Caroline Braud

Characterisation of a high Reynolds number boundary layer subject to pressure gradient and separation

The flow over a ramp model is characterised in detail in the present study. In the selected configuration, a turbulent boundary layer developed on a flat plate is first accelerated in a curved contraction. It is then submitted to a mild adverse pressure gradient on a flat plate followed by a separation above a flap. Inlet boundary conditions and pressure distribution are provided to allow numerical simulations. The flow in the mild adverse pressure gradient region is characterised with hot-wire anemometry. In this region, the boundary layer thickness is of the order of 20 cm, the momentum Reynolds number is about 11,000 and the Clauser pressure gradient parameter β in the stabilised region is about 0.4. Particular emphasis is laid on the separation to provide quantitative information to evaluate turbulence models. This is achieved through a large streamwise two-dimensional two-component particle image velocimetry (2D2C PIV) plane which contains all the separation bubble and part of the flow upstream and downstream of it. The separation border is detected using the backflow coefficient, resulting in a separation length of about 3.49 Hs (with Hs the flap step height) and a maximum height of about 0.17 Hs. The Reynolds stresses and their main production terms are also determined. A region of high turbulence intensity develops above the separation border for all the measured components. The production of dominates the production of turbulent kinetic energy which implies a redistribution from to to explain the increase observed. The production term drives the production of in the first part of the flap which is not the case for zero pressure gradient boundary layers. Finally, a high similarity is observed between and as the production of the latter is dominated by . This flow appears as a challenging test case for Reynolds averaged Navier–Stokes (RANS) and large eddy simulation (LES) validation.

The flow structure behind vortex generators embedded in a decelerating turbulent boundary layer

The objective of the present work is to analyse the behaviour of a turbulent decelerating boundary layer under the effect of both passive and active jets vortex generators (VGs). The stereo PIV database of Godard and Stanislas [1,2] obtained in an adverse pressure gradient boundary layer is used for this study. After presenting the effect on the mean velocity field and the turbulent kinetic energy, the line of analysis is extended with two points spatial correlations and vortex detection in instantaneous velocity fields. It is shown that the actuators concentrate the boundary layer turbulence in the region of upward motion of the flow, and segregate the near-wall streamwise vortices of the boundary layer based on their vorticity sign.

Vortex Generator Induced Flow in a High Re Boundary Layer

Stereoscopic Particle Image Velocimetry measurements have been conducted in cross-planes behind three different geometries of Vortex Generators (VGs) in a high Reynolds number boundary layer. The VGs have been mounted in a cascade producing counter-rotating vortices and the downstream flow development was examined. Three VG geometries were investigated: rectangular, triangular and cambered. The various VG geometries tested are seen to produce different impacts on the boundary layer flow. Helical symmetry of the generated vortices is confirmed for all investigated VG geometries in this high Reynolds number boundary layer. From the parameters resulting from this analysis, it is observed at the most upstream measurement position that the rectangular and triangular VGs produce vortices of similar size, strength and velocity induction whilst the cambered VGs produce smaller and weaker vortices. Studying the downstream development in the ensemble and spanwise averaged measurements, it is observed that the impact from the rectangular and triangular VGs differs. For the rectangular VGs, self-similarity in the streamwise component was confirmed.

Influence of light sheet separation on SPIV measurement in a large field spanwise plane

Stereoscopic particle image velocimetry (SPIV) is nowadays a well-established measurement technique for turbulent flows. However, the accuracy and the spatial resolution are still highly questionable in the presence of complex flow with both strong gradients and out-of-plane motions. To give guidelines for both setup and measurements of such flow configurations, a large region of overlap between two SPIV systems on the same laser light sheet is acquired in a plane normal to the streamwise direction of a high Reynolds turbulent boundary layer flow. A simple separation of the two light sheets is used to improve the accuracy of the measurements by increasing the velocity dynamic range especially. It also presents the enhancement of accuracy due to the light sheet separation for characterizing streamwise vortices (i.e. perpendicular to the sheet). The proposed technique has been demonstrated in the Laboratoire de Mecanique de Lille wind tunnel facility which has been specially designed to study fully developed turbulent boundary layers at high Reynolds numbers. The outlook is to study in detail the physics of the streamwise vortices generated from vortex generators taking advantage of the large scales of this turbulent boundary layer.

Large-eddy simulations of control of a separated flow over a 2D bump by means of pulsed jets

A numerical study of active flow control of a separated converging-diverging channel flow is presented. Several geometrical and operating parameters of the continuous and pulsed jets are investigated using large-eddy simulations (LES), which are highly resolved in the actuation region. The LES without control is validated with a direct numerical simulation of the same flow. Configurations with both counter-rotating and co-rotating jets are tested. The main statistical results are in general agreement with the literature. Thanks to the unsteady highly resolved simulation, both temporal and spatial organizations of the generated vortices are investigated. An optimal frequency associated with the convective time as well as an optimal duty cycle are obtained, leading to a control efficiency comparable to the cases with continuous jets and the same operating velocity and, consequently, to a significant saving in mass flow rate.

Analysis and Modelling of a Fluidic Actuator

This paper deals with the fluidic actuators in the pulsed mode used in turbulent boundary layer control, in particular as vortex generator (VG) in order to delay separation. Recently the study by Kostas et al (2007) has shown the importance of the transient phase of the VG actuators. In particular, an enhancement of the vortex-generation mechanism has been observed during the transient period, that is responsible for an increase of turbulence stress levels up to 200% relatively to the non actuated case. A large dependency of the exit velocity on the physical characteristic of the feed tube has been detected. This dependency suggests that a precise quantification of the pulsed jet dynamic during the transient period is necessary. In this work the transient behavior of the fluidic actuators used by Kostas et al (2007) is analyzed and experimented. A model is developed to explain the dynamics of the flow inside the actuator. On the whole, experiments show that the role of all physical parameters is consistent with the foreseen properties. The results obtained help to separate the input dynamic of the controller from the controlled flow. Another perspective of this work is to provide a guide for the design of fluidic actuators.Copyright

Real-time processing methods to characterize streamwise vortices

We propose to combine the active vortex generators with the particle image velocimetry (PIV) measurements and post-processing streamwise vortex characterization algorithms into a feedback based closed-loop control system for wind turbine applications. We develop two vortex identification and characterization methods that use PIV realizations for the purpose of a real-time (online or on-the-fly) feedback-based control. Both methods can extract centers and strengths of streamwise vortices generated behind active vortex generators in a turbulent boundary layer flow, and we show how to integrate those in a closed-loop control strategy. For demonstration purposes we use stereoscopic PIV measurements at the wind tunnel facility obtained in the transverse-wall-normal plane behind active vortex generators. A robust algorithm is using the

Open and closed-loop experiments to identify the separated flow dynamics of a thick turbulent boundary layer

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FLOW CONTROL OVER A RAMP USING ACTIVE VORTEX GENERATORS

-criteria and the integration of vorticity of each extracted vortex. Results show that a moving window average of a small number of instantaneous fields is nevertheless needed for increased robustness. The robust method requires the full field PIV computation followed by spatial derivatives calculations. A faster method is developed, which, using only horizontal lines of vertical velocity, has a high potential to significantly cut down the computational effort relative to the robust method. We compare the two methods and discuss their shortcomings and the potential for the real-time, online, closed-loop control of turbulent boundary layers of the wind turbine blades.