scholarly journals Sound generation in a mixing layer

1997 ◽  
Vol 330 ◽  
pp. 375-409 ◽  
Author(s):  
TIM COLONIUS ◽  
SANJIVA K. LELE ◽  
PARVIZ MOIN
Keyword(s):  
Acoustic Field ◽  
Stokes Equations ◽  
Near Field ◽  
Mixing Layer ◽  
Navier Stokes ◽  
Far Field ◽  
Source Terms ◽  
Acoustic Analogy ◽  
Navier Stokes Equations ◽  
Acoustic Sources

The sound generated by vortex pairing in a two-dimensional compressible mixing layer is investigated. Direct numerical simulations (DNS) of the Navier–Stokes equations are used to compute both the near-field region and a portion of the acoustic field. The acoustic analogy due to Lilley (1974) is also solved with acoustic sources determined from the near-field data of the DNS. It is shown that several commonly made simplifications to the acoustic sources can lead to erroneous predictions for the acoustic field. Predictions based on the quadrupole form of the source terms derived by Goldstein (1976a, 1984) are in excellent agreement with the acoustic field from the DNS. However, despite the low Mach number of the flow, the acoustic far field generated by the vortex pairings cannot be described by considering compact quadrupole sources. The acoustic sources have the form of modulated wave packets and the acoustic far field is described by a superdirective model (Crighton & Huerre 1990). The presence of flow–acoustic interactions in the computed source terms causes the acoustic field predicted by the acoustic analogy to be very sensitive to small changes in the description of the source.

Direct computation of the sound generated by vortex pairing in an axisymmetric jet

1999 ◽  
Vol 383 ◽  
pp. 113-142 ◽  
Author(s):  
BRIAN E. MITCHELL ◽  
SANJIVA K. LELE ◽  
PARVIZ MOIN
Keyword(s):  
Mach Number ◽  
Stokes Equations ◽  
Near Field ◽  
Prediction Method ◽  
Axial Direction ◽  
Navier Stokes ◽  
Far Field ◽  
Source Terms ◽  
Vortex Pairing ◽  
Field Sound

The sound generated by vortex pairing in axisymmetric jets is determined by direct solution of the compressible Navier–Stokes equations on a computational grid that includes both the near field and a portion of the acoustic far field. At low Mach number, the far-field sound has distinct angles of extinction in the range of 60°–70° from the jet's downstream axis which can be understood by analogy to axisymmetric, compact quadrupoles. As the Mach number is increased, the far-field sound takes on a superdirective character with the dominant sound directed at shallow angles to the jet's downstream axis. The directly computed sound is compared to predictions obtained from Lighthill's equation and the Kirchhoff surface method. These predictions are in good agreement with the directly computed data. The Lighthill source terms have a large spatial distribution in the axial direction necessitating the introduction of a model to describe the source terms in the region downstream of the last vortex pairing. The axial non-compactness of the quadrupole sources must be adequately treated in the prediction method.

Analytical and Numerical Investigations on the Aeroacoustical Oscillation of Flow Past the Cavity

2015 ◽  
Author(s):  
Weidong Shao ◽  
Jun Li
Keyword(s):  
Stokes Equations ◽  
Near Field ◽  
Navier Stokes ◽  
Far Field ◽  
Navier Stokes Equations ◽  
Flow Past ◽  
Tonal Frequency ◽  
Number Flow ◽  
Unsteady Characteristics ◽  
Flow Flux

Noise radiated by aeroacoustical oscillation of low Mach number flow past a two-dimensional cavity has been investigated analytically and numerically using electro-acoustical analogy and a hybrid scheme combining CFD with an implementation of the porous Ffowcs Williams-Hawkings equation. The noise generation mechanism is illustrated and the interaction between flow and cavity as well as key factors of resonant frequency is discussed. The 2D compressible unsteady Reynolds averaged Navier-Stokes equations (URANS) are solved to obtain near field acoustic source and unsteady characteristics of cavity flow. A buffer domain is exerted along all external boundaries to suppress boundary wave reflection. Computed tonal frequency and amplitude of pressure oscillations demonstrate good agreement with previous computational simulations and experiments. The influences of the length and shape of the neck and porous inserts on the noise radiated to the far field are also investigated. The 3D far field numerical results show that at a certain incoming flow velocity and shear layer thickness the frequency of the dominant oscillation increases with the length of the neck and the magnitude in the downstream far field is 8dB greater than that in the upstream far field. The increasing chamfer decreases the resonance frequency and changes the effective streamwise opening length resulting in significant differences in acoustic pressure fluctuation. The porous inserts on the floor of the cavity reduce the mass flow flux through the cavity neck and accordingly suppress the amplitude of dominant oscillation. The preliminary simulations reveal promising methods for sound radiation control.

