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Flow entrained with permeable particles is frequently encountered in natural and engineering applications. However, the lack of knowledge about the drag on the porous spheres and the corresponding complex flow structures has been a hindrance for better understanding of these flow systems In this work, we perform a comprehensive numerical simulation of flow past a porous particle with a wide range of Reynolds number (5 ≤ Re ≤ 200) and porosity (0.1 ≤ ɛ ≤ 0.95). Furthermore, considering that few studies has reported the flows with moving porous spheres, the simulation of both a single and two porous spheres settling in an enclosure is conducted for further evaluating the effect of moving porous media on the fluid–solid flows. The lattice Boltzmann method (LBM) is applied to the fluid flow, and particularly, the immersed moving boundary (IMB) scheme describes the interaction between the fluid and porous solid body. The accuracy of that the hybrid IMB-LBM for modeling porous sphere flows, to our knowledge, is first explored and established by the present study. The results suggest that both Re and ɛ have a significant influence on the drag and recirculation flow of porous particles. At Re ≤ 75, the porous particle experiences a lower resistance than that of the impermeable counterpart. When Re exceeds 75, the drag ratio is higher than unity for ɛ ≤ 0.85. With increasing porosity, the recirculation wake penetrates the particle, then detaches from the particle, and eventually disappears in the particle rear. Based on the simulated results, a new drag correlation for porous spheres is developed. As for the moving porous spheres, they fall slower than the solid counterparts, and their drafting, kissing and tumbling (DKT) process can also be slowed down. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a method for stability analysis of N-cell cascade step-down buck converters, which is a kind of complex nonlinear system. A nonlinear model in the form of time-variant state equations is derived, and then the loop gain describing the overall system is introduced. With the help of them, we obtain the equations for phase cross over frequency and gain margin by small-signal perturbation technique, harmonic balance method, and inverse iteration method. Based on the equations, the overall cascade system stability can be analyzed regardless of the number of converters cascaded. Finally, the cascade buck converter with three power stages is used to demonstrate the effectiveness of the proposed stability analysis method. [Copyright &y& Elsevier]

An immersed boundary-discrete unified gas kinetic scheme (IB-DUGKS) is developed in this paper for interface-resolved simulation of particle suspension flows. Compared with the conventional IB methods which consider the interface as a generator of external force, the no-slip boundary condition in the present scheme is implemented directly by correcting the distribution functions near the interface. Therefore, it makes good use of the intrinsic nature for the DUGKS as a kinetic method, and removes simultaneously the necessity to construct the models to evaluate and adsorb the external force in DUGKS. Furthermore, the present IB-DUGKS promotes the accuracy of the IB methods from first- to second-order, which is verified reasonably in the cylindrical Couette flow. After that, several well-established particle-fluid flows are simulated, including the motion of a neutrally buoyancy particle moving in the Poiseuille and Couette flows, and sedimentation of a particle in an enclosure. In all test cases, the results are in good agreement with the data available in the literature. The robust of the present IB-DUGKS is also demonstrated in the simulations. [ABSTRACT FROM AUTHOR]