Papers by Christophe Bogey
Proceeding of Seventh International Symposium on Turbulence and Shear Flow Phenomena
Five round jets at Mach number 0.9 and diameter-based Reynolds number 10 5 originating from a pip... more Five round jets at Mach number 0.9 and diameter-based Reynolds number 10 5 originating from a pipe nozzle are computed by Large-Eddy Simulations using grids of 252 million points. In the pipe, the boundary layers are tripped, in order to obtain, at the exit section, laminar mean velocity profiles of momentum thickness equal to 1.8% of the jet radius, and peak turbulence intensities of 0, 3, 6, 9 and 12% of the jet velocity. The influence of initial turbulence on flow development is thus investigated. As the nozzle-exit turbulence level increases, the coherent structures typically found in initially laminar jets gradually disappear, which leads to shear layers spreading at lower rate with strongly reduced rms fluctuating velocities. The jets also develop farther downstream, resulting in longer potential cores.
7th AIAA/CEAS Aeroacoustics Conference and Exhibit, 2001
Lighthill's equation is used to compute the noise produced by subsonic flows. The objective is to... more Lighthill's equation is used to compute the noise produced by subsonic flows. The objective is to show that, although this equation is based on a wave equation in a medium at rest, meanflow effects on sound propagation, included in Lighthill's source term, can be properly taken into account by numerical approches. The source terms are evaluated from the unsteady compressible flow motion equations, which provide also a reference sound field. By this way, all mean flow -sound waves interactions are included into the source terms. Two-dimensional cases are first considered with the radiation of a monopole in a sheared mean flow and with the sound generated by a mixing layer. They show that mean flow effects on propagation are correctly predicted with Lighthill's equation. The sound produced by pairings of axisymmetric vortices in a three-dimensional circular jet with a Mach number of 0.9 and a Reynolds number of 65000 is then investigated. Solving Lighthill's equation provides an acoustic radiation quite consistent with the sound field obtained directly from LES.
21st AIAA/CEAS Aeroacoustics Conference, 2015
Six planar supersonic jets are computed by compressible large-eddy simulations (LES) using low-di... more Six planar supersonic jets are computed by compressible large-eddy simulations (LES) using low-dissipation schemes. At the exit of a nozzle of height h, they are ideally expanded and have an exit velocity ue, yielding a Mach number of Me = 1.28, and a Reynolds number of Reh = ueh/ν = 5 × 10 4 . Four jets impinging on a flat plate at distances L from the nozzle lips ranging from 3.94h up to 9.1h, with an angle θ between the jet direction and the plate of 90 degrees, are first considered. Two other jets with L = 5.5h and θ = 60 and θ = 75 degrees are also examined. In this way, the effects of both the nozzle-to-plate distance and the angle of impact on the flow and acoustic fields of the jet are studied. Mean velocity flows and snapshots of density, pressure and vorticity are shown. The pressure fields are also described by computing sound pressure levels and using Fourier decomposition. Several tones are obtained in certain cases and their corresponding Strouhal numbers and symmetric or antisymmetric natures are in agreement with experimental data and theoretical models. They are due to an aeroacoustic feedback mechanism occurring between the nozzle lips and the flat plate. This mechanism generates an hydrodynamicacoustic standing wave revealed by using Fourier decomposition.
Direct and Large-Eddy Simulation VI
A round free jet at Reynolds number ReD = 11000 is computed by Large Eddy Simulation on a grid of... more A round free jet at Reynolds number ReD = 11000 is computed by Large Eddy Simulation on a grid of 44 million nodes containing a part of the selfsimilarity region of the flow. Turbulence properties in this region, including secondand third-order moments of velocity, pressure-velocity correlations and kinetic energy budget, are calculated. They are compared to, and complement, the experimental results of Panchapakesan & Lumley [1] for a jet at the same Reynolds number.
