Malyuga V. Unsteady flow problems with allowance for effects of sound radiation

The thesis is devoted to the study of the stable self-oscillation generation in the course of the interaction between the flow and solid bodies, as well as to the study of the sound radiation processes by the flow. The mechanisms of the self-oscillation generation and the hydrodynamic feedback channels are described. The characteristics of the sound fields generated when the flow dashes against a solid body are estimated. The mathematical models and methods for accurate estimation of the energy and spatial characteristics of the sound fields generated when the flow interacts with solid obstacles of various forms are developed. We develop the methods for description of the vortex sound sources in both the external flows and the internal flows in channels of complex geometry. We also develop the technique for estimation of the sound fields generated by the flow. The technique is based on the modern approaches of computer fluid mechanics and allows us to perform numerical calculations at cluster supercomputers. The algorithm of solution which belongs to the class of hybrid methods, is constructed. The hybrid methods allow us to decompose the problem of sound generation by a flow into two stages. At the first stage the hydrodynamics of the flow is calculated, at the second stage the characteristics of the acoustic field generated by the flow are evaluated. For numerical simulation of two-dimensional fluid flows we used the DNS technique. The required accuracy of calculations was achieved by improving the computational grid. In the case of three-dimensional flows, the DNS methods were used only in laminar flow regimes. In turbulent regimes, we used the LES technique. The reason is that the three-dimensional problems require an order of magnitude more computer resources and consequently it is impossible to achieve the required accuracy of numerical simulation by simply increasing the number of nodes in the computational grid. The MPI technology was used for parallel computing. The parallelization was carried out on the principle of geometry parallelism. When performing numerical calculations we used the computational procedures of the open source toolbox OpenFOAM. At the second stage of the constructed hybrid method the acoustic problems were solved. For this purpose, the analytical solutions of the acoustic problems were constructed. For the purpose of verification of the constructed hybrid numerical algorithm we solved a number of problems. First, the sound field of the Aeolian tones was numerically calculated. Then, the problem of sound generation (wedge tones) by a submerged jet running against a sharp wedge was solved. And then the problem of the self-oscillations generation in the flow past a cylinder with a flat splitter attached to the rear side is solved. Since the vortex formation and shedding behind a cylinder can lead to undesirable vibrations of the body and even to the destruction of structures, it is necessary to have possibility to control the process of vortex shedding. One of the methods for controlling the flow is placement of a flat splitter behind the cylinder. The problem of sound generation by the flow past a sphere is solved in the wide range of the Reynolds number. Although the problem is three-dimensional, the modern methods of parallel computations at cluster supercomputers allow us to obtain adequate numerical solutions of the problem. At small Reynolds numbers the DNS technique is used, at large Reynolds numbers the technique of LES. The constructed numerical algorithm allowed us to adequately describe the flow in all known regimes. It was shown that our results are in good agreement with the experimental and computational results of others authors. The range of Reynolds numbers within which self-sustained oscillations arise in the flow and therefore the flow can generate sound was determined. It was found that in the laminar regime the amplitude of oscillation of the lateral force applied to the sphere is approximately five times greater than the amplitude of the drag force oscillation. The flow of viscous incompressible fluid in a plane duct with two stenoses located sequentially was studied. Nature of the flow in the region between the stenoses, depending on the Reynolds number, is analyzed. In particular, it was shown that in a certain range of Reynolds number the vertical structures appear in the shear layers between the jet and the cavities formed by the stenoses. These oscillations can fundamentally serve as a source of sound vibrations in the channel. The problem of sound generation by the flow in a duct with two successive stenoses is considered. The method of partial domains was used for solution of this problem. The methods of controlling both the flow and the sound energy emitted by the flow in an irregular duct containing two stenoses are considered. It is shown that one of the effective methods is the rational choice of the domain geometry.