ANALYSIS OF A HETEROGENEOUS DOMAIN DECOMPOSITION FOR COMPRESSIBLE VISCOUS FLOW

2001 ◽  
Vol 11 (04) ◽  
pp. 565-599 ◽  
Author(s):  
CRISTIAN A. COCLICI ◽  
WOLFGANG L. WENDLAND
Keyword(s):  
Domain Decomposition ◽  
Stokes Equations ◽  
Domain Decomposition Method ◽  
Near Field ◽  
Outer Region ◽  
Navier Stokes ◽  
Far Field ◽  
Computational Domain ◽  
Linearized Euler Equations ◽  
Naca0012 Airfoil

We analyze a nonoverlapping domain decomposition method for the treatment of two-dimensional compressible viscous flows around airfoils. Since at some distance to the given profile the inertial forces are strongly dominant, there the viscosity effects are neglected and the flow is assumed to be inviscid. Accordingly, we consider a decomposition of the original flow field into a bounded computational domain (near field) and a complementary outer region (far field). The compressible Navier–Stokes equations are used close to the profile and are coupled with the linearized Euler equations in the far field by appropriate transmission conditions, according to the physical properties and the mathematical type of the corresponding partial differential equations. We present some results of flow around the NACA0012 airfoil and develop an a posteriori analysis of the approximate solution, showing that conservation of mass, momentum and energy are asymptotically attained with the linear model in the far field.

Modeling of the Vortex-Structure in a Particle-Laden Mixing-Layer

2005 ◽  
Author(s):  
Jens A. Melheim ◽  
Stefan Horender ◽  
Martin Sommerfeld
Keyword(s):  
Vortex Structure ◽  
Stokes Equations ◽  
Fluid Velocity ◽  
Mixing Layer ◽  
Equation Model ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Calculated Concentration ◽  
Langevin Model ◽  
Particle Velocities

Numerical calculations of a particle-laden turbulent horizontal mixing-layer based on the Eulerian-Lagrangian approach are presented. Emphasis is given to the determination of the stochastic fluctuating fluid velocity seen by the particles in anisotropic turbulence. The stochastic process for the fluctuating velocity is a “Particle Langevin equation Model”, based on the Simplified Langevin Model. The Reynolds averaged Navier-Stokes equations are closed by the standard k-epsilon turbulence model. The calculated concentration profile and the mean, the root-mean-square (rms) and the cross-correlation terms of the particle velocities are compared with particle image velocimetry (PIV) measurements. The numerical results agree reasonably well with the PIV data for all of the mentioned quantities. The importance of the modeled vortex structure “seen” by the particles is discussed.

An efficient method for predicting zero-lift or boundary-layer drag including aeroelastic effects for the design environment

2015 ◽  
Vol 119 (1221) ◽  
pp. 1451-1460
Author(s):  
J. A. Camberos ◽  
R. M. Kolonay ◽  
F. E. Eastep ◽  
R. F. Taylor
Keyword(s):  
Boundary Layer ◽  
Stokes Equations ◽  
Near Field ◽  
Field Method ◽  
Navier Stokes ◽  
Spanwise Direction ◽  
Design Environment ◽  
Navier Stokes Equations ◽  
Induced Drag ◽  
Flight Regimes

AbstractOne of the aerospace design engineer’s goals aims to reduce drag for increased aircraft performance, in terms of range, endurance, or speed in the various flight regimes. To accomplish this, the designer must have rapid and accurate techniques for computing drag. At subsonic Mach numbers drag is primarily a sum of lift-induced drag and zero-lift drag. While lift-induced drag is easily and efficiently determined by a far field method, using the Trefftz plane analysis, the same cannot be said of zero-lift drag. Zero-lift drag (CD,0) usually requires consideration of the Navier-Stokes equations, the solution of which is as yet unknown except by using approximate numerical techniques with computational fluid dynamics (CFD). The approximate calculation of zero-lift drag from CFD is normally computed with so-called near-field techniques, which can be inaccurate and too time consuming for consideration in the design environment. This paper presents a technique to calculate zero-lift and boundary-layer drag in the subsonic regime that includes aeroelastic effects and is suitable for the design environment. The technique loosely couples a two-dimensional aerofoil boundary-layer model with a 3D aeroelastic solver to compute zero-lift drag. We show results for a rectangular wing (baseline), a swept wing, and a tapered wing. Then compare with a rectangular wing with variable thickness and camber, thinning out from the root to tip (spanwise direction), thus demonstrating the practicality of the technique and its utility for rapid conceptual design.

