Junwei Su

The nature of a universal subgrid eddy viscosity model in a turbulent channel flow

Large-eddy simulation, which is a widely used method of computational fluid dynamics, can accurately simulate wall-bounded turbulent flows due to near-wall treatment. Through analyzing the similarities of the eddy viscosity formulations for large-eddy simulation and Reynolds-averaged Navier-Stokes, we propose a universal subgrid eddy viscosity model (USM) for large-eddy simulation of turbulent flows. The subgrid eddy viscosity in the model is related not only to the norm of the strain rate tensor of the smallest resolved scales but also to the mixing length associated with the subgrid scale, while the subgrid tensor is associated with the strain rate tensor of large scales like in the classical Smagorinsky model. With the friction-velocity–based Reynolds number, 395, the channel flow simulations by USM show that this subgrid eddy viscosity model can physically illustrate the eddy viscosity in the near-wall region and directly simulate the wall-bounded turbulent flow, compared with the classical Smagorinsky model, the dynamic Smagorinsky model, and Moser et al.s direct numerical simulations (DNS).

An Efficient RIGID Algorithm and Its Application to the Simulation of Particle Transport in Porous Medium

RIGID algorithm was recently proposed to identify the contact state between spherical particles and arbitrary-shaped walls, demonstrating significantly improved robustness, accuracy and efficiency compared to existing methods. It is an important module when coupling computational fluid dynamics with discrete element model to simulate particle transport in porous media. The procedure to identify particle and surface contact state is usually time-consuming and takes a large part of the CPU time for discrete element simulations of dense particle flow in complex geometries, especially in cases with a large number of particle–wall collisions (e.g. particle transport in porous media). This paper presents a new version of RIGID algorithm, namely ERIGID, which further improves the efficiency of the original algorithm through a number of new strategies including the recursive algorithm for particle-face pair selection, angle-testing algorithm for determining particle-face relations and the smallest index filter for fast rejection and storage of time invariant. Several specially designed numerical experiments have been carried out to test the performance of ERIGID and verify the effectiveness of these strategies. Finally, the improved algorithm is used to simulate particle transport in a rock treated as a porous medium. Our numerical results reveal several important flow phenomena and the primary reason for particle trapping inside the rock.

Local fixed pivot quadratue method of moment for bubble population balance equation including coalescence and breakage

Population balance equation as an essential tool to describe micro‐behaviors and resulting bubble size distribution has received considerable attention in scientific and engineering fields. Numerical solution is the only choice in most cases due to its complexity. However, it is almost impossible for the existing numerical methods to predict both bubble size distribution and its moments exactly. In this work, a new numerical method basing on the idea of short time Fourier transformation, namely local fixed pivot quadrature method of moment, is proposed for bubble coalescence and breakage. A continuous summation of Dirac Delta function as trial functions in the local domain and monomials as the weighted functions to conserve the local moments were adopted. The moments and the bubble size distribution were constructed based on the moments in the local domain. Numerical tests including pure coalescence, pure breakage and coalescence and breakage combined processes showed that both the moments and bubble size...