Near wake interactions and drag increase regimes for a square-back bluff body

Abstract

The three-dimensional near wake of a square-back bluff body in ground proximity is experimentally perturbed by placing a pair of D-shaped obstacles under the body. Five obstacle widths $d$ , from 12 % to 26 % of the height of the body, are used to perform a sensitivity study of the body's pressure drag by varying the relative distance $l$ between the obstacle pair and the base. Two successive drag-sensitive regimes are identified for obstacle-to-base distances $l/d < 2.5$ , where the pressure drag of the body is increased up to 22 %. In different regimes, the flow dynamics measured downstream of the obstacles are found to be very different. When the obstacles are the closest to the base, $l/d<1.5$ , the pressure drag changes of the main body are driven by mean merging between the wakes of the obstacles and of the main body, and scale with $d$ . Contrarily, when the obstacles are located farther from the base, $1.5< l/d<2.5$ , the wakes of the obstacles are isolated from the main body wake. Here the dynamics of the obstacle wake drive the pressure drag changes of the main body, which scale with $d^{2}$ . In both regimes, we measure a mean mass transfer from the wake of the main body to the wakes of the obstacles. This is the main mechanism responsible for the pressure drag changes. Using our results and reference studies describing the effects of base suction on the pressure drag of bluff bodies, a physical model is proposed to explain the contrasting scalings of the pressure drag increase in the different regimes observed in this study.

Comparison of flow characteristics behind squareback bluff-bodies with and without wheels

“…However, here, it seems possible that the experimental results were overcorrected by blockage, e.g., the C b of the wW case was corrected from 0.291 (Urquhart et al, 2020) to 0.184 (the reference area here is the projected frontal area including the wheels). The contribution of the base suction to the total drag for the nW model in this study is about 84%, which is higher than the value reported by Varney (2020), but is consistent with that achieved from the experiment of Bao et al (2022). More convincing evidence is that the center of the vortices in the wake fits well in the match between current simulations and the PIV data, as shown in Fig.…”

“…Note that less switching occurs in case wW, suggesting a more stable asymmetry state than in case nW. The same trend has also been identified by Bao et al (2022) that the wheels (or the obstacles) have the ability to promote the appearance of bimodal dynamics in some cases. This will be explained in Sec.…”

“…Note the underbody flow rate in case wW2 is only 8% less than in case wW, but the inclination angle of the bottom flow (measured by the tangent to the bold streamline at x à ¼ 1) increases from 19 to 26 . This emphasizes the high sensitivity of bi-stability to upstream perturbations (Grandemange et al, 2013b;Plumejeau et al, 2020;and Bao et al, 2022).…”

“…As a result of the randomness of the long-time dynamics, the symmetry breaking position of case nW and wW happens to predominate on the right side (state R) in the current time window. Such statistical asymmetry induced by insufficient acquisition time is very common in the literature(Grandemange et al, 2013a;Cadot et al, 2015;Dalla Longa et al, 2019;Pavia et al, 2020;Plumejeau et al, 2020;Kang et al, 2021;Burton et al, 2021;and Bao et al, 2022) when bi-stability is involved. This is because achieving perfect statistical symmetry requires a recording time of over 60 times the long timescale T l $ 10 3 H=U 1(Volpe et al, 2015), which is time costly even for experiments.…”

Wake bi-modality: the effect of upstream boundary layer dynamics

“…Previous studies have shown that the wake switching is sensitive to the underbody flow. Experimental studies have triggered wake switching by disturbing the underbody flow, for example using a passive device in the clearance between the body and the ground (Barros et al 2017;Bao et al 2022). Here, we consider that the disturbances in the underbody flow may in part be caused by the interaction of smaller vortices from other surfaces due to the boundary layer dynamics.…”