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

Abstract

A 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 axisymmetric bodies with base cavity.

Evolution of the bi-stable wake of a square-back automotive shape

“…15a) shows that a drag reduction can be achieved any time that the lateral symmetry of the wake is restored; namely, a 1 (t) → 0 . A similar conclusion was drawn by Evrard et al (2016) and Li et al (2016). A deeper understanding of the origin of this drag reduction can be achieved by looking at the scatter plot between C (POD) D Rear (t) and a 2 (t) (Fig.…”

“…This vortical structure forms parallel to one of the two vertical sides of the base and then splits into two legs that bend horizontally while moving downstream, until they reach the opposite side of the base, with the top leg eventually prevailing over the bottom one. This topology presents some analogies with that speculated by Evrard et al (2016), but in this case, all the vortices captured by the PIV planes have been included in the structure, thanks to a more complete set of data. A similar organisation has been seen in the wake of a sphere for Re > 280 (Chrust et al 2013).…”

“…This is in line with the 8% decrease estimated by Li et al (2016) and the 6% reduction obtained by Grandemange et al (2014b) stabilising the wake using a vertical control cylinder. A slightly higher improvement of C D Rear was obtained by Evrard et al (2016) applying a cavity at the base of a similar geometry, arguably as a consequence of the further weakening of the interaction between the model base and the vortical structure due to the depth of the cavity. The time-averaged value of the pressure drag (Table 2) ) data set is 0.004 higher than that reported for the same geometry in Pavia et al (2016), due to the coarser distribution of pressure taps used in the present study.…”

“…Since the flow field analysed in the present study is highly sensitive to any asymmetry present in the experimental setup, the model was yawed to the onset flow until the most symmetric base pressure distribution was achieved, following a similar procedure to that adopted by Evrard et al (2016). The resulting value of yaw angle was assumed to be where the model axis and onset flow axis were aligned.…”

“…Applying a cavity to the base, Evrard et al (2016) found that the strength of the vortex system associated with the reflectional symmetry breaking mode decreased as the cavity depth was increased, yielding a reduction of the drag until a saturation was reached for a depth d > 0.24H . The stabilising effect of the cavity was modelled using a Langevin equation.…”