Research On Noise Prediction Of Submerged Jets And Turbulent Boundary Layer

This paper focuses on mechanism of hydrodynamic noise and its prediction methods. And it includes two parts. In the first part, theoretical and experimental investigation is done on the subjects of noise prediction methods and noise mechanism of submerged free jest, which is a typical instance of free turbulence. In the second part, noise prediction methods of turbulent boundary layers is discussed, some available results is presented.In the first part, theory of acoustic analogy, Kirchhoff formulation and theory of vortex sound are applied to analyze and predict the radiated noise of submerged jets. And the applicability of each theoretical approach is also discussed. Then a pressure tank is used as a reverberation chamber to test noise radiation of submerged jets experimentally. The results are used to perfect theoretical approaches.Lighthill acoustic analogy and Lilley acoustic analogy are introduced to the area of hydrodynamic noise prediction. They are the most popular approaches in fields of aeroacoustics. The application methods of such acoustic analogy and applicability are discussed in detail. Because of the adjoint equation of free wave equation is simple and can be obtained easily, the Lighthill equation is solved with methods of adjoint equation, which is much convenient the traditional nethods. Aiming at submerged jets, we rededuced Lilley's equation which takes the interaction between mean flow and sound propagation into account and leads to a much better directivity prediction of radiated noise. But it is difficult to solve out. Some examples are employed to illustrate the means of prediction noise radiated from submerged free jets, and useful results are presented for practical use.Kirchhoff formulation is introduced to the area of hydrodynamic noise prediction, the most advantage of which is that the attribute of sound source is unnecessary to know in advance. As a cost, the flow field must be simulated by DNS or LES methods, which is the biggest obstacle of practical application. The key point of Kirchhoff formulation usage is the placement of control surface, which should locate in the linear region. To evaluate the importance of the nonlinear sound source outside the control surface, a nonlinear correction model is introduced. Examples are used to illustrate and application of Kirchhoff formulation in the fields of noise prediction.Powell's vortex sound equation and Howe's vortex sound equation are introduced and are solved by means of Green's function. With examples the vortex sound theory application are displayed, and the difference of the two theories are compared. Results show that Howe's equation has much better performance in the prediction directivity, which can take the interaction between mean flow and sound propagation into accounts.A pressure tank is taken as a reverberation chamber to test noise radiation of submerged jets experimentally. Three kinds of nozzles (straight nozzle, shrinkage nozzle and expansion nozzle) are used to investigate submerged jet noise. The test results are used to determine the constants contained in Lighthill acoustic analogy. Radiation directivity can not be considered in reverberation chamber test, and the small dimension of pressure tank makes about 1 KHz low frequency limitation. These are the two disadvantages of our experimental efforts.In the second parts, six kinds of widely applicable wavenumber-frequency model of turbulent boundary layer pressure fluctuation are introduced and compared to research the noise prediction method of TBL. The results and comments are available for choice of wavenumber-frequency model in engineering practice.