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Sanjiva K. Lele - One of the best experts on this subject based on the ideXlab platform.
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linear and nonlinear processes in two dimensional mixing layer dynamics and sound radiation
Journal of Fluid Mechanics, 2009Co-Authors: Lawrence C Cheung, Sanjiva K. LeleAbstract:In this study, we consider the effects of linear and nonlinear Instability Waves on the near-field dynamics and aeroacoustics of two-dimensional laminar compressible mixing layers. Through a combination of direct computations, linear and nonlinear stability calculations, we demonstrate the significant role of nonlinear mechanisms in accurately describing the behaviour of Instability Waves. In turn, these processes have a major impact on sound generation mechanisms such as Mach Wave radiation and vortex pairing sound. Our simulations show that the mean flow correction, which is required in order to accurately describe the dynamics of large-scale vortical structures, is intrinsically tied to the nonlinear modal interactions and accurate prediction of saturation amplitudes of Instability Waves. In addition, nonlinear interactions are largely responsible for the excitation and development of higher harmonics in the flow which contribute to the acoustic radiation. Two flow regimes are considered: In supersonic shear layers, where the far-field sound is determined by the Instability Wave solution at sufficiently high Mach numbers, it is shown that these nonlinear effects directly impact the Mach Wave radiation. In subsonic shear layers, correctly capturing the near-field vortical structures and the interactions of the subharmonic and fundamental modes become critical due to the vortex pairing sound generation process. In this regime, a method is proposed to combine the Instability Wave solution with the Lilley–Goldstein acoustic analogy in order to predict far-field sound.
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The dynamics of nonlinear Instability Waves in laminar heated and unheated compressible mixing layers
Physics of Fluids, 2009Co-Authors: Lawrence C Cheung, Sanjiva K. LeleAbstract:Previous studies have shown that when using Instability Wave theories to model compressible laminar mixing layers, explicitly accounting for nonlinear processes such as the mean flow correction and modal interactions is necessary in order to properly capture vortex roll-up and pairing phenomena. In this study, we examine the effects of heating when using the nonlinear Instability Wave formulation on two-dimensional compressible mixing layers. We compare the results of both heated and unheated, supersonic and subsonic laminar mixing layer calculations using both the nonlinear parabolized stability equations (PSE) and Navier–Stokes equations. For all supersonic mixing layers, we find that the nonlinear stability method adequately captures the roll-up process and agrees well with direct calculations. While the same is true for the unheated subsonic mixing layer, in the heated subsonic mixing layer, we find that baroclinic vorticity generation, in addition to nonlinear interactions, plays a significant role i...
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Identification of Instability Waves in High-Speed Turbulent Jets
13th AIAA CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), 2007Co-Authors: Jaiyoung Ryu, Sanjiva K. Lele, K. ViswanathanAbstract:Instability Waves have been frequently invoked to explain the dominant noise from highspeed jets which radiates at 30∼50 from the jet axis; the Mach-Wave radiation from supersonically convected Instability Waves and the acoustic radiation from amplitude modulation of subsonically convected Instability Waves are two commonly quoted mechanisms of lowfrequency noise generation in jets. However, current methods for predicting jet noise do not, as yet, use the Instability Wave formalism. It is very difficult to determine in laboratory experiments the extent to which the turbulent flows of high speed jets contain Instability Waves, and the degree to which such a description is quantitatively useful in predicting the noise at shallow angles from the jet axis, where ‘large-turbulence structures/Instability Waves’ are customarily invoked as playing a major role in the sound generation mechanism. We decompose the results of the large-eddy simulation of high-speed jets (Bodony and Lele, 2005) by Fourier and adjoint methods to extract the Instability Wave contribution to the fluctuating jet flow. Jets with three different operating conditions are analyzed to obtain the contribution of jet Instability modes at different frequencies and azimuthal mode number as a function of downstream position. This LES database includes a new set point of a supersonic heated jet. The deduced Instability Wave amplitude and phase dynamics are compared with the predictions of linear jet Instability computed from the parabolized stability equations. The decomposed LES database shows “the physics of Instability Waves” to a limited extent. Least square and adjoint methods are used to provide the amplitude estimate for the linear PSE results. The comparison between the results from LES and PSE shows a limited agreement. The agreement is best for the lowest frequency considered (St = 0.1) and for the first azimuthal mode (n = 1). For higher St and other mode orders (n 6= 1) larger discrepancies are observed.