Directivity and intermittency in the nearfield of a Mach 1.3 jet

2017 ◽  
Vol 16 (3) ◽  
pp. 135-164 ◽  
Author(s):  
S Unnikrishnan ◽  
Datta V Gaitonde ◽  
Lionel Agostini
Keyword(s):  
Acoustic Field ◽  
Stokes Equations ◽  
Base Flow ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Local Fluctuations ◽  
Mode Decomposition ◽  
Gradual Evolution ◽  
Large Eddy ◽  
High Frequency Content

Local fluctuations in a Mach 1.3 cold jet are tracked to understand the genesis of nearfield directivity and intermittency. A newly developed approach leveraging two synchronized large-eddy simulations is employed to solve the forced Navier–Stokes equations, linearized about the evolving unsteady base flow. The results are summarized by exposing the effect of two acoustically significant turbulent regions: the lip-line and core collapse location. The near-acoustic field displays the clear signature of the two regions. However, for both regions, the nearfield evolution of the perturbation field is characterized by generation of intermittent wavepackets, which propagate into the near-acoustic field and gradually acquire their expected broadband and narrowband characteristics at sideline and downstream angles respectively. The simulations elucidate how higher frequencies are obtained in the sideline directions as lower frequencies are filtered out of the forcing fluctuations. Likewise, shallow-angle acoustic signals arise through filtering of high frequency content in that direction. The directivity and intermittency are connected to the filtering of scales by jet turbulence with empirical mode decomposition. The observations highlight the gradual evolution of seemingly random core turbulence into well-defined intermittent wavepackets in the nearfield of the jet. The manner in which centerline fluctuations are segregated into upstream, sideline, and downstream components is examined through narrowband correlations. A similar analysis for the lipline contribution shows primarily upstream and downstream patterns because of the larger structures in the shear layer.

Dynamics of Pulsed Jet in Crossflow

2007 ◽  
Author(s):  
M. Arienti ◽  
M. C. Soteriou
Keyword(s):  
Stokes Equations ◽  
Near Field ◽  
Numerical Approach ◽  
Mie Scattering ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Pulsed Jet ◽  
Jet In Crossflow ◽  
Lagrangian Equations ◽  
Phase Doppler Interferometry

We examine the effect of time-dependent forcing on jet-in-crossflow atomization in the case of pulsed liquid injection and uniform crossflow. The dynamics of the jet is captured by a numerical approach that blends interface tracking of the liquid surface with an empirical description of the atomization process. The unsteady Reynolds-Averaged Navier-Stokes equations for the gas and the continuous (i.e., preceding breakup) liquid phase are solved simultaneously with the Lagrangian equations for the droplet trajectories. This approach captures the near field transient due to the opening (closing) of the fuel valve, as well as the convective delay of the spray in the far field. Validation is carried out with Phase Doppler Interferometry (PDI) and Mie scattering measurements at standard conditions for pulsed jets of water and ethanol in crossflow air. The discussion is focused on the shape of the convecting spray pulse and on the trends due to variations in crossflow and jet velocities.

Computational Analysis of a Inverted Double-Element Airfoil in Ground Effect

2006 ◽  
Vol 128 (6) ◽  
pp. 1172-1180 ◽  
Author(s):  
Stephen Mahon ◽  
Xin Zhang
Keyword(s):  
Flow Field ◽  
Stokes Equations ◽  
Near Field ◽  
Turbulence Models ◽  
Ground Effect ◽  
Wake Flow ◽  
Navier Stokes ◽  
Main Element ◽  
Ride Height ◽  
Navier Stokes Equations

