Summary

The flow past open cavities is a phenomenon that is encountered in many engineering applications and can result in intense acoustic tones. Earlier investigations based on full scale aircraft have indicated that there are many sources that contribute to airframe noise, with one of them being cavity noise. For example, during the landing, there is an increase in the sound pressure level from aircraft that can be associated with the presence of a cavity created by the open landing gear (Shieh, 2000). The flow-induced cavity noise can also occur in automotive vehicles, in which the most relevant aerodynamic noise sources to occupants are from gaps in doors, windows and sunroof, as well as from the air conditioning and ventilation systems. Despite its geometric simplicity, the physics associated with the flow over cavities is a complex phenomenon. For instance, the interaction of vortices with the cavity trailing edge generates a recirculating flow region in the cavity and results the instabilities in the shear layer, yielding both broadband and tonal noise. In this study, the noise radiated from a two-dimensional cavity with a lip over the upstream portion of the cavity submerged in a turbulent subsonic flow is numerically analyzed by using a hybrid scheme that couples flow computations with an implementation of the Ffowcs Williams-Hawkings equation. The turbulent near field is computed through Large Eddy Simulation. The model verification was carried out by comparing results for pressure obtained with different levels computational grid refinement. The physical model was validated with reference to experimental and numerical results available in the literature. The numerical results revealed some significant effects on the cavity- flow due to presence of a neck or a lip over the cavity. Besides the shear layer mode commonly found in cavities an oscillating mechanism characterized by low and high amplitudes as also been observed. Although this new mechanism occurs in a short period, it significantly affects the frequency and amplitude of the different modes. Furthermore, it has been noticed that the neck or lip over cavity is a viable alternative to reduce the acoustic noise associated with cavities.

Material for download