Erwan Collin

Polynomial identification of POD based low-order dynamical system

Low-order modelling based on POD approach has proved to be an efficient tool to analyze turbulent flows as well as to build control systems. In this paper, a novel method to identify low-order dynamical system (LODS) is proposed. This approach relies on the fact that all the POD-Galerkin LODS based on Navier–Stokes equations can be written in polynomial form. One proposes here to estimate the polynomial coefficients arising in such a formalism by the following. The projection coefficients of the flow field onto its POD basis and their time derivatives being known, a statistical approach involving correlations between these quantities, are used to provide an estimate of the coefficients of the dynamical system. The identification method is described and tested in the case of the analytical Lorenz system. Finally, the LODS identification is performed in the case of experimental data of a supersonic jet-mixing layer interaction. Dynamical systems based on flow visualizations are derived and lead to relevant short-time and long-term predictions.

Analysis of a jet–mixing layer interaction

Abstract The improvement of mixing in free-shear flows via external jets has been proven efficient in subsonic and supersonic flows as well. However, the hyper-mixing process is not well known. The present study deals with an experimental and a numerical approach of the interaction of an external control jet with a turbulent mixing layer. The main conclusion is that an intermittent penetration of the control jet occurs both in supersonic and subsonic configurations. Moreover, all results tend to show that the control jet flapping frequency and the spacing between the structures involved downstream of the interaction are respectively very close to the frequency and wavelength of the Kelvin–Helmholtz structures at the impact location. Two hypotheses are provided in order to explain the mechanism of the interaction. The first one is based upon the interaction with the passage of Kelvin–Helmholtz structures in the mixing layer, the other deals with an intrinsic instability of such a flow configuration.

ANALYSIS OF A JET – MIXING LAYER INTERACTION

The improvement of mixing in jets via external jets has been proven to be efficient in subsonic and supersonic flows as well. However, the hyper-mixing process is not well known. The present study deals with an experimental and a numerical approach of the interaction of an external control jet with a turbulent axisymmetric mixing layer. The main conclusion of a dual experimental/numerical study is that an intermittent penetration of the control jet occurs both in supersonic and subsonic configurations. Moreover, all results tend to show that the control jet flapping frequency and the spacing between the structures involved downstream from the interaction are respectively very close to the frequency and wavelength of the Kelvin-Helmholtz structures at the impact location. Two hypotheses are provided in order to explain the mechanism of the interaction. The first one is based upon the interaction with the passage of Kelvin-Helmholtz structures in the mixing layer, the other one deals with an intrinsic instability of such a flow configuration.

Experimental study of a supersonic jet-mixing layer interaction

An experimental study is performed to analyze the interaction between a control jet (CJ) and a moderately supersonic main jet. Flow visualizations and laser Doppler velocimetry methods are used. A strong instability of the CJ has been identified. The dynamic of this instability corresponds to that of the local mixing layer. Two stability scenarios are proposed, one corresponding to the local Kelvin–Helmholtz instability of the main jet, the other linked to a local absolute instability of the interaction. The impact on the turbulent quantities is analyzed. It is shown that a strong modification of the Reynolds stress is manifest but that this extends only a small distance from the interaction.

Analysis of a plane turbulent mixing layer manipulated by a localized forced separation

The purpose of this experimental study is to analyze the effects of a forced separation, near the trailing edge of a splitter plate, on the development of a turbulent plane mixing layer. The separation was driven by a steady pneumatic injection, and is considered here as an actuator for controlling the spreading efficiency of the mixing layer. Particle image velocimetry and hot-wire measurements were performed for the natural and the manipulated regimes. The results highlight the capabilities of this control strategy to enhance mixing. Analysis of the turbulent field and coherent structure organization offers insight into the mechanisms responsible for this mixing enhancement.

Modification of Axisymmetric Jet Expansion by Forced Localized Separation on the Lip

An experimental study is performed to investigate the mixing enhancement induced by forced localized separation on the trailing edge of a subsonic round jet. Two different configurations are tested: the first one with two forced-localized-separation diametrically opposed, and the second one with eight forced-localized-separation regularly distributed. Each forced localized separation, considered here as the actuator, is driven by steady pneumatic blowing. Particle-image-velocimetry measurements are used with two optical setups in order to analyze the flow features in cross sections and meridian planes. In each case, a strong deformation of the mean velocity field is observed, with a substantial reduction of the potential core length of the jet. The turbulent field is shown to be highly modified as well, with a significant raise of all turbulent components. Analysis of the mixing layer region reveals different flow control mechanisms, depending on the distribution of forced localized separations.