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Sound generated by Instability Wave/shock-cell interaction in supersonic jets
2006Co-Authors: Prasun K. Ray, Sanjiva K. LeleAbstract:Broadband shock-associated noise is an important component of the overall noise generated by modern airplanes. In this study, sound generated by the weakly nonlinear interaction between linear Instability Waves and the shock-cell structure in supersonic jets is investigated numerically in order to gain insight into the broadband shock-noise problem. The model formulation decomposes the overall flow into a mean flow, linear Instability Waves, the shock-cell structure and shock-noise. The mean flow is obtained by solving RANS equations with a k - ∈model. Locally parallel stability equations are solved for the shock structure, and linear parabolized stability equations are solved for the Instability Waves. Then, source terms representing the Instability Wave/shock-cell interaction are assembled and the inhomogeneous linearized Euler equations are solved for the shock-noise. Three cases are considered, a cold under-expanded Mj = 1.22 jet, a hot under-expanded M j = 1.22 jet, and a cold over-expanded M j = 1.36 jet. Shock-noise computations are used to identify and understand significant trends in peak sound amplitudes and radiation angles. The peak sound radiation angles are explained well with the Mach Wave model of Tam & Tanna (J. Sound Vib. Vol. 81, 1982, p. 337). The observed reduction of peak sound amplitudes with frequency correlates well with the corresponding reduction of Instability Wave growth with frequency. However, in order to account for variation of sound amplitude for different azimuthal modes, the radial structure of the Instability Waves must be considered in addition to streamwise growth. The effect of heating on the M j = 1.22 jet is shown to enhance the sound radiated due to the axisymmetric Instability Waves while the other modes are relatively unaffected. Solutions to a Lilley-Goldstein equation show that sound generated by 'thermodynamic' source terms is small relative to sound from 'momentum' sources though heating does increase the relative importance of the thermodynamic source. Furthermore, heating preferentially amplifies sound associated with the axisymmetric modes owing to constructive interference between sound from the momentum and thermodynamic sources. However, higher modes show destructive interference between these two sources and are relatively unaffected by heating.
Meelan M. Choudhari - One of the best experts on this subject based on the ideXlab platform.
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Instability Wave-streak interactions in a supersonic boundary layer.
Journal of fluid mechanics, 2017Co-Authors: Pedro Paredes, Meelan M. ChoudhariAbstract:Optimal initial conditions for transient growth in a two-dimensional boundary layer flow correspond to stationary, counter-rotating vortices that subsequently develop into streamwise elongated streaks, which are characterized by an alternating pattern of low and high streamwise velocity. For incompressible flows, previous studies have shown that boundary layer modulation due to streaks below a threshold amplitude level can stabilize the Tollmien-Schlichting Instability Waves, resulting in a delay in the onset of laminar-turbulent transition. In the supersonic regime, the linearly, most-amplified Waves become three-dimensional, corresponding to oblique, first-mode Waves. This change in the character of dominant instabilities leads to an important change in the transition process, which is now dominated by oblique breakdown via nonlinear interactions between pairs of first-mode Waves that propagate at equal but opposite angles with respect to the free stream. Because the oblique breakdown process is characterized by a rapid amplification of stationary streamwise streaks, artificial excitation of such streaks may be expected to promote transition in a supersonic boundary layer. Indeed, suppression of those streaks has been shown to delay the onset of transition in prior literature. Consistent with those findings, the present study shows that optimally growing stationary streaks indeed destabilize the first-mode Waves, but only when the spanwise Wavelength of the Instability Waves is equal to or smaller than twice the streak spacing. Transition in a benign disturbance environment typically involves first-mode Waves with significantly longer spanwise Wavelengths, and hence, these Waves are stabilized by the optimal growth streaks. Thus, as long as the amplification factors for the destabilized, short Wavelength Instability Waves remain below the threshold level for transition, a significant net stabilization is achieved, yielding a transition delay that is comparable to the length of the laminar region in the uncontrolled case.
