Jean-Luc Aider

Transitions in the wake of a flapping foil

We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using two-dimensional particle image velocimetry. An analysis in terms of a flapping frequency-amplitude phase space allows the identification of (i) the transition from the well-known Bénard-von Kármán (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and (ii) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. We also show that the transition from a BvK wake to a reverse BvK wake precedes the actual drag-thrust transition and we discuss the significance of the present results in the analysis of flapping systems in nature.

A model for the symmetry breaking of the reverse Bénard–von Kármán vortex street produced by a flapping foil

The vortex streets produced by a flapping foil of span to chord aspect ratio of 4:1 are studied in a hydrodynamic tunnel experiment. In particular, the mechanisms giving rise to the symmetry breaking of the reverse Benard–von Karman (BvK) vortex street that characterizes fishlike swimming and forward flapping flight are examined. Two-dimensional particle image velocimetry (PIV) measurements in the midplane perpendicular to the span axis of the foil are used to characterize the different flow regimes. The deflection angle of the mean jet flow with respect to the horizontal observed in the average velocity field is used as a measure of the asymmetry of the vortex street. Time series of the vorticity field are used to calculate the advection velocity of the vortices with respect to the free stream, defined as the phase velocity U phase , as well as the circulation Γ of each vortex and the spacing ξ between consecutive vortices in the near wake. The observation that the symmetry-breaking results from the formation of a dipolar structure from each couple of counter-rotating vortices shed on each flapping period serves as the starting point to build a model for the symmetry-breaking threshold. A symmetry-breaking criterion based on the relation between the phase velocity of the vortex street and an idealized self-advection velocity of two consecutive counter-rotating vortices in the near wake is established. The predicted threshold for symmetry breaking accounts well for the deflected wake regimes observed in the present experiments and may be useful to explain other experimental and numerical observations of similar deflected propulsive vortex streets reported in the literature.

Drag reduction of a bluff body using adaptive control methods

A classical actuator is used to control the drag exerted on a bluff body at large Reynolds number (Re=20000). The geometry is similar to a backward-facing step whose separation point is modified using a rotating cylinder at the edge. The slow fluctuations of the total drag are directly measured by means of strain gauges. As shown by visualizations, the actuator delays the separation point. The size of the low-pressure region behind the body is decreased and the drag reduced. It is found that the faster the rotation of the cylinder, the lower the drag. In a first study, the goal of the control is for the system to reach a drag consign predetermined by the experimentalist. The control loop is closed with a proportional integral correction. This adaptive method is shown to be efficient and robust in spite of the large fluctuations of the drag. In the second method, the system finds itself its optimal set point. It is defined as the lowest cost of global energy consumption of the system (drag reduction versus...

Large-eddy simulation applied to study the influence of upstream conditions on the time-dependant and averaged characteristics of a backward-facing step flow

The unsteady backward-facing step flow is investigated using large-eddy simulation. Two different inlet conditions are tested to study the sensitivity of the separated flow to a modification of the upstream boundary layer. The first inlet condition consists of a mean turbulent profile perturbed by a white noise. The second relies upon a more realistic condition, in which fully turbulent inflow data are derived from an auxiliary simulation of a quasi-temporal boundary layer. The temporal pressure spectra in different locations downstream of the step exhibit four different characteristic frequencies in both cases. Pressure and velocity statistics, supplemented by visualizations, demonstrate how the flow in the shear layer is strongly influenced by the upstream conditions. The turbulent boundary layer triggers a rapid destabilization of the mixing layer, resulting in a shortening of the mean recirculation length. The temporal spectra reveal that the precursor simulation leads to an increase of the characteri...

Coherent-vortex dynamics in large-eddy simulations of turbulence

We present a review of coherent-vortex dynamics obtained thanks to large-eddy simulations (LES) associated with simple and effective vortex- identification and animation techniques. LES of a large class of constant-density or weakly compressible three-dimensional flows have been carried out. In isotropic turbulence, we present the formation and evolution of spaghetti-type vortices, seen thanks to Q, vorticity and pressure, together with the time evolution of the kinetic energy, enstrophy and skewness. In a spatially growing boundary layer on a flat plate, one observes during transition big Λ vortices lying on the wall (with very well correlated oblique induced low- and high-speed streaks) shedding smaller hairpin vortices around their tips. In the developed boundary layer, we show animations of the purely longitudinal low- and high-speed streaks, as well as animations of low-pressure regions. In a backwards-facing step, we examine the influence of upstream conditions upon the flow structure, by comparing two inflow conditions: a white noise superposed on a mean velocity profile and a realistic turbulent boundary layer. The latter three-dimensionalizes the flow downstream of the step and reduces the reattachment length. In both cases big staggered arch vortices form, impinge the lower wall and are carried away downstream. In a two-dimensional (2D) square cavity, spanwisely oriented vortices are shed behind the upstream edge, and impinge the downstream edge, transforming into arch vortices very similar to the back-step case. These arch vortices are also found behind a 2D rectangular obstacle with wall effect. We discuss the relevance of the vortices found with respect to reality. All these eddies are very important in terms of drag and noise reduction in aerodynamics and aeroacoustics.