17th AIAA/CEAS Aeroacoustics Conference (32nd AIAA Aeroacoustics Conference), 2011
In this paper, the data obtained by LES for an initially laminar and overexpanded jet at Mach num... more In this paper, the data obtained by LES for an initially laminar and overexpanded jet at Mach number 3.3 and Reynolds number 10 5 are reexamined in order to investigate the nonlinear effects on the propagation of the acoustic waves, and the normalized flow/acoustics cross-correlations. To study the non-linear propagation effects at the direction φ = 60 • and up to 240 radii from the nozzle exit, the LES near field is propagated in far-field by solving either the isentropic linearized Euler equations or the full Euler equations. The comparisons of the acoustic data obtained from the two methods clearly show that the non-linear effects are strong up to about 240 radii from the nozzle exit. Using the far-field acoustic results from the non-linear propagation, the normalized cross-correlations between the turbulent flow quantities and the acoustic pressure signals at the direction φ = 60 • are then evaluated to give some information on sound generation. A sound source which may be similar to that one observed in subsonic jets is first found on the jet axis in the vicinity of the end of the potential core. Other sound sources attributed to the supersonic convection of turbulent vortices are noticed. Finally, the normalized cross-correlations between the fluctuating density along the jet axis and the acoustic pressure display correlation bands between the 3rd and the 5th shock cells. These bands might be linked with the screech generation mechanism.
Notes on Numerical Fluid Mechanics and Multidisciplinary Design
Jets with complex shock-cell structures appear in numerous technological applications. Most super... more Jets with complex shock-cell structures appear in numerous technological applications. Most supersonic jets used in aeronautics will be imperfectly expanded in flight, even those from carefully designed convergent-divergent nozzles. The adaption to the ambient pressure takes place in a sequence of oblique shocks which interact with the free shear layers and produce noise. The shock/shearlayer interaction emanates a broadband noise component. This may trigger the young shear layer at the nozzle, forming a feedback loop which results in a discrete noise component called screech. Both components are undesirable from structural and environmental (cabin noise) points of view. Screech tones produce sound pressure levels of 160 dB and beyond. The focus of this research project lies in the minimization of supersonic jetnoise and in particular in the minimization of jet-screech. Since screech -a phenomenon which is not yet understood in all details -seems to be affected by the presence of the jet-nozzle, we will perform a shape optimization of the nozzle geometry to minimize the emanated noise. The optimization technique is based on the adjoints of the compressible Navier-Stokes equations. These equations have to be solved backwards in time to get the gradient information of the objective function. Results of a direct numerical simulation of a full supersonic, over-expanded jet, its acoustic field and its sound generation mechanisms was performed on a computational grid containing more than 300 million points and with a Reynolds number of 30 000 and will be presented at the conference.
50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012
The effects of the Reynolds number on initially highly disturbed isothermal round jets at Mach nu... more The effects of the Reynolds number on initially highly disturbed isothermal round jets at Mach number M = 0.9 and at diameter-based Reynolds numbers ReD = 2.5 × 10 4 , 5 × 10 4 , 10 5 and 2 × 10 5 are investigated using Large-Eddy Simulation. The jets originate from a pipe nozzle of radius r0, in which a tripping procedure is applied to the boundary layers. At the nozzle exit, laminar-like mean velocity profiles of momentum thickness δθ ≃ 0.018r0, yielding Reynolds numbers Reθ varying from 251 to 1830 depending on ReD, and peak turbulence intensities around 9% of the jet velocity, are thus obtained. With increasing Reynolds number, the turbulence spectra close to the nozzle exit and in the mixing layers broaden, as expected, while remaining dominated by the large-scale components naturally observed in turbulent boundary layers and shear layers, respectively. The mixing layers however develop more slowly, with reduced levels of velocity fluctuations. The axial profiles of turbulence intensities become smoother, showing in particular a clear overshoot two radii downstream of the nozzle exit at ReD = 2.5 × 10 4 , but a monotonical growth at ReD = 2 × 10 5 . The jet potential core moreover lengthens slightly with ReD, but the flow properties do not change significantly farther downstream. The jets at higher Reynolds numbers are finally found to generate lower sound levels, with a decrease of about 2 dB over the range of ReD considered.
