Nicolas de Cacqueray

Bio: Nicolas de Cacqueray is an academic researcher from École centrale de Lyon. The author has contributed to research in topics: Jet (fluid) & Mach number. The author has an hindex of 8, co-authored 12 publications receiving 534 citations.

Investigation of a High-Mach-Number Overexpanded Jet Using Large-Eddy Simulation

Abstract: a n = relative contribution of azimuthal modes n c = local sound speed c1 = ambient field sound speed f = frequency fc = cutoff frequency fshock = central frequency of the broadband shockassociated noise fup = frequency of the upstream-propagating shockassociated noise LVS = first shock length from vortex sheet, 2 M j 1 rj= 1, 1 2:40483 L1 = first shock length measured on the jet axis Ma = acoustic Mach number Mc = convection Mach number Me = exit Mach number Mj = equivalent fully expanded exit Mach number, uj=c1 n = azimuthal mode pe = exit static pressure pj = equivalent fully expanded exit static pressure p0 = fluctuating static pressure hpi = mean static pressure Re = Reynolds number based on exit conditions re = nozzle radius rj = equivalent fully expanded nozzle radius Stc = cutoff Strouhal number Ste = Strouhal number based on exit conditions Stup = Strouhal number of the upstream-propagating shock-associated noise Te = exit temperature uaxis = centerline mean axial velocity uc = convection velocity ue = exit velocity uj = equivalent fully expanded exit velocity huzi = mean axial velocity hu02 z i = mean-square axial velocity fluctuations hu02 z in = mean-square axial velocity fluctuations due to mode n juz j Ste; n = two-dimensional power spectral densities z1 = first shock location on the jet axis = specific heat ratio r = mesh size in the radial direction z = mesh size in the axial direction = boundary-layer thickness in the pipe nozzle

A computational study of shear stress in smooth rectangular channels

Abstract: The assumption of uniform flow permits the computation of the bed mean shear stress in open channels from the knowledge of the bed slope, hydraulic radius and fluid density alone. It is however important to distinguish between bank and bed shear values, in particular for sediment transport applications and research on this issue has been ongoing since the 1930s. This line of work was revived recently by Yang and Lim (1997) and Guo and Julien (2005) who have proposed division methods and formulations for the calculations of the shear stress values. This paper examines their work and the formulation of Guo and Julien in particular, using detailed Computational Fluid Dynamics simulation results. Numerical results for the various components of shear are presented and the type of division lines computed here compared with the existing propositions, in particular those of Leighly (1932), Keulegan (1938) and the recent findings ofYang and Lim, and Guo and Julien.

Noise of an Overexpanded Mach 3.3 Jet: Non-Linear Propagation Effects and Correlations with Flow

Abstract: The noise emitted by an overexpanded round jet at a Mach number of 3.3 and a Reynolds number of 105, computed in a previous study using large-eddy simulation (LES), is investigated. In a first step, the non-linear sound propagation effects are quantified by performing two far-field wave extrapolations from the LES near-field data. The extrapolations are carried out by solving the linearized Euler equations in one case and the full Euler equations in the other, without atmospheric absorption, up to a distance of 240 radii from the jet nozzle exit. The non-linear effects are shown to be quite significant, resulting in a series of N-shaped waves in the pressure signals, and in weaker mid-frequency components and stronger high-frequency components in the spectra. Close to the peak directivity radiation angle, for instance, they lead to about a 8 dB loss and a 6 dB gain at the Strouhal numbers of 0.2 and 1, respectively. In a second step, noise generation mechanisms are discussed by calculating correlations be...

Linear and nonlinear waves, by G. B. Whitham. Pp.636. £50. 1999. ISBN 0 471 35942 4 (Wiley).

Abstract: to be done in this area. Face recognition is a problem that scarcely clamoured for attention before the computer age but, having surfaced, has involved a wide range of techniques and has attracted the attention of some fine minds (David Mumford was a Fields Medallist in 1974). This singular application of mathematical modelling to a messy applied problem of obvious utility and importance but with no unique solution is a pretty one to share with students: perhaps, returning to the source of our opening quotation, we may invert Duncan's earlier observation, 'There is an art to find the mind's construction in the face!'.

Numerical Methods for High-Speed Flows

Abstract: We review numerical methods for direct numerical simulation (DNS) and large-eddy simulation (LES) of turbulent compressible flow in the presence of shock waves. Ideal numerical methods should be accurate and free from numerical dissipation in smooth parts of the flow, and at the same time they must robustly capture shock waves without significant Gibbs ringing, which may lead to nonlinear instability. Adapting to these conflicting goals leads to the design of strongly nonlinear numerical schemes that depend on the geometrical properties of the solution. For low-dissipation methods for smooth flows, numerical stability can be based on physical conservation principles for kinetic energy and/or entropy. Shock-capturing requires the addition of artificial dissipation, in more or less explicit form, as a surrogate for physical viscosity, to obtain nonoscillatory transitions. Methods suitable for both smooth and shocked flows are discussed, and the potential for hybridization is highlighted. Examples of the application of advanced algorithms to DNS/LES of turbulent, compressible flows are presented.

Influence of nozzle-exit boundary-layer conditions on the flow and acoustic fields of initially laminar jets

Abstract: Round jets originating from a pipe nozzle are computed by large-eddy simulations (LES) to investigate the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets. The jets are at Mach number 0.9 and Reynolds number 105, and exhibit exit boundary layers characterized by Blasius velocity profiles, maximum root-mean-square (r.m.s.) axial velocity fluctuations between 0.2 and 1.9% of the jet velocity, and momentum thicknesses varying from 0.003 to 0.023 times the jet radius. The far-field noise is determined from the LES data on a cylindrical surface by solving the acoustic equations. Jets with a thinner boundary layer develop earlier but at a slower rate, yielding longer potential cores and lower centreline turbulent intensities. Adding random pressure disturbances of low magnitude in the nozzle also increases the potential core length and reduces peak r.m.s. radial velocity fluctuations in the shear layer. In all the jets, the shear-layer transition is dominated by vortex rolling-ups and pairings, which generate strong additional acoustic components, but also amplify the downstream-dominant low-frequency noise component when the exit boundary layer is thick. The introduction of inlet noise however results in weaker pairings, thus spectacularly reducing their contributions to the sound field. This high sensitivity to the initial conditions is in good agreement with experimental observations.

Unstructured Large-Eddy Simulations of Supersonic Jets

Abstract: Experience gained from previous jet noise studies with the unstructured large-eddy simulation flow solver “Charles” is summarized and put to practice for the predictions of supersonic jets issued f...