Experimental study on the effect of adaptive flaps on the aerodynamics of an Ahmed body

Abstract

We perform an experimental study on the turbulent flow around a square-back Ahmed body of height, , at varying , where 𝑢�∞ is the free-stream velocity and 𝜈� is the incoming flow kinematic viscosity, under different yaw angles 𝛽�=(0∘,−5∘,−10∘), to analyze the use of rear flexibly hinged parallel plates as a control strategy to reduce the drag in a self-adaptive manner under changing flow conditions. The model implements rear parallel rigid flaps of depth 𝑑�=0.5⁢ℎ, which are mounted with torsional joints through embedded flexible foils of calibrated thickness. This holding system restricts the motion of the plates to a rotary displacement, 𝜃�. The fluid-structure dynamics is characterized by the reduced velocity, defined as 𝑈�*=𝑢�∞/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�∞ 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 𝛽�=−10∘ (13.5% in comparison with a body with fixed rigid plates of the same depth). Such variations are shown to be associated with a passive reconfiguration process of rear flaps. Additionally, hinged flaps are shown to interact with the reflectional-symmetry-breaking (RSB) modes, typically present in the wake of three-dimensional bodies. At aligned conditions, the interaction with the RSB modes is characterized by two regimes, in such a way that the hinged flaps manage to partially stabilize the RSB modes, and consequently to inhibit the bistable behavior at low values of 𝑈�* (in a similar manner to rigid flaps), while at high values of 𝑈�*, they respond dynamically to switches between the opposite wake deflections of the RSB modes, deviating themselves accordingly.