Identification of very large scale structures in the boundary layer over large roughness elements

International audienceThe dynamics of a turbulent boundary layer developing over a cube array is investigated based on stereoscopic PIV meas-urements performed in a boundary layer wind tunnel. The fluctuating streamwise velocity component u is analyzed via the recently introduced Spectral Proper Orthogonal Decomposition method (S-POD). A modification of the definition of the spatial S-POD modes is proposed so that they are orthonormal to each other. This results in spatio-temporal modes which reflect the statistical organization of the most energetic structures both in space and time. It is first demonstrated that the first S-POD modes of u correspond to large-scale elongated coherent structures of low or high momentum. The existence of very large scale structures whose length spans several boundary layer thickness is demonstrated, in agreement with the recent similar findings for flows over smooth wall. These structures are found to be non-negligible contributors to the Reynolds shear stress and the turbulent kinetic energy. Their relationship with the smaller scales of the flow is investigated via the computation of joint third-order statistics and is shown to be of non-linear nature

Data Assimilation for control of a plane mixing layer

International audienc

Transient Characterization of the Reattachment of a Massively Separated Turbulent Boundary Layer Under Flow Control

International audienceThe transient dynamics of a high Reynolds number separated flow over a two-dimensional ramp submitted to pulsed fluidic control is investigated. A spanwise array of 22 round jets, located upstream of the flap leading edge, is used as actuator to generate co-rotating vortical structures. Simultaneous measurements of wall friction using hot-film anemometry and phase-averaged velocity using 2D2C PIV are conducted. The PIV plane encompasses the incoming boundary layer upstream the flap leading edge, the separation bubble and the natural reattachment region. The dynamics of the separated flow is studied for successive sequences of pulsed actuation. Pockets of turbulence are periodically generated by the separation process and pushed downstream. After the transition period, the controlled flow shows large amplitude oscillations around a steady mean, particularly for the separation area. The transient dynamics of the flow at the actuation activation is also studied. The separated flow is strongly modified by the actuation from the first pulse. Characteristic times of the transient dynamics can be determined by fitting a first-order model with delay on the data. For the reattachment, the dimensionless characteristic rising times defined as τ+r=τr U0 / Lsep of 11.7 for the friction gain, 4.8 for the separation length and 4.1 for the first mode of a Conditional Proper Orthogonal Decomposition analysis of the phase-averaged velocity fields were found. These values are in good agreement with previous studies and are of particular interest for modeling the transients and for further closed-loop control applications

Data Assimilation applied to a numerical 2D cylinder wake flow

International audienc

Caractérisation statistique des sources aéroacoustiques en jets supersoniques par vélocimétrie laser à effet Doppler (application à la prédiction du bruit de jet)

Le travail présenté s'inscrit dans le contexte particulier des écoulements en régime supersonique à l'origine de niveaux de bruit très importants et pouvant induire des phénomènes de vibrations significatives des structures solides présentes dans le champ acoustique rayonné. L'étude préalable de techniques de réduction de bruit et des mécanismes vibro-acoustiques nécessite, entre autres, une connaissance rigoureuse du bruit rayonné ainsi que de ces différentes sources. Pour un écoulement libre quelconque, une approche possible d'estimation du bruit généré consiste à relier la dynamique du champ turbulent intrinsèque de l'écoulement au champ acoustique rayonné. Ceci est rendu possible grâce au concept d'{\it analogie aéroacoustique} proposé initialement par Lighthill (1952). La détermination des organisations spatiale et temporelle du champ turbulent à partir d'un tenseur de corrélation des fluctuations de vitesse permet de rendre compte, statistiquement, de l'évolution au sein de l'écoulement des mécanismes de conversion de l'énergie cinétique turbulente en énergie acoustique et indirectement, de celle des sources aéroacoustiques. Des mesures par Vélocimétrie Laser à Effet Doppler (technique de mesure optique) en jets supersoniques successivement froid et chaud sont ainsi ici réalisées afin de déterminer une des composantes principales de ce tenseur de corrélation. Une modélisation simplifiée du tenseur, basée sur l'estimation d'échelles spatiales et temporelles dites intégrales et caractéristiques des mécanismes sources, est ensuite proposée dans la cas d'une turbulence homogène. La composition spectrale du champ turbulent dans le cas du jet supersonique froid est également ici rapportée et renseigne sur sa dynamique. Ces données expérimentales sont finalement couplées à un code de calculs aéroacoustiques dans le but de prédire le spectre d'intensité acoustique rayonné par un jet supersonique chaud.The prediction of jet mixing noise using an acoustic analogy and based on experimental measurements is here investigated in the case of a supersonic jet. The basis of any acoustic analogy is the representation of the specifics spatial and temporal organisations of the turbulent field as a spatiotemporal correlation tensor of the turbulence velocity. This quantity gives informations on the dynamic and efficiency of the mechanisms of conversion of the turbulence kinetic energy into acoustic energy. Turbulence and source properties such as integral length and time scales, as well as convection velocity are commonly used to characterised the mixing processes and used for accurately modelling the spatiotemporal correlation tensor. The accuracy of the noise prediction is therefore related to the accuracy with which this tensor is modelled. Modelling of this quantity, based on experimental two-point Laser Doppler Velocimetry measurements in the case of a supersonic jet is here proposed. Modelling is based on an assumption of a homogeneous and isotropic turbulence field so that the tensor can be reduced to one component only. The prediction of the noise radiated by the flow requires also the spatial distribution of the energy of the sources to be known. An aerodynamic code is here used and validated by experimental measurements in a hot supersonic jet. Additional measurements have been performed to study the spectral content of the turbulent field and its evolution along the flow in such typical jets. This highlight on how the turbulent kinetic energy is efficiently used by the different components of the turbulent field. The noise radiated by the hot supersonic jet is finally predicted using a numerical code based on the Lighthill's acoustic analogy and the model established for the correlation tensor obtained from the experimental measurements.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Dynamic reconstruction of a numerical 2D cylinder wake flow using Data Assimilation

International audienc

Jet Turbulance Characteristics Associated with Downstream and Sideline Sound Emission

International audienc

EFFECT OF DBD PLASMA ACTUATORS ON VORTICALSTRUCTURES IN A TURBULENT MIXING LAYER

International audienc

Effect of DBD plasma actuators on vortical structures in a turbulent mixing layer

The influence of linear (spanwise-uniform) forcing applied by a DBD plasma actuator on the growth of a turbulent mixing layer and the dynamics of large-scale spanwise vortices, are investigated experimentally. Furthermore, the freestream turbulence intensity in the low-velocity stream is maintained at a high level to examine its impact on the control-authority of the applied forcing. The influence of the applied forcing on the dynamics and interactions of the spanwise vortices in the mixing layer is apparent. However, the growth rate of the mixing layer is not affected, suggesting that the high level of freestream turbulence diminishes the control-authority of the applied forcing.</p

A bilinear input-output model with state-dependent delay for separated flow control

International audience— It is proposed a first step to the model-based closed-loop control of a separated flow. In such situations, fluid mechanics phenomena are highly nonlinear and can be represented by means of the Navier-Stokes equations. However, such a model still rises difficult issues for control practice. This paper proposes an alternative, bilinear and delayed model, the accuracy of which is studied. The identification technique combines least-square technique with a Mesh Adaptive Direct Search (MADS) algorithm. The main feature of the model is state dependent structure of input delay