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Instability-Wave propagation in boundary-layer flows at subsonic through hypersonic Mach numbers
Mathematics and Computers in Simulation, 2004Co-Authors: Li Jiang, Meelan M. Choudhari, Chau Lyan Chang, Chaoqun LiuAbstract:Direct numerical simulations (DNS) form an important ingredient to physics-based prediction of laminar-turbulent transition in boundary-layer flows, particularly in applications where it is desirable or even essential to model the various stages of transition process in an integrated manner. This paper addresses two building-block issues towards such capability: application to Instability-Wave propagation in boundary layers over curvilinear surfaces and robust outflow boundary conditions across the speed regime. In particular, detailed comparisons of linear and nonlinear development of Instability Waves in a range of boundary-layer flows are used to cross-validate a high-order direct numerical simulation algorithm against the approximate but computationally more efficient technique of parabolized stability equations (PSE). Three separate flow configurations are investigated in this study: (i) development of a Tollmien-Schlichting (TS) Instability Wave over a two-dimensional (2D), symmetric, low-speed airfoil, (ii) both first and second-mode development in a self-similar, flat plate boundary layer at Mach 4.5, and (iii) amplification of first and second modes of Rayleigh Instability and a stationary Gortler vortex in the hypersonic, axisymmetric boundary layer over a flared cone. The satisfactory agreement between the DNS and PSE predictions for both amplitudes and mode shapes of the Instability Waves confirms the overall efficacy of the DNS algorithm, while underscoring the accuracy of predictions based on the PSE approximation.
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ICCSA (2) - Direct numerical simulations of Instability-Wave generation and propagation in supersonic boundary layers
Computational Science and Its Applications — ICCSA 2003, 2003Co-Authors: Li Jiang, Meelan M. Choudhari, Chau Lyan Chang, Chaoqun LiuAbstract:Stringent requirements for component performance plus economic and environmental challenges for supersonic aircraft leave little room for inefficiencies in airframe design [1], making it necessary to have accurate and reliable prediction tools for boundary-layer transition to turbulence. To help develop and calibrate such tools, direct numerical simulations (DNS) of various stages during transition over both model and realistic flow configurations are necessary. This paper describes the application of a DNS solver based on high-order compact differences to supersonic swept-wing configurations. Specifically, spatial simulations of crossflow Instability in swept-wing boundary layers have been performed and validated against predictions based on the simpler but nearly equally accurate parabolized stability equations. The excitation of crossflow modes due to localized surface disturbances is also investigated. To help increase the efficiency of crossflow simulations for infinite-span swept wings, a simple modification to the DNS procedure is shown to significantly reduce the total grid requirement.
G. A. Faranosov - One of the best experts on this subject based on the ideXlab platform.
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Instability Wave CONTROL IN TURBULENT JET BY ACOUSTICAL AND PLASMA ACTUATORS
Progress in Flight Physics – Volume 7, 2015Co-Authors: G. A. Faranosov, Nikolay Ostrikov, V. F. Kopiev, Ivan Belyaev, M. Yu. Zaytsev, Yu. S. Akishev, M. E. Grushin, Nikolay Trushkin, Valentin BityurinAbstract:It was recently demonstrated by direct experiment in subsonic jets that an Instability Wave in jet shear layer generated by pure-tone acoustic excitation could be suppressed by another acoustic excitation, which generates an Instability Wave with the same properties. It was suggested that Instability Waves could be generated by any oscillating field near the nozzle exit. This paper presents the results of experimental investigations of Instability Wave suppression by other types of periodic excitation. Three types of plasma actuators are considered: (i ) high-frequency (HF) dielectric barrier discharge (DBD); (ii ) slipping discharge; and (iii ) corona discharge. Control authority of the plasma actuators over Instability Waves is demonstrated. For high-speed hot jets where Instability Waves are the dominant noise source, Instability Wave control is equivalent to noise control.
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Instability Wave control in turbulent jet by plasma actuators
Journal of Physics D: Applied Physics, 2014Co-Authors: Victor F. Kopiev, G. A. Faranosov, I. V. Belyaev, V. F. Kopiev, Yu. S. Akishev, M. E. Grushin, Valentin Bityurin, N. K. Berezhetskaya, A I Klimov, I. A. KossyiAbstract:Instability Waves in the shear layer of turbulent jets are known to be a significant source of jet noise, which makes their suppression important for the aviation industry. In this study we apply plasma actuators in order to control Instability Waves in the shear layer of a turbulent air jet at atmospheric pressure. Three types of plasma actuators are studied: high-frequency dielectric barrier discharge, slipping surface discharge, and surface barrier corona discharge. Particle image velocimetry measurements of the shear layer demonstrate that the plasma actuators have control authority over Instability Waves and effectively suppress the Instability Waves artificially generated in the shear layer. It makes these actuators promising for application in active control systems for jet noise mitigation.
