An experimental and numerical study is conducted on a rectangular open cavity with a length to depth ratio of 2 at Mach number 1.71 by placing a subcavity at different locations. The subcavity at the front wall has already been established as a passive control device experimentally. In addition, it has been observed that it can act as a passive resonator. However, in the current study, it is found that the location of the subcavity and its dimensions play a crucial role in determining the types of oscillations existing inside the cavity. Cavity models with a subcavity length to main cavity length of 0.2 (l/L = 0.20) were investigated by placing the subcavity at the front wall, aft wall, and simultaneously at both front and aft walls. High speed schlieren visualization revealed the presence of different shock features associated with the cavity flow field. Statistical techniques such as fast Fourier transform, spectrogram, coherence, and correlation are employed to analyze the unsteady pressure data. Numerical computations are carried out to validate the experimental results and also to explore the flow physics. The front wall subcavity acts as a passive control device with a maximum reduction of 34.1 dB in the sound pressure level for the most dominant tone, and there is also a notable reduction in the overall sound pressure level by 11.7 dB. In the case of front wall subcavity, the acoustic wave gets inclined as it interacts with the subcavity, thereby displacing the shear layer to form a dome-shaped structure. The aft wall subcavity acts as a passive resonator with distinct fluid-resonant oscillations and the respective modal frequencies differ widely from those predicted using Rossiter’s expression. The shear layer interacts with a recirculation region formed inside the subcavity at the aft wall, thereby mitigating the effect due to direct impingement of the shear layer on the aft wall. The subcavity at both walls acts as a passive suppression device with a reduction of 34.9 dB in the sound pressure level for the most dominant mode and also with a reduction of 14.5 dB in the overall sound pressure level.
The effects of a Trapped Vortex Cavity (TVC) on the aerodynamic performance of a NACA 0024 airfoil at a constant angle of attack (AoA) of 14 • were investigated in this study. It was observed that mass suction (MFR) was required to stabilise the vortex within the cavity segment. Lift to drag ratio (L/D) and MFR were chosen as performance objectives, along with a fully attached flow constraint (flow separation at X/c ≥ 95% ). Parametric analysis was carried on the baseline airfoil with and without suction and compared to the airfoil with TVC with and without suction. It was observed that L/D increases as MFR increases for a baseline airfoil, and flow separation is delayed at high suction values (MFR = 0.2 kg/s). The TVC modifies the pressure distribution on the baseline airfoil when MFR is applied to the cavity section and there is a significant increase in lift; thus, L/D increases and flow separation is delayed. A lower value of MFR = 0.08 kg/s is sufficient to stabilise the vortex and improve the efficiency of the TVC airfoil. The findings of these parametric studies were used to do a multi-objective optimisation using a genetic algorithm to attain the desired cavity shape while achieving the largest L/D and the lowest MFR (that is proportional to the power required for control) with a fully attached flow constraint. It was found that mass suction and cavity shape both had an equal influence on flow control. The Pareto optimal front yielded a series of optimum designs. One of them was subjected to an off-design analysis in order to validate its performance at other incidences. It was observed that it performs better than the baseline airfoil, with an improved L/D and an increase in stall angle from 10 • to 14 • .
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