Experimental scaling law for the subcritical transition to turbulence in plane Poiseuille flow.

We present an experimental study of the transition to turbulence in a plane Poiseuille flow. Using a well-controlled perturbation, we analyze the flow by using extensive particle image velocimetry and flow visualization (using laser-induced fluorescence) measurements, and use the deformation of the mean velocity profile as a criterion to characterize the state of the flow. From a large parametric study, four different states are defined, depending on the values of the Reynolds number and the amplitude of the perturbation. We discuss the role of coherent structures, such as hairpin vortices, in the transition. We find that the minimal amplitude of the perturbation triggering transition scales asymptotically as Re(-1).

Transition scenario of the round jet in crossflow topology at low velocity ratios

We study experimentally a round Jet In CrossFlow (JICF) at low values of the jet-to-crossflow velocity ratio R using instantaneous and time-averaged three-dimensions three-components velocimetry. The difference between instantaneous and time-averaged swirling structures of the JICF is emphasized. Through the analysis of spatial distribution of instantaneous transverse and longitudinal vortices the main transitions of the JICF are characterized for 0.15 1.25), the classic JICF topology is recovered. In between, a deformation of the classical JICF topology is observed consisting in a progressive disappearance of the leading-edge vortices, a bending of the jet trajectory toward the wall and thus a strengthened interaction with the boundary layer. Thanks to a state-of-the-art review on the JICF topology and using visualizations of the flow structures extracted from our experimental volumetric velocimetry mea...

Turbulent spots in channel flow: An experimental study - Large-scale flow, inner structure and low-order model

We present new experimental results on the development of turbulent spots in channel flow. The internal structure of a turbulent spot is measured, with Time Resolved Stereoscopic Particle Image Velocimetry. We report the observation of travelling-wave-like structures at the trailing edge of the turbulent spot. Special attention is paid to the large-scale flow surrounding the spot. We show that this large-scale flow is an asymmetric quadrupole centred on the spot. We measure the time evolution of the turbulent fluctuations and the mean flow distortions and compare these with the predictions of a nonlinear reduced order model predicting the main features of subcritical transition to turbulence.Graphical abstract

Control of the separated flow downstream of a backward-facing step using visual feedback

The separated flow downstream of a backward-facing step is controlled using visual information for feedback. This is done when looking at the flow from two vantage points. Flow velocity fields are computed in real time and used to yield inputs to a control loop. This approach to flow control is shown to be able to control the detached flow in the same way as has been done before by using the area of the recirculation region downstream of the step as the input for a gradient descent optimization scheme. Visual feedback using real-time computations of two-dimensional velocity fields also allows for novel inputs in the feedback scheme. As a proof of concept, the spatially averaged value of the swirling strength λCi is successfully used as the input for an automatically tuned proportional–integral–derivative controller.The separated flow downstream a backward-facing step is controlled using visual information for feedback. This is done when looking at the flow from two vantage points. Flow velocity fields are computed in real-time and used to yield inputs to a control loop. This approach to flow control is shown to be able to control the detached flow in the same way as has been done before by using the area of the recirculation region downstream the step as input for a gradient descent optimization scheme ([1]). Visual feedback using real-time computations of 2D velocity fields also allows for novel inputs in the feedback scheme. As a proof of concept, the spatially averaged value of the swirling strength λci is used successfully as input for an automatically tuned PID controller.

Pressure-sensitive paint for automotive aerodynamics

Measurement of static pressure over an automotive body is tedious work. The preparation of the model can be long and the spatial resolution in high curvature regions, where separations occur, is poor. To overcome these intrinsic drawbacks it is necessary to use another nonintrusive experimental technique. We show in this paper that the Pressure Sensitive Paint (PSP) fulfills the requirements and is reliable for automotive applications. We consider a simplified 1/4-scale model of a Peugeot 206 with a modified back slant to enhance the longitudinal vortices. The PSP measurements are compared to standard pressure ports measurements and to CFD computations. We find good quantitative agreement between pressure ports and PSP measurements and a good qualitative and quantitative agreement between PSP and the computations over the entire slant surface. The high spatial resolution and high sensitivity of this technique allows very good description of the flow over any complex surfaces even at low velocity. Considering that PSP does not need any particular instrumentation of the car, it is well adapted to the constraints of the automotive industry and appears to be a very attractive improvement of the traditional pressure taps.