Aerodynamic Noise In Moving/Steadic Device And Experimental Research

The development of numerical methods and experimental techniques encourage people work hard to research the complex unsteady flow phenomena and sound induced by air flow and aerodynamic noise reduction problem in order to adapt to the increasingly serious energy and environmental requirements of the times.The low-Mach movement/ static devices are the research objecst and the physical models in this paper. vortex sond theory express the physical mechanism that vortex(flow field) induce sound. In the flow field, vortex structure tensile deformation produe sound.the areodynamic noise come fromvortex stretching and deformation, and ruputure. vortex as the source of sound field. sound and vortex may transform eacher other. well understandign and grasp of vortex sound theory and their interaction are the theoretical premise of the research work later in this article.considering the flow field and sound field experiments in the low-speed, quiet wind tunnel and anechoic chamber, the two experimental environment were introdued. To he wind tunnel, first some of the existing wind tunnel and related technologies were summaryed. then different parts of the wind tunnel design methods were analyzed, especially focuse on how to get low background noise for test section. To anechoic chamber some important concepts were introduced, such as cut off frequency.Based on in front of the vortex sound theory and relevant experimental conditions, using large-eddy simulation and the wall function method the flow field around square cylinder with the reducing of the gap between cylinder and wall was investigated. With the gap reducing, the symmetrical structure o flow field rear of the square cylinder has been the gradual destruction and low-speed region is expanding. As a result in the trailing edge the velocity gradient changes were reduced that is root of vorticity. And thus periodic vortex shedding was restrained. In low-speed muting wind tunnel flow, using visualization technique through a smoker we got a clear flow field structure seeing the behavior of vortex shedding clearly. Wall gap is sufficiently small, the vortex structure beneath of square cylinder is locked in a certain small area, and don't spread to downstream, so the periodic Karman vortex street disappears. Finally, by sound pressure test system, we research the characteristic frequency and sound pressure level with the different gap. Numerical and Experimental results are agreed with each other, and they illustrate When the gap is sufficiently small, the shed vortex is suppressed, thereby suppressing the generation of the characteristic frequency. Through control the wall distance to control vortex shedding, so as to achieve the purpose of reducing noise.Another basic physical model of this study is flow cross cavity which is typical vortex sound interaction problem based on that research of sunroof aerodynamic noise and active control were investigated. Steady flow simulations reveal flow structure and vorticity distribution of sunroof. At trailing edge area vortex impinged, broke up, and attach again. So here is the source of low frequency buffeting. Using hotwire, two different positions velocity distribution were measured. The results explain the reason of the sunroof aerodynamic noise. From this point, the active control method was introduced to control the velocity distribution in boundary layer to reduce the noise. In the experiment, one interest was found, that is with the speed increase the characteristic frequency has a regular form no to exist then disappear again. Using optimized deflector the sound pressure level was reduced 10 dB(A), at speed 25m/s.To the eccentric vortex unsteady flow and aerodynamic noise of low speed cross flow fan. In anechoic chamber the near field pressure and far field sound pressure of two different impellers were measured. The result illustrate that the noise of cross flow fan is broadband noise instead of discrete noise. To research the external characteristics a fan test system was design. Through flow field computational simulation, we conducted error analysis. Based the works above, the method of two-stage flow passages was set up the we put forward the aerodynamic noise prediction mode...