by applying them to the aeroacoustics of flow past a square cylinder. It is expected that the outcome of this study could facilitate the development of a unified and versatile closed-loop contr...

Aeroacoustics of flow merging at duct junction

Although merging flow at duct junction is always encountered in fluid-transporting systems, previous works were mainly devoted to study the acoustics of duct junction but the aeroacoustics of flow merging has received little attention. Therefore, this paper aims at revealing the mechanism of sound generation by a merging flow at a 30-degree duct junction. The flow problem is investigated numerically by solving the unsteady compressible Navier-Stokes equations and the gas equation of state simultaneously, thus allowing the acoustic field and the aerodynamic field to be determined without modelling the source terms in the wave equation. The Conservation Element and Solution Element (CE/SE) method is chosen as the solver, which has been successfully applied in tackling many aeroacoustic problems. The results show that a shear layer is created between the two flows at duct junction due to the velocity gradient across the flow. Furthermore, another shear layer is also formed at downstream to the edge of duct j...

Evaluation of barrier performance by direct environmental noise simulation

After a city has experienced rapid urbanization, it is usually left with many pollution problems (e.g. air, noise, etc.) that hinder its further sustainable growth. Worsen by the rapid increase in the demands of land transportation for high population mobility; the traffic noise inevitably becomes a serious environmental problem. In countermeasure to the traffic noise problem and to compromise between limited space and costing, noise barriers are commonly implemented to protect sensitive land uses from noise pollution by mean of stopping, deflecting or reducing the noise propagation. Although recent studies have shown that the use of two or more screens in the barrier profile can enhance the diffraction efficiency of plane barriers in noise reduction without increasing its height. However, ground reflections are seldom included in the analysis, which could be a critical factor in the practical point of view. In this paper, a numerical technique derived from the high-fidelity time-domain computational aero...

Computational Duct Aeroacoustics using CE/SE Method

One-step duct aeroacoustic simulation has received attention from aerospace and mechanical high-pressure fluid-moving system manufacturers for quite some time. They aim to simulate the unsteady flow and acoustic fiel d in the duct simultaneously in order to investigate the aeroacoustic generation mechanisms. Because of the large length and energy scale disparities between the acoustic far field an d the aerodynamic near field, highly accurate and high-resolution simulation scheme is r equired. This involves the use of high order compact finite difference and time advancement schemes in simulation. However, in this situation, large buffer zones are always neede d to suppress the spurious numerical waves emanating from computational boundaries. This further increases the computational resources to yield accurate results. On the other hand, for such flow device as turbocharger, the sudden release of high pressure might results i n the occurrence of propagating pressure discontinuity (shock wave) with local supersonic Ma ch number. Usually numerical aeroacoustic scheme that is good for low Mach number flow is not able to give satisfactory simulation results. Therefore, the aeroacoustic re search community has been looking for a more efficient one-step duct aeroacoustic simulatio n scheme that has the comparable accuracy to the finite-difference approach with sma ller buffer regions, yet is able to give accurate solutions from subsonic to low supersonic flows. The conservation element and solution element (CE/SE) scheme is one of the possi ble schemes satisfying the above requirement. This paper aims to report the development of a CE/SE scheme for one-step duct aeroacoustic simulation and illustrate its rob ustness and effectiveness with two selected benchmark problems. One is the shock propagation at duct junctions and the trailing edge noise problem (Problem 2 in Category 4, the Fourth CAA Workshop of AIAA).