Wake transitions and steady -instability of an Ahmed body in varying flow conditions

Abstract

The paper investigates experimentally the flow past a flat-back, taller than wide Ahmed body having rectangular base aspect ratio $H/W=1.11$ in the context of ground vehicle aerodynamics. Parametric studies at Reynolds number $2.1\times 10^5$ explore the sensitivity of the aerodynamic force and body pressure distribution with respect to varying flow conditions defined from variable ground clearance $C$ (taken at mid-distance from the front and rear axles of the body), pitch angle $\alpha$ , and yaw angle $\beta$ (equivalent to a crosswind). Two-dimensional parametric maps in the parametric spaces $(\beta,C)$ and $(\beta,\alpha )$ are obtained for parameter ranges covering real road vehicle driving conditions. The study of the base pressure gradient reveals non-trivial and sharp transitions identified as significant changes of near wake orientation in both parametric spaces. All unsteady transitions correspond to fluctuation crises of the vertical gradient component only. These transitions are summarized in phase diagram representations. Both phase diagrams in $(\beta,C)$ and $(\beta,\alpha )$ parametric spaces can be unified at large yaw in the $(\beta,C_r)$ space, where the rear clearance $C_r$ separating the lower edge of the base from the ground is a simple function of the pitch $\alpha$ and the clearance $C$ . The impacts of the wake transitions are clearly identified in the base drag, drag, lift and side force coefficients. The contribution of the steady near wake $z$ -instability (Grandemange et al., Phys. Fluids, vol. 25, 2013a, pp. 95–103) is assessed by repeating the sensitivity analysis with a rear cavity. As reported previously, the rear cavity suppresses the steady instability by symmetrizing the wake. A domain for the existence of the instability is finally proposed in the attitudes map $(\beta,\alpha )$ defined from regions where the mean lateral force coefficients are significantly decreased by the presence of the rear cavity. In addition, it is found that the steady instability forces the wake to be less unsteady for all attitudes that do not correspond to unsteady transitions.

Towards adaptive drag reduction of a flat-back 3D bluff body in variable pitch and cross-wind configurations by flow orientation at the base separation

“…The high sensitivity of the aerodynamics with the body attitude at all pitch angles and small yaw is due to the steady wake instability observed for squareback design. It is known to produce a permanent asymmetry of the wake (Grandemange et al, 2012) and transitions of the wake asymmetry orientation in the (α, β) attitudes plane (Fan et al, 2022). It is observable from the base pressure gradient as reported in Fan et al (2022) and in Fig.…”

“…Pressure is measured with a Scanivalve ZOC33/64PX pressure scanner placed inside the body. The sampling frequency is 1 kHz per channel with an accuracy better than ±0.5 Pa on converged mean values (see Fan et al 2022 for further details). The static pressure p ∞ of the test section is used to compute the instantaneous pressure coefficient:…”

“…For each of the 36 rear designs (ϕ t , ϕ b ) that are manually fitted, the aerodynamic force and the base pressure distribution are recorded for the 57 body attitudes using an automated displacement system. The experimental technique used to avoid the force balance drift during the total duration of the automated experiment is fully described in Fan et al (2022). The acquisition time for each attitude is 50 s insuring a good convergence of the mean values as shown in Fan et al (2022).…”

“…While the first transition could be rare as it requires large yaw (about 10 • ), the latter occurs at few degrees only. The steady wake instability is also sensitive to the pitch as shown by the comprehensive experimental investigation of Fan et al (2022), emphasizing that pitch plays a significant role on the drag variation within a small yaw range.…”

The influence of vehicle body roll motion on aerodynamic characteristics under crosswind condition

“…Due to the influence of a complex surrounding environment, climate conditions and other factors, crosswinds are frequently generated in the vicinity of highways (McTavish and McAuliffe, 2021; Wang et al , 2023; Wu et al , 2023). When vehicles move at high speeds under the influence of these crosswinds, the airflow around the vehicle becomes asymmetric (Lorite-Díez et al , 2020; Fan et al , 2022). This asymmetry leads to more pronounced changes in the vehicle’s body roll attitude and increases vulnerability to rollover accidents.…”