This study aims to clarify the flow characteristics and wake structure of convertible vehicles. Numerical simulations are performed to obtain a preliminary visualization, and the potential vortical motion characteristics are investigated by examining the Q-criterion across multiple cross-sections. Comparisons between numerical and experimental results validate the reasonableness of our numerical model. The predominant wake topology of a two-seat convertible is obtained in terms of the location, shape, and spin direction of the vortices. We observe a "nook" vortex that is triggered by the flow acceleration induced by the pressure gradient near the windshield step, provoking undesirable aeroacoustic noise and degrading the cabin comfort. Complicated A-pillar vortex dynamics are revealed, with small vortices that are shed into the cabin and impinge the seats, eventually forming a long tail structure above the back of the vehicle. Moreover, periodic fluctuations of the windshield vortex are induced by the Kelvin-Helmholtz instability, significant impacting the streamwise wake. Ultimately, the combined motion characteristics of the A-pillar and windshield vortices exert undesirable effects on the aeroacoustic noise and drag, suggesting fundamental mechanisms for achieving optimal energy-saving and acoustic convertibles in the future. Based on the wake topology and the vortical generating mechanism, approaches are proposed to reduce the drag and aeroacoustic noise by impeding the flow over the door into the cabin and modifying the shape of windshield step, and lengthening the windshield in stream direction.

Characteristics of the generation and propagation of aerodynamic noise are unique to conventional ground vehicles or aircraft due to the wing-in-ground (WIG) phenomenon. This study numerically examines the aeroacoustic characteristics of a multi-directional wing under the WIG effect with different values of clearance. The flow field was simulated by using a large eddy simulation for six groups of wings with different clearances and one group of wings in free space, at a freestream velocity of 0.3 Ma and an angle of attack of 5°. Acoustic simulations were carried out for each condition by using Möhring's analogy, and the acoustic analysis was based on details of the flow field. A multi-vortex system was generated at the trailing edge of the wing when the clearance was lower than 0.2 C. It consisted of an oscillating attached vortex that expanded and shrank with continuous changes in energy, and two periodically shedding vortices were formed that generated a strong wall-bound vortex street downstream of the wing after shedding. An analytical approach was used to simultaneously analyze different dominant frequencies in each region of the sound field. We conclude that different intensities of the WIG effect led to remarkable differences in the characteristic frequency of aerodynamic noise induced by the multi-directional wing. The characteristic frequencies of aerodynamic noise of the multi-directional wing with a low clearance (clearance ≤ 0.2 C) were mainly caused by the wall-bound vortex street while those at a high clearance (clearance ≥ 0.4 C) were mainly caused by vortices on the wingtip.