Mechanics of bluff body drag reduction during transient near-wake reversals

The influence of background turbulence on Ahmed-body wake bistability

“…This hypothesis can explain why the frequency of switching is proportional to the free-stream turbulence. We also note that this seems to accord with the observations of Haffner et al (2020) who linked the stability of each mode to a mechanism whereby large-scale structures are formed from the forcing and amplification of the shear layer by an interaction between the strong recirculating flow and the opposite shear layer. Given the stability of the mode relies on the strength of the recirculation, it follows that perturbations that disrupt the recirculation will also reduce the stability of the mode.…”

“…It is also particularly noteworthy that, despite the range of incoming flow profiles and turbulence levels, the lateral symmetry breaking modes are still favoured for all cases, and the distribution of the conditionally time-averaged base-pressure profiles are almost unchanged. As a corollary, while the perturbations here are disruptive, they do not prevent the interaction between opposite shear layers, a condition that has been identified during the period of transition between symmetry states with associated increased base pressure, as described by Haffner et al (2020).…”

“…Barros et al (2017) were able to control the wake flows through applying grids and cylinders to an underbody, while Bonnavion & Cadot (2018) examined the effects of ground proximity and inclination. More recently, Haffner et al (2020) have applied a combination of passive and active control methods to re-examine the drag reduction in the intermediate state, with ramifications for flow control, while Dalla Longa, Evstafyeva & Morgans (2019) employed large-eddy simulations to model bistability, and Kang et al (2021) numerically examined suppression of bistability through immersing the body in a thick boundary layer. Efforts to understand the mechanism responsible for the bistability have found a strong correlation between the strength of the recirculation and the stability of the barycentre of pressure (Barros et al 2017;Varon et al 2019;Haffner et al 2020).…”

Unsteady Coanda effect and drag reduction for a turbulent wake

“…The unsteady Coanda effect involving a strong interaction between high-frequency forcing and small-scale curved surfaces produces a strong cross-flow momentum. It is thus also currently being investigated as an active fluidic flap device in order to act on the asymmetries of the Ahmed body by controlling the shear layer interaction mechanism described by Haffner et al (2020) and extend and generalize the recent findings about yaw asymmetries control of Li et al (2019).…”

Rare Event-Triggered Transitions in Aerodynamic Bifurcation

“…Such phenomena have been investigated in laboratory experiments, including thermal convection [1][2][3], Couette flows [4], von Kármán swirling flows [5][6][7], rotating turbulence [8], or the dynamo instability [9]. It is also observed in the atmosphere [10][11][12], with dramatic consequences on climate, or in industrial systems [13][14][15][16][17][18], with implications on energy efficiency and fatigue of the structures. In spite of their variety, these systems share the feature of exhibiting spontaneous transitions between multistable large-scale modes, with waiting times ahead of transitions exponentially distributed [1][2][3]6,7,19], and characteristic times several orders of magnitude larger than any hydrodynamic time of the underlying flow.…”