The self-excited vibration of runner is an important factor affecting the stable operation of hydro-turbines. Due to the particularity of structure, there is always a gap between the runner and the shell of the tubular turbine, and the eccentricity of the runner often occurs due to manufacturing or installation. The uneven clearance caused by runner eccentricity will lead to Alford effect and may induce the self-excited vibration. Based on a model tubular turbine, the Alford effect and Alford force of different runner eccentricity is deeply explored through the combination of experiment and numerical simulation. Based on the investigation under a common high-efficiency condition, it is found that the difference of runner eccentricity will cause obvious changes in performance. With the increase of runner eccentricity, the average value of runner radial force increases gradually. The overall frequency of radial force shows several frequency bands and the frequency may shift with the increase of eccentricity level. The runner eccentricity causes obvious changes of internal flow and tip leakage vortex in the blade tip clearances. In the narrowest clearance due to eccentricity, the turbulence kinetic energy and vortex intensity obviously rise up. The pressure in the clearances with different widths are also different. The influence of flow field inside clearance will be the main reason of Alford effect. This study is meaningful for tubular turbines for the operation stability and security.

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Axial-flow turbomachinery is widely used in low head water transfer and electricity generation projects. As there is a gap between the impeller and casing of the tubular flow unit, the fluid will cross the gap to form tip leakage flow, which may induce intense pressure pulsation, noise and mechanical vibration, and even threaten the safe operation of the unit. In order to ensure the efficient and stable operation of hydropower units, the influence factors of tip clearance flow and its formation and development mechanism have been deeply studied in this paper. In this paper, the impact of gap width, angle of attack and inlet velocity on tip leakage flow of hydrofoil with clearance are studied by orthogonal experiment method. The results suggest that the gap width has the greatest influence on tip clearance flow, the incidence angle takes the second place, and the inlet velocity has the least effect on tip clearance flow. Then the fractal characteristics of tip leakage vortices with different gap widths are studied. The results demonstrate that the fractal dimension of tip leakage vortices in large gaps was significantly larger than that in small gaps; The fractal dimension of the leakage vortex decreases gradually along the flow direction.

The wall boundary is important in computational-fluid-dynamics simulations. If extremely small leakage, changing leakage or a moving body exists in the simulation case, the difficulty in meshing and solving near-wall flow increases. The immersed-solid method, which inserts a rigid, solid body into the entire fluid domain, was a choice to solve the wall-boundary-solution problems mentioned above, without considering mesh deformation. The purpose of this paper is to verify the effectiveness of the immersed-solid method in the simulation of extremely small leakage, changing leakage or a moving body, and to provide a theoretical basis for the use of the submerged-solid method in engineering. In this study, the NACA0015 hydrofoil was used to check the hydrodynamic characteristics in using the immersed-solid method. The comparative study was conducted at the incidence angle of 8 degrees and a Reynolds number of 5.0 × 105, by using the immersed-solid and traditional no-slip-wall boundary. The results show that the flow striking and separation with pressure rise and drop can be correctly captured using an immersed-solid setup with boundary tracking. However, the accuracy of pressure and velocity field using the immersed-solid method was insufficient. The turbulence-kinetic energy was much higher around the immersed-solid foil body. Generally, the immersed-solid method can qualitatively predict the correct hydrodynamic characteristics. Its convergence ability is better, and it can save approximately 20% of CPU time, even if the grid density is 4.39 times of the traditional no-slip wall. Therefore, the immersed-solid method can be a good choice for engineering-flow cases with complex wall problems.