Edouard Boujo


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Boujo

First Name

Edouard

ORCID

0000-0002-4448-6140

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2015 - 20206
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Publications1 - 6 of 6
  • Saturation of a turbulent mixing layer over a cavity: response to harmonic forcing around mean flows
    Item type: Journal Article
    Boujo, Edouard; Bauerheim, Michael; Noiray, Nicolas (2018)
    Journal of Fluid Mechanics
    Turbulent mixing layers over cavities can couple with acoustic waves and lead to undesired oscillations. To understand the nonlinear aspects of this phenomenon, a turbulent mixing layer over a deep cavity is considered and its response to harmonic forcing is analysed with large-eddy simulations (LES) and linearised Navier–Stokes equations (LNSE). The Reynolds number is Re=150000. As a model of incoming acoustic perturbations, spatially uniform time-harmonic velocity forcing is applied at the cavity end, with amplitudes spanning the wide range 0.045–8.9 % of the main channel bulk velocity. Compressible LES provide reference nonlinear responses of the shear layer, and the associated mean flows. Linear responses are calculated with the incompressible LNSE around the LES mean flows; they predict well the amplification (both measured with kinetic energy and with a proxy for vortex sound production in the mixing layer) and capture the nonlinear saturation observed as the forcing amplitude increases and the mixing layer thickens. Perhaps surprisingly, LNSE calculations based on a monochromatic (single-frequency) assumption yield a good agreement even though higher harmonics and their nonlinear interaction (Reynolds stresses) are not negligible. However, it is found that the leading Reynolds stresses do not force the mixing layer efficiently, as shown by a comparison with the optimal volume forcing obtained from a resolvent analysis. Therefore they cannot fully benefit from the potential for amplification available in the flow. Finally, the sensitivity of the optimal harmonic forcing at the cavity end is computed with an adjoint method. The sensitivities to mean flow modification and to a localised feedback (structural sensitivity) both identify the upstream cavity corner as the region where a small-amplitude modification has the strongest effect. This can guide in a systematic way the design of strategies aiming at controlling the amplification and saturation mechanisms.
  • Second-order sensitivity of parallel shear flows and optimal spanwise-periodic flow modifications
    Item type: Journal Article
    Boujo, Edouard; Fani, Andrea; Gallaire, François (2015)
    Journal of Fluid Mechanics
  • Processing time-series of randomly forced self-oscillators: The example of beer bottle whistling
    Item type: Journal Article
    Boujo, Edouard; Bourquard, Claire; Xiong, Y.; et al. (2020)
    Journal of Sound and Vibration
  • A self-consistent formulation for the sensitivity analysis of finite-amplitude vortex shedding in the cylinder wake
    Item type: Journal Article
    Meliga, Philippe; Boujo, Edouard; Gallaire, François (2016)
    Journal of Fluid Mechanics
    We use the adjoint method to compute sensitivity maps for the limit-cycle frequency and amplitude of the Bénard–von Kármán vortex street in the wake of a circular cylinder. The sensitivity analysis is performed in the frame of the semi-linear self-consistent model recently introduced by Mantič et al. (Phys. Rev. Lett., vol. 113, 2014, 084501), which allows us to describe accurately the effect of the control on the mean flow, but also on the finite-amplitude fluctuation that couples back nonlinearly onto the mean flow via the formation of Reynolds stress. The sensitivity is computed with respect to arbitrary steady and synchronous time-harmonic body forces. For a small amplitude of the control, the theoretical variations of the limit-cycle frequency predict well those of the controlled flow, as obtained from either self-consistent modelling or direct numerical simulation of the Navier–Stokes equations. This is not the case if the variations are computed in the simpler mean flow approach overlooking the coupling between the mean and fluctuating components of the flow perturbation induced by the control. The variations of the limit-cycle amplitude (that falls out the scope of the mean flow approach) are also correctly predicted, meaning that the approach can serve as a relevant and systematic guideline to control strongly unstable flows exhibiting non-small, finite amplitudes of oscillation. As an illustration, we apply the method to control by means of a small secondary control cylinder and discuss the obtained results in the light of the seminal experiments of Strykowski & Sreenivasan (J. Fluid Mech., vol. 218, 1990, pp. 71–107).
  • Quantifying stochastic limit-cycle parameters from the adjoint Fokker-Planck equation
    Item type: Conference Paper
    Boujo, Edouard; Noiray, Nicolas (2016)
    Contributions to the Foundations of Multidisciplinary Research in Mechanics: Papers presented during the 24th International Congress of Theoretical and Applied Mechanics (ICTAM 2016), Montreal 22-26, 2016
  • Quantifying acoustic damping using flame chemiluminescence
    Item type: Journal Article
    Boujo, Edouard; Denisov, Alexey; Schuermans, Bruno; et al. (2016)
    Journal of Fluid Mechanics
    Thermoacoustic instabilities in gas turbines and aeroengine combustors fall within the category of complex systems. They can be described phenomenologically using nonlinear stochastic differential equations, which constitute the grounds for output-only model-based system identification. It has been shown recently that one can extract the governing parameters of the instabilities, namely the linear growth rate and the nonlinear component of the thermoacoustic feedback, using dynamic pressure time series only. This is highly relevant for practical systems, which cannot be actively controlled due to a lack of cost-effective actuators. The thermoacoustic stability is given by the linear growth rate, which results from the combination of the acoustic damping and the coherent feedback from the flame. In this paper, it is shown that it is possible to quantify the acoustic damping of the system, and thus to separate its contribution to the linear growth rate from the one of the flame. This is achieved by postprocessing in a simple way simultaneously acquired chemiluminescence and acoustic pressure data. It provides an additional approach to further unravel from observed time series the key mechanisms governing the system dynamics. This straightforward method is illustrated here using experimental data from a combustion chamber operated at several linearly stable and unstable operating conditions.
Publications1 - 6 of 6