Adjoint-based shape optimization using lattice Boltzmann method for flow and sound control in tandem cylinders

Aerodynamic noise control for flows with multiple bluff bodies is important in applications such as the pantographs of high-speed trains and landing gears of aircraft. In this study, aeroacoustic shape optimization is performed to develop an effective passive control technique for mitigating the flow-induced sound generated by a cylinder in the wake of another cylinder, focusing on twodimensional laminar flow past two tandem cylinders at Reynolds and Mach numbers of 160 and 0.2, respectively. The shape optimization aimed at minimizing sound generation employs the lattice Boltzmann method and the unsteady adjoint method. The results highlight the benefits of diminishing the front surface curvature and adding protrusions to the side surfaces of the downstream cylinder. These changes suppress flow acceleration and negative pressure fluctuations when the stagnation point shifts owing to upstream wake oscillation, while mitigating positive pressure fluctuations through an increased flow velocity near the shifted stagnation point. Consequently, the modifications lead to a reduction in lift fluctuations and dipole sound generation, achieving a sound reduction of 2.4 dB compared to the original circular shape. However, the optimized shape significantly increases the mean drag force, indicating a trade-off in the passive control strategy.