Nicolas Mazellier

Closed-loop separation control over a sharp edge ramp using genetic programming

Abstract We experimentally perform open and closed-loop control of a separating turbulent boundary layer downstream from a sharp edge ramp. The turbulent boundary layer just above the separation point has a Reynolds number

Biomimetic bluff body drag reduction by self-adaptive porous flaps

Re_{\theta }\approx 3500

Modelling of the subgrid scale wrinkling factor for large eddy simulation of turbulent premixed combustion

Reθ≈3500 based on momentum thickness. The goal of the control is to mitigate separation and early re-attachment. The forcing employs a spanwise array of active vortex generators. The flow state is monitored with skin-friction sensors downstream of the actuators. The feedback control law is obtained using model-free genetic programming control (GPC) (Gautier et al. in J Fluid Mech 770:442–457, 2015). The resulting flow is assessed using the momentum coefficient, pressure distribution and skin friction over the ramp and stereo PIV. The PIV yields vector field statistics, e.g. shear layer growth, the back-flow area and vortex region. GPC is benchmarked against the best periodic forcing. While open-loop control achieves separation reduction by locking-on the shedding mode, GPC gives rise to similar benefits by accelerating the shear layer growth. Moreover, GPC uses less actuation energy.

Self-adaptive control of a bluff body wake by means of porous flaps

The performances of a new data processing technique, namely the empirical mode decomposition, are evaluated on a fully developed turbulent velocity signal perturbed by a numerical forcing which mimics a long-period flapping. First, we introduce a “resemblance” criterion to discriminate between the polluted and the unpolluted modes extracted from the perturbed velocity signal by means of the empirical mode decomposition algorithm. A rejection procedure, playing, somehow, the role of a high-pass filter, is then designed in order to infer the original velocity signal from the perturbed one. The quality of this recovering procedure is extensively evaluated in the case of a single tone perturbation (sine wave) by varying both the amplitude and the frequency of the perturbation. An excellent agreement between the recovered and the reference velocity signals is found, even though some discrepancies are observed when the perturbation frequency overlaps the frequency range corresponding to the energy-containing eddies as emphasized by both the energy spectrum and the structure functions. Finally, our recovering procedure is successfully performed on a non-stationary perturbation (linear chirp) covering a broad range of frequencies.

Wake of super-hydrophobic falling spheres: influence of the air layer deformation

Abstract The performances of an original passive control system based on a biomimetic approach are assessed by investigating the flow over a bluff body. This control device consists of a couple of flaps made from the combination of a rigid plastic skeleton coated with a porous fabric mimicking the shaft and the vane of the birdʼs feathers, respectively. The sides of a square cylinder have been fitted with this system so that each flap can freely rotate around its leading edge. This feature allows the movable flaps to self-adapt to the flow conditions. Comparing both the uncontrolled and the controlled flow, a significant drag reduction ( ≈ 22 % on average) has been obtained over a broad range of Reynolds numbers. This improvement is related to the increase of the base pressure in the controlled case. The investigation of the mean flow reveals a noticeable modification of the flow topology at large scale in the vicinity of the controlled cylinder. Meanwhile, the study of the relative motion of both flaps highlights that their dynamics is sensitive to the Reynolds number. Furthermore, the analysis of the flow dynamics at large scale suggests a lock-in coupling between the flap motion and the vortex shedding.

Experimental and Numerical Study of the Frequency Response of a Fluidic Oscillator for Active Flow Control

25 Ahmed model for the purpose of controlling aerodynamics. The application of steady microjet actuators in multiple congurations are investigated through a series of experimental studies. Each arrays’ location is selected to target key ow features, e.g. the slant separation bubble, the trailing wake vortices, and the c-pillar vortices. These are independently targeted in an eort to understand their response to control as well as their impact on overall aerodynamics. The ow’s response is characterized by aerodynamic force measurements, mean surface pressures, and velocity eld measurements. Actuation of a control array just downstream of the slant edge was shown to be one of the most eective control congurations achieving a reduction in drag of up to 13%. A tandem control method, where two arrays are actuated simultaneously was able to further reduce the maximum drag reduction for a single array by up to an additional 2%.

Flow Control of a Bluff Body Wake by Means of Self-Adaptive Flaps

We propose a model for assessing the unresolved wrinkling factor in the large eddy simulation of turbulent premixed combustion. It relies essentially on a power-law dependence of the wrinkling factor on the filter size and an original expression for the ‘active’ corrugating strain rate. The latter is written as the turbulent strain multiplied by an efficiency function that accounts for viscous effects and the kinematic constraint of Peters. This yields functional expressions for the fractal dimension and the inner cut-off length scale, the latter being (i) filter-size independent and (ii) consistent with the Damköhler asymptotic behaviours at both large and small Karlovitz numbers. A new expression for the wrinkling factor that incorporates finite Reynolds number effects is further proposed. Finally, the model is successfully assessed on an experimental filtered database.

Turbulence characterization of a high-pressure high-temperature fan-stirred combustion vessel using LDV, PIV and TR-PIV measurements

The effects resulting from the use of an innovating self-adaptive passive control system are investigated. The system studied here, is constituted of a couple of hinged porous flaps positioned symmetrically on each side of a bluff body. Its originality consists on its design which relies on a biomimetic approach, as well as its ability to self-adapt to the flow conditions. The comparison between the results obtain for the controlled model and the uncontrolled one, over the range of Reynolds numbers 2.10 4 to 8.10 4 , lead to an average mean drag reduction of 22%. An investigation of the mean pressure distribution in the near wake of the body confirmed that flow topology at large scale is strongly modified. This study focus on the attenuation of pressure fluctuations observed at the centre of the cylinder base. This parameter is considered a valid indicator of a strong reduction of the drag fluctuations.