Flow control of D-shaped bluff bodies using attached dual membranes

In this study, a numerical investigation is conducted on the flow and aerodynamic performance of a D-shaped bluff body passively controlled by two flexible membranes affixed to its trailing. Despite the promise of passive flow control using deformable structures, this specific configuration has received limited attention in prior research. To address this gap, we systematically explore the influence of control parameters, i.e., membrane length and stiffness, on wake dynamics and force characteristics at a Reynolds number of 300. The results reveal that, relative to the uncontrolled bluff body, the integration of rigid membranes leads to notable reduction in both the time-averaged drag coefficient (C_d‾) and the root-mean-square lift coefficient (C_{l_rms}). Moreover, control effectiveness improves with increasing membrane length, primarily by delaying flow separation and suppressing vortex shedding. Beyond rigid configurations, membranes with optimized flexibility exhibit even greater aerodynamic benefits, arising from qualitatively different fluid–structure interaction mechanisms that depend on the dynamic flapping behavior of the membranes. Within the explored parameter space, three distinct flapping modes are identified: chaotic flapping, contact flapping, and periodic flapping. Each mode exhibits characteristic kinematic behaviors and aerodynamic responses that significantly affect flow control performance. Among them, the contact flapping mode-defined by contact between two filaments yields the optimal performance gains locally, achieving a 23.0 % decrease in C_d‾ and a 92.6 % decrease in C_{l_rms} at a non-dimensional membrane length of l* = 2.0 and bending stiffness of k* = 0.01. The periodic flapping mode, characterized by sustained and regular flapping motion without filament contact, also demonstrates considerable performance improvements, achieving a 17.3 % reduction in C_d‾ and a 53.8 % reduction in C_{l_rms} at l* = 1.75 and k* = 0.1. To elucidate the underlying mechanisms, a detailed investigation of the flow structures, pressure fields, membrane kinematics, and aerodynamic force components is conducted. This study provides the first systematic mapping and analysis of three distinct flapping modes in a D-shaped bluff body with dual membranes, establishing clear correlations with aerodynamic forces and flow structures. The insights gained from this study may enhance the understanding of fluid-structure interaction in passive flow control and offer valuable guidelines for aerodynamic optimization in related engineering applications.