Gilles Harran

The three-dimensional transition in the flow around a rotating cylinder

The flow around a circular cylinder rotating with a constant angular velocity, placed in a uniform stream, is investigated by means of two- and three-dimensional direct numerical simulations. The successive changes in the flow pattern are studied as a function of the rotation rate. Suppression of vortex shedding occurs as the rotation rate increases (>2). A second kind of instabilty appears for higher rotation speed where a series of counter-clockwise vortices is shed in the upper shear layer. Threedimensional computations are carried out to analyse the three-dimensional transition under the effect of rotation for low rotation rates. The rotation attenuates the secondary instability and increases the critical Reynolds number for the appearance of this instability. The linear and nonlinear parts of the three-dimensional transition have been quantified by means of the amplitude evolution versus time, using the Landau global oscillator model. Proper orthogonal decomposition of the three-dimensional fields allowed identification of the most energetic modes and three-dimensional flow reconstruction involving a reduced number of modes.

Near-Wake Turbulence Properties around a Circular Cylinder at High Reynolds Number

The present study investigates the turbulent properties of the flow around a circular cylinder in the near-wake and in the near-wall upstream region at the Reynolds number 140,000. A detailed cartography of the mean and turbulent velocity fields using a moderate blockage and aspect ratio is provided in order to use the present results for direct comparisons with realisable 3D Navier-Stokes computations. The flow structure is analysed by means of two experiments using respectively the LDV and the PIV techniques, both providing a refined grid of measurement points. The dynamics of the separation region, the growth and decay of turbulence in the near wake, as well as the spatial growth of the organised mode are analysed.

Anisotropic eddy-viscosity concept for strongly detached unsteady flows

The accurate prediction of the flow physics around bodies at high Reynolds number is a challenge in aerodynamics nowadays. In the context of turbulent flow modeling, recent advances like large eddy simulation (LES) and hybrid methods [detached eddy simulation (DES)] have considerably improved the physical relevance of the numerical simulation. However, the LES approach is still limited to the low-Reynolds-number range concerning wall flows. The unsteady Reynolds-averaged Navier–Stokes (URANS) approach remains a widespread and robust methodology for complex flow computation, especially in the near-wall region. Complex statistical models like second-order closure schemes [differential Reynolds stress modeling (DRSM)] improve the prediction of these properties and can provide an efficient simulationofturbulent stresses. Fromacomputational pointofview, the main drawbacks of such approaches are a higher cost, especially in unsteady 3-D flows and above all, numerical instabilities.

Unsteady Flow Around a NACA0021 Airfoil Beyond Stall at 60° Angle of Attack

The flow around thick and symmetric NACA0021 airfoil at angle of attack 60°, is studied at the Reynolds number based on the chord length \(Re = \frac{{U_\infty C}}{\nu } = 2.7 \times 10^5 \). Hybrid Detached Eddy Simulation (DES) approaches are used, with improvement by means of Organised Eddy Simulation (OES) in the context of the URANS part.

Capturing coherent structures and turbulent interfaces in wake flows by means of the Organised Eddy Simulation, OES and by Tomo-PIV

The present study aims at a physical analysis of the coherent and chaotic vortex dynamics in the near wake around a flat plate at incidence, to provide new elements in respect of the flow physics turbulence modelling for high-Reynolds number flows around bodies. This constitutes nowadays a challenge in the aeronautics design. A special attention is paid to capture the thin shear layer interfaces downstream of the separation, responsible for aeroacoustics phenomena related to noise reduction and directly linked to an accurate prediction of the aerodynamic forces. The experimental investigation is carried out by means of tomographic PIV. The interaction of the most energetic coherent structures with the random turbulence is discussed. Furthermore, the POD analysis allowed evaluation of 3D phase averaged dynamics as well as the influence of higher modes associated with the finer-scale turbulence. The numerical study by means of the Organised Eddy Simulation, OES approach ensured a reduced turbulence diffusion that allowed development of the von Karman instability and of capturing of the thin shear-layer interfaces, by using appropriate criteria based on vorticity and dissipation rate of kinetic energy. A comparison between the experiments and the simulations concerning the coherent vortex pattern is carried out.