18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference), 2012
Large-Eddy Simulations of isothermal round jets at a Mach number of 0.9 are performed in order to... more Large-Eddy Simulations of isothermal round jets at a Mach number of 0.9 are performed in order to investigate the influence of the nozzle-exit boundary-layer thickness on initially highly disturbed subsonic jets at moderate Reynolds numbers. The jets are originating from a pipe nozzle of radius r0, and exhibit, at the exit section, peak disturbance levels of 9 per cent of the jet velocity, and mean velocity profiles similar to laminar boundary-layer profiles of thickness δ0 = 0.09r0, 0.15r0, 0.25r0 or 0.42r0, yielding momentum thicknesses δθ(0) between 0.012r0 and 0.05r0. Four jets at a diameter Reynolds number ReD = 5 × 10 4 , providing momentum-thickness Reynolds numbers Reθ = 304, 486, 782 and 1288 depending on δ0, are first considered. Four jets at Reynolds numbers ReD = 8.3 × 10 4 , 5 × 10 4 , 3 × 10 4 and 1.8 × 10 4 , with δ0 = 0.09r0, 0.15r0, 0.25r0 and 0.42r0, respectively, giving Reθ ≃ 480 in all cases, are then examined. The effects of δ0/r0 and Reθ on the jet flow and sound fields can thus be distinguished. At a constant ReD, thickening the initial shear layers mainly results in lower turbulence intensities in the mixing layers and weaker sound levels at all emission angles due to the variations of Reθ. Different trends are therefore obtained at a nearly identical Reθ. Increasing the ratio δ0/r0 in this case leads to a shorter potential core, higher centerline velocity fluctuations, and stronger noise in the downstream direction.
16th AIAA/CEAS Aeroacoustics Conference, 2010
Compressible Large-Eddy Simulations of five isothermal round jets at a Mach number of 0.9 and a d... more Compressible Large-Eddy Simulations of five isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number of 10 5 originating from a pipe are reported, to examine the feasibility of computing the flow and acoustic fields of initially nominally turbulent jets. In the pipe the boundary layers are tripped in order to obtain at the exit section laminar mean velocity profiles of momentum thickness δθ = 0.018 times the jet radius, yielding a Reynolds number Reθ = 900, as well as peak turbulent intensities around 9% of the jet velocity. Two methods of boundary-layer tripping, and four grids containing from 50 to 252 million points, are considered. The results are found to vary negligibly with the tripping procedure, but appreciably with the grid resolution. A high azimuthal resolution appears in particular necessary to simulate the initial shear-layer development properly. For the present tripped jet, nθ = 256 points in the azimuth are for instance insufficient, which has led us to specify nθ = 1024 points to reach a high level of confidence in the numerical solutions. A fine discretization of the mixing layers also seems to be required from the nozzle exit to the end of the potential core to get jet features similar to those measured at high Reynolds numbers.
11th AIAA/CEAS Aeroacoustics Conference, 2005
Noise sources are investigated in subsonic jets at Mach numbers M = 0.6 and M = 0.9, with Reynold... more Noise sources are investigated in subsonic jets at Mach numbers M = 0.6 and M = 0.9, with Reynolds numbers ReD = 1700 and ReD ≥ 10 5 , using data provided by Large Eddy Simulations (LES). The cross-correlations between signals of the radiated sound pressure and turbulence signals along jet axis and shear layer are in particular calculated. The normalized correlations are found to be significant, around 0.10, between the pressure radiated in the downstream direction and the centerline flow quantities. In the cases involving the sideline pressure or flow quantities along the shear layer, the correlations are much smaller. The maximum correlations are observed on the jet axis just at the end of the potential core, and fall at large emission angles. Furthermore the correlations appear to be lower as the Mach number is reduced, and to be enhanced as the Reynolds number is decreased. These correlations can be expected to be mostly due to the noise source radiating downstream, which may thus be located on the jet centerline at the end of the potential core. This flow region is moreover characterized by a dominant frequency over a large axial distance and by a high level of intermittency.
