Christophe Sicot

Finite time Lagrangian analysis of an unsteady separation induced by a near wall wake

Following the Lagrangian theory of unsteady flow separation on slip boundaries proposed by Lekien and Haller [“Unsteady flow separation on slip boundaries,” Phys. Fluids 20, 097101 (2008)], we use finite time Lagrangian analysis in order to educe large scale, unsteady flow separation downstream a near wall obstacle at a significant Reynolds number. By large scale flow separation, we mean here the ejection of fluid and vorticity outside a neighborhood of the wall at the scale of the obstacle. Indeed, while the separation point at the wall is not spatially resolved by the high speed particle image velocimetry measurements, free-slip boundary conditions are applied before educing unstable manifolds in the near wall region using finite time Lyapunov exponents. For this turbulent flow, conditional statistics are presented in order to discuss the relative contributions of the unsteady aerodynamics and of the turbulence in the separation region. The dynamics of the corresponding separation point has a very clear link with the fluctuating wall pressure induced by this unsteady turbulent flow.Following the Lagrangian theory of unsteady flow separation on slip boundaries proposed by Lekien and Haller [“Unsteady flow separation on slip boundaries,” Phys. Fluids 20, 097101 (2008)], we use finite time Lagrangian analysis in order to educe large scale, unsteady flow separation downstream a near wall obstacle at a significant Reynolds number. By large scale flow separation, we mean here the ejection of fluid and vorticity outside a neighborhood of the wall at the scale of the obstacle. Indeed, while the separation point at the wall is not spatially resolved by the high speed particle image velocimetry measurements, free-slip boundary conditions are applied before educing unstable manifolds in the near wall region using finite time Lyapunov exponents. For this turbulent flow, conditional statistics are presented in order to discuss the relative contributions of the unsteady aerodynamics and of the turbulence in the separation region. The dynamics of the corresponding separation point has a very clear...

Passive Control of Flow in an A-Pillar Region in the Presence of Uniform or Turbulent Upstream Flow

The flow at the level of A-pillar region is characterized by complex and unsteady three-dimensional separated areas. They are mainly responsible for pressure fluctuations which are the source aerodynamic noises radiated outward or transmitted in the passenger compartment. This noise can make the driver tired and stress the passengers in the cabin. Therefore, one of the technological challenges for ground vehicles is the reduction of noise generated by these aerodynamic sources. At the present time, active control devices are wanted to control the vortex and its interaction with the wall (lateral windows). However, fixed devices can also be installed to a form of passive control which will play on the relative position of the vortex and its dynamics. In the present work, simple geometric devices were installed on the A-pillar strut of a model body to modify the development of the conical vortex and its fluctuating pressure footprint. Two strategies were tested. The first consisted in a narrow plate running along the A-pillar, expected to modify the A-pillar strength and its position relative to the side wall. The second consisted in generating a secondary vortex, intense enough to interact with the main structure. The efficiency of these devices has been tested in the presence of a uniform upstream flow and a turbulent upstream flow. The isotropic and homogeneous turbulent flow was generated using a grid placed in the entry of test section, generating an intensity of 4% and an integral length scale of the magnitude of the A-pillar vortex diameter.For a uniform upstream flow, it was observed that the simple geometric devices used made the vortex core more energetic and coherent. Moreover, the group of devices acting as a vortex generator was able to very significantly reduce the coefficient of fluctuating pressure. Such an improvement was however not obtained in the presence of upstream turbulence.Copyright

Aerodynamic performances of rounded fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models. Additional tests were conducted with geometry modifications as spoilers and underbody diffusers.