Successive Steps of 2D and 3D Transition in the Flow Past a Rotating Cylinder at Moderate Reynolds Numbers

The flow past a rotating circular cylinder, placed in a uniform stream, is investigated by means of 2D and 3D direct numerical simulations, using the finite-volume version of the code ICARE/IMFT. The flow transition is studied for Reynolds numbers from 40 to 500, and for rotation rates ˛ (ratio of the angular and the free-stream velocities) up to6. For a fixed Reynolds number, different flow patterns are observed as˛ increases: Von-Karman vortex shedding for low rotation rates, suppression of the vortex shedding at higher˛, appearing of a second mode of instability for a high interval of˛ where only counter clockwise vortices are shedd, and steady state flow for very high rotation speeds where the rotation effects keep the vortex structure near the wall and inhibit detachment. Three dimensional com- putations are carried out showing that the secondary instability is attenuated under the rotation effect. The linear and non-linear growth of the 3D flow transition are quantified using the Ginzburg-Landau global oscillator model. The analysis of the coherent structures under the rotation effect is performed by the proper orthogonal decomposition, as well the pattern reconstruction using the first POD modes.

Low-Order Modeling for Unsteady Separated Compressible Flows by POD-Galerkin Approach

A low-dimensional model is developed on the basis of the unsteady compressible Navier-Stokes equations by means of POD-Galerkin methodology in the perspective of physical analysis and computational savings. This approach consists in projecting the complex physical model onto a subspace determined to reach an optimal statistical content conservation. This leads to a drastic reduction of the number of degrees of freedom while preserving the main flow dynamics. The high-order system formulation is modified and an inner product which couples the contributions of both kinematic and thermodynamic state variables is selected. The associated reduced order model is a quadratic polynomial ordinary differential equation system which presents an inherent sensitivity to POD basis truncation for long-term prediction. A calibration process based on the minimisation of the prediction error with respect to reference dynamics is implemented. The predictive capacities of the low-order approach are evaluated by comparison with results issued from the 2D Navier-Stokes simulation of a transonic flow around a NACA0012 airfoil, at zero angle of incidence. This configuration is characterised by a complex unsteadiness caused by a von Karman instability mode induced by shock/vortex interaction, and a low frequency buffeting mode.

Fluid-Structure Interaction and Control Around Vibrating and Morphing Configurations at High Reynolds Number

This study analyses the fluid-elastic instability occurring in turbulent wakes around tandem cylinders as well as the morphing effect in order to control the instability amplification and to improve the aerodynamic performances particularly concerning the Airbus-A320 wing-flap configuration. These studies have been carried out by means of High-Fidelity numerical simulations by using the NSMB—Navier Stokes MultiBlock code (Grossi et al. AIAA J 52(10):2300–2312, 2006) [1], as well as by refined physical experiments, after having built advanced morphing wing prototypes and tested in wind tunnel by using Time-Resolved PIV (TRPIV) and aerodynamic balance measurements. It has been shown that the electroactive morphing concepts are able to provide significant improvements of the aerodynamic performances and to decrease the rms of the displacement and of the forces in the case of the tandem cylinders configuration undergoing Flow Induced Vibration.

Experimental Investigation of a Hybrid Morphing NACA4412 Airfoil Via Time-Resolved PIV Measurements

Particle image velocimetry (PIV) measurements are conducted at the trailing edge of a piezoelectric actuated airfoil in order to investigate the physical effect on the flow via high-frequency low-amplitude actuation. Furthermore the effects of large-amplitude low frequency actuation modifying the airfoil camber are investigated using aerodynamic force measurements. A statistical analysis reveals the reduction of the Reynolds stress tensor components with increasing actuation frequency up to a frequency of \(60\,\text {Hz}\). The modification of the airfoil camber allows real-time control of the desired lift. The feasibility of the designed hybrid morphing mechanism under aerodynamic loads at a Reynolds number of 218,000 was shown for both the large amplitude and the high frequent actuation.

Naturally Separated Turbulent Flow Around Two Cylinders in Tandem at High Reynolds Number by Time-Resolved PIV

In this study the focus is made on the properties of the bistable flow between two cylinder in tandem for a pitch ratio \(L/D = 3.7\) at a Reynolds number of 132,800. Time resolved particle image velocimetry (TRPIV) is used to measure the spatial distribution and time fluctuations of the velocity field in the wake of the upstream cylinder. From velocity spectra and statistics fields, the existence of two distinct seemingly stable flow patterns is shown, having different spatial structures and predominant frequencies.