Experimental study on the drag reduction of a 3D bluff body by rotary plates

We perform an experimental study on the turbulent flow around a squareback Ahmed body of height, h, at varying \(Re=u_{\infty}h/\nu \in [0.5, 1]\times 10^5\), where \(u_\infty\) is the freestream velocity and \(\nu\) is the incoming flow kinematic viscosity, under different yaw angles \(\beta=(0^{\circ},5^{\circ},10^{\circ})\), to analyze the use of rear flexibly-hinged parallel plates as a control strategy to reduce the drag. The model implements rear parallel rigid flaps of depth d = 0.5h, which are mounted with a torsional joint through embedded flexible foils of calibrated thickness. 
This holding system restricts the motion of the plates to rotary displacement, \(\theta\). The fluid-structure dynamics is characterized by the reduced velocity, defined as \(U^*=u_{\infty}/f_n h\), where $f_n$ is the natural frequency of rotary oscillations of the hinged plates, measured in free decay tests of flaps. In fact, we have explored the range of reduced velocity, \(U^*=[0,65]\), varying \(u_\infty\) and consequently Re. We perform force and pressure measurements to quantify the variations of the drag and the base pressure coefficients while laser displacement sensors are used to obtain the angular flaps motion. Results show that the hinged plates decrease the drag coefficient of the original body by nearly 4.4% for flow conditions aligned with the body axis. Under cross-flow conditions, their efficiency is even larger, attaining relative reductions drag of nearly 9.1% at \( \beta =10^\circ\) (13.5% in comparison with a body with fixed rigid plates of the same length). Such variations are shown to be associated to a passive reconfiguration process of rear flaps. Additionally, hinged flaps are shown to interact with Reflectional-Symmetry-Breaking (RSB) modes, typically present in the wake of three-dimensional bodies. The interaction with the RSB modes is characterized by two regimes, in such a way that the hinged flaps manage to suppress bi-stable behavior at low values of \(U^{*} \)(in a similar manner to rigid flaps), while they respond dynamically to switches between deflected states of the RSB modes, deviating themselves accordingly at high values of \(U^{*}\).