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Acoustic control of Instability Waves in a turbulent jet
Acoustical Physics, 2013Co-Authors: V. F. Kopiev, I. V. Belyaev, M. Yu. Zaytsev, V. A. Kopiev, G. A. FaranosovAbstract:The possibility of acoustic control of Instability Waves formed in the mixing layer of a jet is experimentally investigated. The feasibility of suppressing a hydrodynamic Instability Wave in a subsonic turbulent jet by an external acoustic action is demonstrated. This result can be used in designing active control systems for jet noise suppression.
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A theoretical study of the efficiency of Instability Wave excitation in a two-dimensional nozzle edge model
Acoustical Physics, 2012Co-Authors: G. A. FaranosovAbstract:The paper considers a model problem of Instability Wave generation that occurs at a tangential discontinuity leaving the edge of a half-plane because of a specially adjusted external action, which models the basic characteristics of an active action on a jet by means of small-size actuators. The dependence of the Instability Wave amplitude on both the degree of spatial boundedness of the external action and its position with respect to the edge is investigated.
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Theoretical Investigation of the Effectiveness of Artificial Instability Waves Excitation in Subsonic Jets
18th AIAA CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference), 2012Co-Authors: G. A. FaranosovAbstract:In the present paper several model problems are considered in order to assess theoretically the effectiveness of Instability Wave generation in jets by a spatially localized action. Since the concept of Instability Waves as sound sources in high-speed jets seems promising from the viewpoint of the development of active jet noise control systems based on the excitation of the jet by different types of (usually small-scale) actuators, the problem of the effectiveness of artificial Instability Waves excitation in jets becomes important, and along with experimental investigations, theoretical efforts are also needed for qualitative and partly quantitative assessments to be made. In the present work we consider the well-known simplified models of jet flows (two-dimensional and axisymmetric ones with shear layer replaced by a vortex sheet) suited for theoretical approach. The base flow is subjected to weak (acoustic) excitation which is localized in space by an appropriate choice of the spectral function. The problem formulated is solved by the standard Wiener-Hopf technique with focusing on Instability Wave properties and their dependence on the parameters of excitation.
Bruce R Sutherland - One of the best experts on this subject based on the ideXlab platform.
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internal Wave Instability Wave Wave versus Wave induced mean flow interactions
Physics of Fluids, 2006Co-Authors: Bruce R SutherlandAbstract:In continuously stratified fluid, vertically propagating internal gravity Waves of moderately large amplitude can become unstable and possibly break due to a variety of mechanisms including (with some overlap) modulational Instability, parametric subharmonic Instability (PSI), self-acceleration, overturning, and convective Instability. In PSI, energy from primary Waves is transferred, for example, to Waves with half frequency. Self-acceleration refers to a mechanism whereby a Wave packet induces a mean flow (analogous to the Stokes drift of surface Waves) that itself advects the Waves until they become convectively unstable. The simulations presented here show that self-acceleration dominates over parametric subharmonic Instability if the Wave packet has a sufficiently small vertical extent and sufficiently fast frequency.
John O Roads - One of the best experts on this subject based on the ideXlab platform.
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feedback of tropical Instability Wave induced atmospheric variability onto the ocean
Journal of Climate, 2007Co-Authors: Hyodae Seo, Markus Jochum, Raghu Murtugudde, Arthur J Miller, John O RoadsAbstract:The effects of atmospheric feedbacks on tropical Instability Waves (TIWs) in the equatorial Atlantic Ocean are examined using a regional high-resolution coupled climate model. The analysis from a 6-yr hindcast from 1999 to 2004 reveals a negative correlation between TIW-induced wind perturbations and TIW-induced ocean currents, which implies damping of the TIWs. On the other hand, the feedback effect from the modification of Ekman pumping velocity by TIWs is small compared to the contribution to TIW growth by baroclinic Instability. Overall, the atmosphere reduces the growth of TIWs by adjusting its wind response to the evolving TIWs. The analysis also shows that including ocean current (mean TIWs) in the wind stress parameterization reduces the surface stress estimate by 15%–20% over the region of the South Equatorial Current. Moreover, TIW-induced perturbation ocean currents can significantly alter surface stress estimations from scatterometers, especially at TIW frequencies. Finally, the rectification effect from the atmospheric response to TIWs on latent heat flux is small compared to the mean latent heat flux.