Study On The Aeroacoustic Characteristics Of Cavity With Complex Structure

Many aerodynamic investigations have been focused on the study of cavity problems over the past decades and most of them have been carried out on the simple cavity with transonic and supersonic speeds. But the cavity structures are more complex and the flow speeds are usually lower than the sound speed in the common engineering practices. So, the studies on aeroacoustic characteristics of the cavity with complex structure at quite low speeds are more valuable. However, there are few studies and published papers that focus on the problems of cavity with the complex structure, although many investigations have been carried out on the studies of some quite difficult problems for the simple cavity by the researchers at home and abroad, such as the shear layer separation, the aerodynamic noise, the interactions between compressed wave and shear layer, etc..The insert pipe muffler and the oil separator model were selected as the major research objects in present study. The studies on aeroacoustic characteristics of them were carried out and analyzed quite in detail by the experiments, the theory analysis and the FEM analysis. Moreover, the whistle tone frequencies are predicated. Finally, a general study method on the aeroacoustic characteristics of cavity with complex structure is put forward in this dissertation. The major research contents are presented as follows,1,The experimental test rig of study on the aeroacoustic characteristics of cavity with complex structure was set up and the flow was supplied by an open circuit centrifugal fan driven wind tunnel with low pressure and low speed. The effects of flow, tailpipe, cavity, flow-acoustic feedback loop inside the cavity, etc. were detailed analyzed.2,Three types of the whistle tones are included mainly in the aeroacoustic characteristics of insert pipe muffler. The first one appears both inside and outside the mufflers and it is closely related with the acoustic modes of tailpipe. The second one is not captured both inside and outside the muffler with small length-to-diameter ratio (L/D), but appears both inside and outside the muffler with middle L/D, and only inside the muffler with large L/D. Furthermore, it has a close relationship with the acoustic modes of cavity. The third one is captured only inside the mufflers with small and middle L/Ds, but disappeared both inside and outside the muffler with large L/D. Moreover, it is generated by the flow-acoustic feedback loop inside the cavity.3,The transmission loss (TL) formula of insert pipe muffler is given by the plane wave method, the acoustic transfer matrix method, and the two-dimensional theory method, as well as the comparisons of these methods are also presented. Furthermore, it is proved that the whistle peak frequencies of the first type and the second type whistle tones can be predicated accurately by the plane wave method and the 3D FEM method.4,Two types of the whistle tones are included mainly in the aeroacoustic characteristics of oil separator model. The first one appears both inside and outside the oil separator model and it is closely related with the acoustic modes of cavity. The second one is only captured outside the oil separator model and it has a close relationship with the acoustic modes of tailpipe.5,The study on aeroacoustic characteristics of the oil separator model can be carried out by the plane wave method and the 3D FEM method but not for the acoustic transfer matrix method and the two-dimensional theory method at present time. Moreover, it is found that the whistle peak frequencies of the first type whistle tones can be predicated accurately by the plane wave method and the 3D FEM method and for the second type whistle peaks, the plane wave method can be used efficiently but the 3D FEM method are not suitable for predication of the whistle peak frequencies.6,The study on aeroacoustic characteristics of the cavity with complex structure has been carried out by the Rossiter's re-modified formula, which is revised based on the Rossiter's modified formula that is widely used in the investigations of simple cavity problem.7,A general study method on the aeroacoustic characteristics of cavity with complex structure is put forward.