The flow around an inverted double-element airfoil in ground effect was studied numerically, by solving the Reynolds averaged Navier-Stokes equations. The predictive capabilities of six turbulence models with regards to the surface pressures, wake flow field, and sectional forces were quantified. The realizable k−ε model was found to offer improved predictions of the surface pressures and wake flow field. A number of ride heights were investigated, covering various force regions. The surface pressures, sectional forces, and wake flow field were all modeled accurately and offered improvements over previous numerical investigations. The sectional forces indicated that the main element generated the majority of the downforce, whereas the flap generated the majority of the drag. The near field and far field wake development was investigated and suggestions concerning reduction of the wake thickness were offered. The main element wake was found to greatly contribute to the overall wake thickness with the contribution increasing as the ride height decreased.

scholarly journals To the issue of evaluating sonic boom overpressure and loudness

2019 ◽  
Vol 304 ◽  
pp. 02003
Author(s):  
Igor G. Bashkirov ◽  
Sergey L. Chernyshev ◽  
Vladlen S. Gorbovskoy ◽  
Andrey V. Kazhan ◽  
Vyacheslav G. Kazhan ◽  
...  
Keyword(s):  
Software Package ◽  
Stokes Equations ◽  
Aerodynamic Drag ◽  
Near Field ◽  
Theoretical Data ◽  
Aerodynamic Characteristics ◽  
Sonic Boom ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Computational Mesh

At present, in the world there is a growing interest in the development of a new generation of supersonic passenger aircraft. One of the main problems of creating such aircraft is to ensure both an acceptable sonic boom level and high aerodynamic characteristics in the supersonic cruising mode. This requires the development of reliable methods for obtaining the near field under the plane with taking into account the influence of the boundary layer, calculation of overpressure signature on the ground and evaluation of sonic boom loudness. In this work four variants of the equivalent body of revolution of minimum sonic boom with different nose sharpening were investigated for an aircraft weighing 19 tons in supersonic cruising flight at Mach number of 1.7 and altitude of 15.5 km using the software package for solving the Reynolds–averaged Navier–Stokes equations (RANS) ANSYS CFX. A macro for calculating the overpressure signature on the ground for the distribution of disturbances in the near field under the aircraft and a program for evaluating the sonic boom loudness in various metrics were developed. Computational mesh verification of the results was carried out, the obtained overpressure signatures were compared with theoretical data and calculation results from the software package for the integration of complete system of Euler equations by finite–difference method X–CODE. The effect of the sharpening of the nose part on aerodynamic drag and sound boom characteristics was shown. The work was done in the interests of the international project RUMBLE (RegUlation and norM for low sonic Boom LEvels).

scholarly journals Comparison of Higher-Order Numerical Schemes and Several Filtering Methods Applied to Navier-Stokes Equations with Applications to Computational Aeroacoustics

2009 ◽  
Vol 3 (3) ◽  
pp. 443-459
Author(s):  
L.S. Lai ◽  
G.S. Djambazov ◽  
C.-H. Lai ◽  
K.A. Pericleous
Keyword(s):  
Stokes Equations ◽  
Small Time ◽  
High Order ◽  
Navier Stokes ◽  
Acoustic Analogy ◽  
Time Step ◽  
Navier Stokes Equations ◽  
Flow Equations ◽  
High Order Schemes ◽  
Fine Mesh

In computational acoustics, fluid-acoustic coupling methods for the computation of sound have been widely used by researchers for the last five decades. In the first part of the coupling procedure, the fully unsteady incompressible or compressible flow equations for the near-field of the unsteady flow are solved by using a Computational Fluid Dynamics (CFD) technique, such as Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) or unsteady Reynolds averaged Navier-Stokes equations (RANS) the CFD predictions are then used to calculate sound sources using the acoustic analogy or solving a set of acoustic perturbation equations (APE) leading to the solution of the acoustic field. It is possible to use a 2-D reduced problem to provide a preliminary understanding of many acoustic problems. Unfortunately 2-D CFD simulations using a fine-mesh-small-time-step-LES-alike numerical method cannot be considered as LES, which applies to 3-D simulations only. Therefore it is necessary to understand the similarities and the effect between filters applied to unsteady compressible Navier-Stokes equations and the combined effect of high-order schemes and mesh size. The aim of this study is to provide suitable LES-alike methods for 2-D simulations. An efficient software implementation of high-order schemes is also proposed. Numerical examples are provided to illustrate these statistical similarities.

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