Self-adaptive elastic flaps with bending and torsion for 3D blunt body drag reduction

TL;DR

This work demonstrates that simple, self-adaptive rear flaps on a square-back Ahmed body can significantly reduce drag in three-dimensional wakes under crosswinds. By comparing top-bottom (TB) and left-right (LR) edge configurations with rigid (RF) and elastic hinged (1HF, 2HF) flaps, the study shows that TB with bending and torsion (2HF) achieves the best wind-averaged drag reduction of $7.62\%$, while also damping wake vortices and turbulence. The results reveal that the flexible TB devices maintain a rear cavity effect that elongates the recirculation region, suppress the Reflectional Symmetry Breaking (RSB) mode, and reduce base suction more effectively than rigid or LR configurations, especially at yaw. These findings suggest that low-DOF, self-adaptive rear devices can offer robust drag reduction in crosswind conditions for blunt-body vehicles, with potential guidance for future design of adaptive aeroelastic control surfaces.

Abstract

This study investigates the potential for drag-reduction of low-mechanical-order, self-adaptive control systems, consisting of hinged flaps attached along the edges of the rectangular base of a canonical blunt body. Comparative experiments are conducted in a wind tunnel under crosswind conditions at a Reynolds number of $Re = 2.13 \times 10^5$. The flaps, made of rigid rectangular panels, are mounted in three configurations: rigidly fixed%(RF) , flexibly hinged with a single degree of freedom in bending, and flexibly hinged with two degrees of freedom, allowing both bending and torsion, the latter representing a novel drag-reduction device, easily tunable to ensure a quasi-steady, stable adaptive reconfiguration. Two geometric arrangements are tested: horizontal flaps attached along the top and bottom (TB) edges, and vertical flaps along the lateral (left and right, LR) edges. The experimental study includes force, pressure, flap deformation and wake velocity measurements at varying yaw angles to simulate crosswind conditions. When the body is aligned with the flow, both arrangements reduce drag due to a rear cavity effect that elongates the recirculating flow. The TB arrangement is found to be much more effective at reducing drag in yawed conditions and its performance is improved using the flexible hinges. In these cases, static deformations correspond to boat-tailing that reduces the induced drag together with the turbulent kinetic energy in the wake. The use of the wind-average drag coefficient (taking into account events of crosswind) to evaluate an effective drag reduction clearly shows the TB arrangement with bending and torsion as the best appendage, with a 7.62\% drag reduction compared to the body with no appendages, proving the good performance of simple, two-degrees-of-freedom control systems to adapt to changing three-dimensional wakes.

Figures (13)

• Figure 1: $(a)$ Lateral, $(b)$ top, and $(c)$ rear views of the squareback Ahmed body inside the wind tunnel. $(d)$ Sketch of the tested configurations for the Top-Bottom (TB) flap arrangement. $(e)$ Sign criteria used for the calculation of deflection angles. $(f)$ Sketch of the flap position for the determination of bending, $\theta_b$, and torsion, $\theta_t$, angles.
• Figure 2: Detailed sketch of the flexibly hinged systems for $(a)$ 1HF and $(b)$ 2HF configurations. In $(b)$ a Triple Lamina Emergent Torsional (LET) joint is used to confer an elastic deformation in bending as in $(a)$ and an additional elastic deformation in torsion.
• Figure 3: Sketch of the performed free-decay tests for bending ($a$) and torsional ($d$) motions. Temporal evolution and power spectral density (PSD) of the response of the flap in terms of bending ($b, c$) and torsional ($e, f$) angles.
• Figure 4: $(a-f)$ Evolution of the time-averaged (cyan symbols) and Probability Density Function (PDF) contours of the force and pressure coefficients with the yaw angle $\beta$ for the baseline case (B) at $Re= 2.13 \cdot 10^5$. Force and pressure coefficients: Drag, $c_x$, base drag, $c_B$, lateral force coefficient, $c_y$, horizontal base pressure gradient, $g_y$, vertical force coefficient, $c_z$, and vertical base pressure gradient, $g_z$. Blue and red points represent the conditionally averaged values for N and P state respectively in $z$-axis.
• Figure 5: For $\beta = [0:5:15]^\circ$ at $Re= 2.13 \cdot 10^5$: Time-averaged contours of $(a)$ streamwise velocity, $U_x^*$, $(b)$ magnitude of streamwise vorticity, $\overline{|\omega_x^*|}$ , and $(c)$ turbulent kinetic energy, $K^*$, at $x^{*}$ = 0.915 for B configuration. Thin white lines illustrate the flow streamlines ($U^{*}_{y}$, $U^{*}_{z}$), while thick white line shows the isoline of $U_x^*=0$.