Henri-Claude Boisson

Physical Eddy Recovery through Bi-Orthogonal Decomposition in an Acoustically Forced Plane Jet

Phase-locked X-probe hot-wire measurements were made in the near field of an acous-tically forced plane jet at a Reynolds number of 5600. They revealed leapfrog pairings during which the trailing eddys convection velocity grew up to 1.2 times the exit velocity. Two-dimensional bi-orthogonal decomposition of the velocity field yielded spatial (topos) and temporal (chronos) eigenmodes which were compared with the afore-mentioned eddies. It appeared that at least five two-dimensional eigenmodes were required for a satisfactory reconstruction. Besides, it was shown that as expected, in such spatially evolving flows, eigenmodes depended on the domain of the measurements. Further, one-dimensional decomposition showed the possibility of eddy detection in a coherent structure education scheme as well as the relevance of bi-orthogonal entropy in the description of flow organization.

Experimental coherent structures extraction and numerical semi-deterministic modelling in the turbulent flow behind a backward-facing step

The coherent structures in a turbulent flow behind a backward-facing step are analysed experimentally by the vorticity-based conditional method, and numerically by the semi-deterministic modelling. The conditional method is applied to hot wires rake measurements in a sub-sonic wind tunnel. The semi-deterministic modelling is based on a simplified approach using a modified k-ϵ model. Comparisons of the time-averaged motions and of the unsteady characteristics are done in the upper part of the shear layer after separation. It is found that semi-deterministic modelling is able to capture the coherent structures and the phenomena relating to their existence although major improvements of the method are still needed.

SIMULATION OF WAKE VORTEX AIRCRAFT IN GROUND EFFECT

The problem developed in this paper is encountered in airplane aerodynamics and concerns the influence of long life longitudinal wake vortices generated by wing tips or by external obstacles such as reactors or landing gears. More generally it concerns 3D bodies of finite extension in cross flow. At the edge of such obstacles, longitudinal vortices are created by pressure differences inside the boundary layers and rotate in opposite senses. It can constitute a danger to another aircraft that flies in this wake, especially during takeoff and landing. In this case the wake vortex trajectories and strengths are altered by ground interactions. This study presents the results of a Large Eddy Simulation of wake vortex in ground effect providing the vorticity flux behavior.

Study of a plane-free jet exhausting from a channel by vortex-in-cell method

A two-dimensional simulation of a plane jet exhausting from a channel has been performed using the vortex in cell algorithm in the Reynolds number range of 100–900. The vorticity is generated on the wall of the entrance channel whose length has been fixed in order to obtain a fully developed velocity profile at the entry of the jet. The transient behaviour of the velocity field starting from rest has been observed until reaching a quasi steady regime. The mean value of the velocity field is compared with the results of a finite volume computation on the same mesh. The velocity fluctuations obtained using this method are analysed. Their effect on the mean flow is estimated to be smaller than the viscous effect. Copyright

Behavior of a Mini Synthetic Jet in a Transverse Wall Flow: Experimental and Numerical Study

A synthetic jet is a time-averaged fluid motion generated by sufficient strong oscillatory flow downstream from a sudden expansion. The study of the interaction between the synthetic jet and an external flow is of great interest in particular for aeronautical applications. A network of such actuators could be used indeed on airplane wings for example to control, with a good energetic efficiency, the boundary layer separation in order to increase the lift or the laminar/turbulent transition for drag reduction. In this paper, the interaction of a sub-millimetric synthetic jet actuator with an external flow is experimentally studied. In these experiments, the actuation is ensured by acoustic excitations produced by a loud-speaker. Hot-wire anemometry is used for measurement of the velocity field in various functioning configurations (velocity of the transversal flow, actuation frequency, ...). In most of the tested configurations, the boundary layer of the transversal flow is significantly modified by the synthetic jet, which shows that these types of actuators could be efficiently used for flow control purposes.Copyright

Conditional Vortical Structures of a Plane Jet Based on the Complementary LSE/POD Technique

The reliability of the complementary LSE/POD technique in an eddy detection scheme was investigated via X-probe hot wire measurements in the near field of a plane jet. Both natural and acoustically forced jets at Reynolds number 5600 were considered. LSE estimations proved to be satisfactory approximations of the velocity field as long as one reference probe was placed at the jet centre while the second was located near the maximum shear line.