International Journal of Aeroacoustics, 2010
The two components of combustion noise, namely the direct noise induced by heat release fluctuati... more The two components of combustion noise, namely the direct noise induced by heat release fluctuations in the flow and the indirect noise caused by acceleration of entropy perturbations through mean flow gradients resulting in acoustic radiation, are examined by considering different types of inhomogeneous wave equations deduced from the balance equations of fluid dynamics. This is accomplished by adapting acoustic analogies initially derived by Lighthill (1952), Phillips (1960) and Lilley (1972) for non-reactive flows to the reacting case. This reformulation is based on a standard set of assumptions borrowed from combustion. It is shown that a consistent formulation of a wave operator and its associated source terms is required.
International Journal of Aeroacoustics, 2007
The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 ... more The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 mm with Mach numbers varying over the range 0.6 ≤ M j ≤ 1.6 are investigated experimentally, and characterized in terms of sound spectra and levels. Properties of near-field jet noise, obtained in particular at 7.5 diameters from the jet centerline, are documented. They differ appreciably from properties of far-field noise, and form a database that can be used for the validation of the acoustic fields determined by compressible Navier-Stokes computations. The near pressure fields originating from simulations can thus be directly compared, without resorting to extrapolation methods which might lead to uncertainties in the far pressure fields. In the present paper, sound source localizations are also carried out from the near-field pressure signals. The experiments provide in addition far-field results evaluated at 52 diameters from the nozzle exit, in good agreement with the data of the l...
International Journal of Aeroacoustics, 2008
The noise generated by isothermal round jets at Mach number M = 0.9 and at the diameter-based Rey... more The noise generated by isothermal round jets at Mach number M = 0.9 and at the diameter-based Reynolds number ReD = 5 × 105, originating from a straight pipe nozzle, is computed directly using Large Eddy Simulation (LES), in order to highlight the potential influence of the inlet boundary conditions on the acoustic predictions. Two jets are simulated, displaying levels of fluctuating axial velocity at the nozzle exit respectively of 1.6% and 9% of the jet velocity, while the momentum thickness of the shear layers is nearly the same. The shear-layer development and the radiated sound fields obtained for the two jets are found to differ significantly. The shear layer of the jet with low initial turbulence levels develops for instance with higher turbulence intensities and a velocity flow field that is more correlated. In addition coherent annular vortices and pairings are clearly observed in this jet. Regarding the radiated noise, the jet with high initial turbulence levels provides s...
12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics Conference), 2006
Subsonic isothermal round jets at Mach number Mj = 0.9 and at the diameter-based Reynolds number ... more Subsonic isothermal round jets at Mach number Mj = 0.9 and at the diameter-based Reynolds number ReD = 5 × 10 5 are computed using compressible Large Eddy Simulation (LES), in order to investigate the effects of nozzle-exit turbulence levels on jet noise. A pipe nozzle is included in the computational domain, and the development of the boundary layer inside the nozzle is calculated. In this way, two jets displaying respectively low and high turbulence levels at the nozzle exit are considered. In the two cases, the levels of fluctuating axial velocity at the nozzle exit are indeed of 0.016 and 0.090 with respect of the jet velocity, while the momentum thickness of the boundary layers is nearly the same. The shear-layer developments and the radiated sound fields obtained for the two jets are found to differ significantly. The shear layer of the jet with low nozzle-exit turbulence levels develops with higher turbulence intensities and a velocity flow field that is more correlated. Coherent annular vortices are also clearly observed only in this jet. Regarding the radiated noise, the jet with high turbulence levels at the nozzle exit provides sound levels and spectra in very good agreement with experimental data obtained for jets at high Reynolds numbers ReD ≥ 5 × 10 5 , which are expected to be initially turbulent. The computed jet with low exit turbulence levels is shown to generate more noise, which results from vortex pairings in the shear layer.
