Mechanism And Control Of Flowinduced Noise Generation From Two Non-parallel Cylinders

The landing gear noise is one of the most important noise sources of fuselage noises.Since the landing gear can be modeled by two non-parallel cylinders,the global researchers have been applying the theories of flow around cylinder and flow control techniques to the exploration of the noise formation mechanism of landing gear and the development of noise suppression techniques.However,there are few studies on the flow around non-parallel tandem cylinders.Therefore,the available theory and techniques are very limited.The formation mechanism of aerodynamic noise of non-parallel cylinders was well studied through wind tunnel experiments.The experiment was carried out at a Reynolds number of 4.3×104.Helical cables were applied as passive noise suppression method.The vortex shedding in the gap and wake regions was measured by hot-wire anemometer.Meanwhile,noise radiated from the tandem non-parallel cylinders was detected by a microphone.Through the flow visualization experiment,the effect of the spacing ratio L/D?where L is the distance between the centers of the two cylinders?and the helical cables on the flow field around the non-parallel tandem cylinder was intuitively compared.A large amount of quantitative flow field data was obtained through PIV experiments.The experimental results show that the aerodynamic noise of tandem non-parallel cylinders is generated by the combination of the flow separation/vortex shedding of the upstream cylinder shear layer and the attachment of the shear layer to the downstream cylinder.The helical cables can change the direction of the gap flow,make the planar flow three-dimensional,and inhibit the attachment of the shear layer from the upstream cylinder to the downstream cylinder.The optimal noise control case is d/D=0.05,?/D=1.0?where d is the cable diameter,? is the thread pitch?.In this control case,the maximum noise reduction value ?SPL_max=11 dB is obtained at L/D=2.03.The optimal number of helical cable varies with the thread pitch?N=1.0 for ?/D ?1.0,N=2.0 for ?/D>1.0?.The optimal rotation of helical cable is U?RL?D?R??U represents upstream cylinder,D represents downstream cylinder,L represents left-handed helical cable,R represents right-handed helical cable?,which obtained an overall noise reduction by 12 dB.