Fluid force and symmetry breaking modes of a 3D bluff body with a base cavity

International audienceA cavity at the base of the squareback Ahmed model at Re 4 × 10 5 is able to reduce the base suction by 18% and the drag coefficient by 9%, while the geometry at the separation remains unaffected. Instantaneous pressure measurements at the body base, fluid force measurements and wake velocity measurements are investigated varying the cavity depth from 0 to 35% of the base height. Due to the reflectional symmetry of the rectangular base, there are two Reflectional Symmetry Breaking (RSB) mirror modes present in the natural wake that switch from one to the other randomly in accordance with the recent findings of Grandemange et al. (2013b). It is shown that these modes exhibit an energetic 3D static vortex system close to the base of the body. A sufficiently deep cavity is able to stabilize the wake toward a symmetry preserved wake, thus suppressing the RSB modes and leading to a weaker elliptical toric recirculation. The stabilization can be modelled with a Langevin equation. The plausible mechanism for drag reduction with the base cavity is based on the interaction of the static 3D vortex system of the RSB modes with the base and their suppression by stabilization. There are some strong evidences that this mechanism may be generalized to axisym-metric bodies with base cavity

Asymmetry and global instability of real minivans' wake

© 2018 Elsevier Ltd Three minivan cars tested in real flow conditions are investigated through base pressure distribution, force balance and velocity measurements. Discontinuous transitions of permanent wake reversals and bistable dynamics between two well-defined states are observed varying ground clearance, pitch and yaw, with open and closed air-intake. These transitions prove that the cars undergo the same global z-instability as the square-back Ahmed body (M. Grandemange, M. Gohlke and O. Cadot, Physics of Fluids, 25, 2013). The contribution of the global instability to the lift coefficient is estimated to 0.012 and 0.024 depending on the wake state. Eventually, the potential of direct passive control of the z-instability is demonstrated by improving from 4.6% to 8.3% the drag reduction obtained by closing the front air-intake of the car

Effect of a base cavity on the wake of the squareback Ahmed body at various ground clearances and application to drag reduction

Recently, Evrard et al. (2016, Journal of Fluids and Structures, 61) achieved drag reduction by almost 9 % by means of a base cavity on a three-dimensional bluff body, the squareback Ahmed body. The authors associate drag reduction with the suppression of the static asymmetric modes of the wake identified by Grandemange et al. (2013, Journal of Fluid Mechanics, 722) leading to its symmetrization. The beneficial effect of a base cavity on the drag has been known for decades on axisymmetric bluff bodies in the context of aerospace engineering (Morel, 1979, Aeronautical Quarterly, 30) but the phenomenon has not been fully elucidated yet. The present work aims at showing experimentally that the decrease of the asymmetry of the near wake flow is responsible for drag reduction regardless of the ground clearance. With this aim in mind, we do two parametric studies of the ground clearance of a squareback Ahmed body in an industrial wind-tunnel; one without and one with a base cavity. We want to compare the two bifurcations of the wake operated by the ground clearance (Grandemange et al., 2013, Physics of Fluids, 25) depending on the rear geometry. Far enough from the ground i.e. after the bifurcation, the modes indeed disappear in agreement with the work of Evrard et al. the flow is symmetrized and the base pressure increases by 24 %. In the vicinity of the ground however, two new results are reported in this paper. The modes governing the wake are not the same as at higher ground clearances but are rather vertical modes due to the presence of the ground. Besides, even if the cavity does not fully symmetrize the flow in this case, its asymmetry is reduced. Consequently, an important drag reduction of the same order of magnitude as in the far ground regime is observed (7 % associated with a base pressure increase by 20 %). A six-components aerodynamic balance and twenty-one instantaneous base pressure measurements are used to record the data at a high sampling rate to study the dynamics of the changes

Vortices enable the complex aerobatics of peregrine falcons

The peregrine falcon (Falco peregrinus) is known for its extremely high speeds during hunting dives or stoop. Here we demonstrate that the superior manoeuvrability of peregrine falcons during stoop is attributed to vortex-dominated flow promoted by their morphology, in the M-shape configuration adopted towards the end of dive. Both experiments and simulations on life-size models, derived from field observations, revealed the presence of vortices emanating from the frontal and dorsal region due to a strong spanwise flow promoted by the forward sweep of the radiale. These vortices enhance mixing for flow reattachment towards the tail. The stronger wing and tail vortices provide extra aerodynamic forces through vortex-induced lift for pitch and roll control. A vortex pair with a sense of rotation opposite to that from conventional planar wings interacts with the main wings vortex to reduce induced drag, which would otherwise decelerate the bird significantly during pull-out. These findings could help in improving aircraft performance and wing suits for human flights

Transonic shock wave-boundary-layer interactions

Introduction to Transonic Interactions By definition, transonic shock wave-boundary layer interactions (SBLIs) feature extensive regions of supersonic and subsonic flows. Typically, such interactions are characterized by supersonic flow ahead of the shock wave and subsonic flow downstream of it. This mixed nature of the flow has important consequences that make transonic interactions somewhat different from supersonic or hypersonic interactions. The key difference between transonic interactions and other SBLIs is the presence of subsonic flow behind the shock wave. Steady subsonic flow does not support waves (e.g., shock waves or expansion fans), and any changes of flow conditions are gradual in comparison to supersonic flow. This imposes constraints on the shock structure in the interaction region because the downstream flow conditions can feed forward and affect the strength, shape, and location of the shock wave causing the interaction. The flow surrounding a transonic SBLI must satisfy the supersonic as well as subsonic constraints imposed by the governing equations. The interaction also is sensitive to downstream disturbances propagating upstream in the subsonic regions. In contrast, supersonic interactions are "shielded" from such events by the supersonic outer flow