| Flow noise is being closely watched in many fields of engineering industries, its in-fluence is not only on the human senses, but also affect the hidden nature of some military means of delivery, such as helicopters, submarines, etc.. In the field of aerodynamic noise, Lighthill firstly did the research of aircraft engine jet noise by acoustic analogy theory, to open aeroacoustics research. After decades of development, computational aeroacoustics (CAA) is widely used in aircraft, automobiles, fans, pumps and other industrial fields. For the field of underwater noise, due to the sensitivity of its military, we have rarely seen pub-lic research. As the sound waves are the only form of energy in the ocean long-distance transmission, the submarine noise has a significant impact on its survival and weaponry, and it is an important indicator of the submarine concealment. Therefore, the study of water downstream noise is of great significance.Currently, according to whether or not to apply the theory of acoustic analogy, noise prediction numerical method is mainly divided into two categories:one is a direct simula-tion, and the other is indirect simulation, also known as hybrid simulation. Direct simula-tion method using the Computational Fluid Dynamics(CFD) method to directly solve the approach of the Navier-Stockes equation to get the flow field and the sound field at the same time. This simulation theory is the most direct and accurate. But numerical methods have to meet the necessary requirement for lower values dispersion and dissipation. Moreover, the calculating the area is usually large and involving turbulence calculation, computing resources are also very high. On the other hand, in the indirect simulation method, the calculation area is usually divided into two regions, i.e. the sound source area and the prop-agation area. The flow field was calculated, and then the resultant fluid velocity fluctuation is brought into acoustic analogy equation to obtain a relatively large spread on noise radi-ation field from a relatively small region of the sound source, using the general method of CFD. The mixed method at the same time greatly reduces the computation, but also can be applied to existing acoustic similarity theory for noise analysis. Nevertheless, indirect simulation, the simulation sound source with the direct method, the calculation method has a higher requirement.Lattice Boltzmann Method (LBM) is a new and efficient computational fluid dynamics method. It is a series of characteristics that makes it a very unique advantage in sound field simulation. Although it has been obtained some of the more successful applications in the sound field simulation especially in CAA field simulation, the development of a number of dedicated to simulate the sound field model and the corresponding boundary treatment methods. However, for underwater noise prediction, it not yet matures. This paper is the use of this advanced tool LBM, to carry out a systematic study. The main work consists of three parts:1. For direct simulation of flow noise prediction methods, we compare the common and emerging moving boundary conditions, including three kinds of bounce-back (BB) methods and four kinds of immersed boundary (IB) methods. Then we analyzed and evaluated them for the adaptability of flow acoustics. In order to reduce the spurious pressure generated by the fresh grid node changing from solid domain to fluid domain for BB methods and sharp IB methods, we proposed four new kinds of treatments and compared them. By simulating the flow over a circular cylinder pipes, acoustic cylindrical reflection, drag the flow over a cylinder and a cylindrical periodic vibration problems, we can get the following conclusions:(1) For a stationary boundary issues, BB and IB can accurately describe the physical phenomenon. In these three BB methods, interpolation-bounce-back (I-BB) and uniformed and interpolation-bounce-back (UI-BB) methods are more accurate than link-bounce-back (L-BB). In several IB mehtods, the implicit direct-forcing (Implicit-IB) method and ghost fluid (Ghost-IB) method are higher accuracy than others.(2) For moving boundary problems, due to the emergence of the fresh node, all of BB and sharp interface IB methods have to deal with the fresh node. In this proposed four new treatments of the fresh node, the local evolutionary iteration (LI) can more effectively reduce spurious pressure waves fluctuation. In addition, the diffused interface IB method for moving boundary problem has a natural advantage, it does not catch the fresh node problem. In the simulation results of several boundary conditions for the moving boundary problems, we can find Implicit-IB, UI-BB and Ghost-IB are better, they are more suitable for simulations of flow noise problems.2. For the direct simulation of flow noise prediction method, we combine two kinds of immersed boundary methods, and propose a new sharp interface immersed boundary, which satisfied geometric continuity conditions. The main idea of this new method is a mixture of two types of immersed boundary methods:sharp and diffused interface immersed boundary methods, where the sharp one we used is ghost fluid (GF) method, and the diffused one we used is the partially saturated computational cell (PSC) method. These two methods have advantages and disadvantages. PSC can meet geometric continuity conditions (GCL), and there is no unknown node in the new fluid. However, its accuracy is so limited, and it does not satisfy the no-slip boundary condition. The GF as a sharp interface immersed boundary is to meet the no-slip boundary condition, and meet the Dirichlet and Neumann boundary conditions, but it has to deal with the fresh node after boundary moves. It is diffi- cult to meet GCL, and it will lead to the spurious pressure fluctuations. It is because of the complementary characteristics of advantages and disadvantages of PSC and GF, we added PSC-like weight strategy into GF method, and made a series of numerical validations in this new method, including flow around a cylinder in a pipe, drag the flow around a cylinder and cylinder periodic vibration problems. From the numerical results of the validation, we can get the following conclusions:(1) For the stationary wall problem, this new method can well describe the physical characteristics of the wall boundary. When the relaxation time associated with the weighting factor, the accuracy is better.(2) For the moving boundary problem, this new approach can more effectively suppress spurious pressure fluctuations compared to traditional BB and GF methods. When using the relaxation time associated with the weighting factor, the accuracy is better. We can find that the calculation of the weighting factors in this method has a very important influence, the situation that the relaxation time associated with the weighting factor is closer to the properties of the real wall.3. For indirect simulation methods of the flow noise prediction, we combined LBM and K-FWH method, which has been widely proven in CAA field, to propose a hybrid LBM/K-FWH acoustic analogy method. Through numerical examples of plane waves, LBM is ver-ified the feasibility and accuracy in direct simulation. Through the simulation of radiation of the sine wave, we discuss the relation between the selection of the control plane and the simulation results. It confirmed the basic principles that the control surfaces must include all of the non-linear flow. By rotating elliptic cylindrical noise emission simulation, the LBM/K-FWH hybrid method is verified effective and efficient.In short, a comparative study of the moving boundary between the fluid and solid on lattice Boltzmann method, then the improvement measures are proposed; and we proposed a new moving boundary approach which meet geometric continuity conditions for acoustic modeling problems; Finally, the lattice Boltzmann and mature acoustic analogy theory combined to verify its effectiveness. |
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