Passive control of aeroacoustic noise generation in transonic cavity flow via cylindrical rod

Abstract

Cavity flow is a phenomenon that is encountered in many applications in various industries. The flow over a cavity generates highly turbulent and complex flow structures which produce a significant level of noise. The noise generation is undesirable for several concerns depending on the application. Weapon bay of a modern fighter is one of the most important instances of cavity flow in aerospace industry. High amplitude tonal pressure fluctuations occurred within a weapon bay may cause damage on sensitive avionic devices or the stores. Additionally, the noise in the weapon bay increases the detectability of aircraft, which means that the primary purpose of internal store carriage is compromised. Therefore, control of cavity flow noise is a crucial issue for a modern fighter aircraft. In the present study, a numerical investigation was carried out about noise generation in transonic cavity flow and its passive control by a cylindrical rod. First of all, the numerical method was validated by an available experimental data in the literature namely, the M219 cavity case which involves transonic flow over a cavity with a length to depth ratio of 5. CFD analysis of the reference case was performed by Star-CCM+ using Detached Eddy Simulation (DES) model and the methodology was validated by comparing with the experimental data. High amplitude tonal noise was observed within the M219 clean cavity. Subsequently, several designs were developed for the aim of noise reduction by introducing a cylindrical rod with different diameters and positions around the leading edge. Using the same CFD methodology, each design was analyzed numerically and their attenuation performance was evaluated by comparing with the clean cavity. Overall sound pressure levels are examined at both cavity floor and rear wall. OASPL distribution is also calculated over the cavity symmetry plane. Moreover, frequency spectrum of the sound pressure level at the rearmost probe was calculated to determine mode frequencies and evaluate tonal noise reduction. Furthermore, the SPL spectrum was computed not only for a single point, but also for entire symmetry plane of the cavity, hence, SPL fields for discrete frequencies are also available. Therefore, unlike previous studies about the topic, the present study provides detailed spectral examination of cavity flow by providing the SPL fields at the mode frequencies. Additionally, Spectral Proper Orthogonal Decomposition method was utilized to reveal dynamically important flow structures that developed coherently in both time and space. Besides, pressure coefficient distributions on the cavity floor are examined to consider the designs also in perspective of loads on the store. The numerical results of each design are compared and their performances are discussed based on different aspects.