est calcule par une Simulation des Grandes Echelles (SGE) compressible. Le champ aerodynamiqueet ... more est calcule par une Simulation des Grandes Echelles (SGE) compressible. Le champ aerodynamiqueet le rayonnementsonore obtenu directement ` a partir de la SGE sont en bon accord avec les donnees exp´ erimentales. En particulier, les spectres et les fonctions de correlation azimutale des fluctuations de pression acoustique correspondent bien au nombre de Reynoldseleve du jet. Abstract : An isothermal circular jet with a Mach number of M and a Reynolds number of Re is computed by a compressible Large Eddy Simulation (LES). The flow and the sound field obtained directly from the LES are in good agreement with the corresponding measurements of the literature. The acoustic radiation displays also spectra and azimuthal correlation functions which behave as expected for a high Reynolds number jet. Mots-clefs : jet ; a´ eroacoustique ; simulation des grandesechelles (SGE)
… Congrès Français de …, 2007
On présente un calcul démonstratif de la propagation de signaux acoustiques basses fréquences sur... more On présente un calcul démonstratif de la propagation de signaux acoustiques basses fréquences sur de grandes distances dans l'atmosphère. Les simulations numériques sont effectuées en résolvant directement les équations d'Euler linéarisées en 2-D. Le solveur est construit à partir de schémas explicites aux différences finies peu dispersifs et peu dissipatifs. Les caractéristiques de l'atmosphère utilisées pour la solution numérique ont été mesurées durant l'expérience « Misty Picture ». Les résultats de calcul sont presentés et interprétés à l'aide d'un tracé de rayons. Ce calcul préliminaire montre qu'il est possible de calculer fidèlement la propagation de signaux basses fréquences sur de grandes distances dans l'atmosphère en résolvant les équations d'Euler linéarisées avec un coût de calcul raisonnable.
Procedia Engineering, 2010
In this paper, the importance of initial conditions on subsonic jet noise is emphasized by showin... more In this paper, the importance of initial conditions on subsonic jet noise is emphasized by showing numerical results obtained by large-eddy simulations for initially laminar round jets at Mach number 0.9 and Reynolds number 10 5 . The near and the far sound pressure elds of the jets are found to signi cantly vary with the ow parameters, namely the boundary-layer thickness and the turbulence levels, at the nozzle exit. With respect to initially turbulent jets, strong additional noise components generated by pairings of coherent vortical structures in the transitional shear layers are in addition observed, in agreement with experiments.
Physics of Fluids, 2006
Transitional round jets at Mach number M=0.9, with identical initial conditions except for the di... more Transitional round jets at Mach number M=0.9, with identical initial conditions except for the diameter, yielding Reynolds numbers over the range 1.7×103⩽ReD⩽4×105, are computed by large eddy simulation (LES) using explicit selective/high-order filtering. The effects of the Reynolds number on the jet flows are first presented. As the Reynolds number decreases, the jets develop more slowly upstream from the end of the potential core, but more rapidly downstream. At lower Reynolds numbers, the decay of the centerline velocity and the jet spreading are indeed faster, and the turbulence intensities are higher after the potential core, in agreement with data of the literature. The integral length scales are also significantly larger. The results suggest moreover that the jet self-similar region is reached at shorter axial distances at lower Reynolds numbers. The influence of the Reynolds number on the energy-dissipation mechanisms involved in the LES, namely molecular viscosity and expli...
Physics of Fluids, 2009
A coaxial jet originating from parallel coplanar pipe nozzles is computed by a compressible large... more A coaxial jet originating from parallel coplanar pipe nozzles is computed by a compressible large eddy simulation (LES) using low-dissipation and low-dispersion schemes in order to determine its acoustic field and to study noise generation mechanisms. The jet streams are at high velocities, the primary stream is heated, and the Reynolds number based on the primary velocity and the secondary diameter is around 106. High levels of turbulence intensity are also specified at the nozzle exit. The jet aerodynamic field and the near-pressure field are both obtained directly from the LES. The far-field noise is calculated by solving the linear acoustic equations, from the unsteady LES data on a cylindrical surface surrounding the jet. A good agreement is observed in terms of directivity, levels, and narrow-band spectra with noise measurements carried out during the EU project CoJeN for a coaxial jet displaying same stream velocities and temperatures, coplanar nozzle outlets with identical a...
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Papers by Christophe Bogey