Papers by Author: Yan Yan

Abstract: To research the leakage characteristics and sealing mechanism of the comb labyrinth seals, three-dimensional solutions of NavierStokes governing equations are utilized to investigate the leakage characteristics of the comb Labyrinth for water sealing in leakage flow passage of Francis turbine. The finite volume method is used to discretize the Navier-Stokes equations. The quick scheme and the second central scheme are applied to solve convection and diffusion terms respectively. Distributions of velocity and streamline as well as some flow structure in leakage flow passage are gained at different Reynolds numbers. The results show that vortex generated in the comb consumes kinetic energy and causes the pressure decrease greatly, which play a major role in decreasing the leakage flux.

Abstract: In the paper, the stability of the double-walled carbon nanotubes (DWCNTs) under the initial axial stress are studied. Galerkin and harmonic balance method are used to analyze the relation between the amplitudes and the frequencies of the tubes. The results show that the critical initial stress plays a significant control role in the amplitude-frequency curves. When the initial axial stress is less than the critical initial stress, DWCNTs have twice dynamical mode transitions corresponding to twice noncoaxial vibrations, however, when the initial axial stress is larger than the critical initial stress, the topological forms of the amplitude-frequency curve are changeable, which are corresponding to coaxial vibration or three possible noncoaxial vibration type.

Abstract: Based on an elastic beam model, potential flow theory and N-mode Galerkin discretization technique, the dynamical stability behaviors of fluid-conveyed double-walled carbon nanotubes (DWCNTs) considering geometry nonlinearity relating to the time as variation of the flow velocity are studied. The results show that the bifurcations happen in turn of pitchfork and Hopf types as the flow velocity increases. The vdW forces do not change the bifurcation types but make the critical velocities increase sharply. Furthermore, the vdW forces play a critical role in keeping the synchronization of the time histories of the amplitudes or the velocities of different layers of the CNTs-fluid system.

Abstract: In the present study, the dynamic Vreman model based on the global equilibrium between the subgrid-scale dissipation and the viscous dissipation with a global model coefficient are applied to large eddy simulation of turbulent flow in complex geometries. Distributions of pressure, velocity and vorticity as well as some flow structure are gained, which is helpful to examine the performance of SGS model and understand the flow characters in complex geometries.

Abstract: The paper studies the nonlinear vibrating mechanical behaviors of double-walled carbon nanotubes (DWCNTs) with the initial axial stress under the temperature field. The DWCNTs are modeled as double elastic shells coupled together through vdW interaction between inner and outer nanotubes. Based on the model, the relation between the amplitudes and the frequencies of the tubes is achieved. The results show that the initial axial stress could crucially affect the topological forms of the amplitude-frequency curve, whereas it is insensitive to the amplitude ratios in DWCNTs. Meanwhile, it is also concluded that the temperature change is significant for natural frequency, but weakly affects intertube frequency and amplitude ratios as well as amplitude-frequency curve.