Fenghua Qin

Generation of cylindrical converging shock waves based on shock dynamics theory

A simple but effective technique is proposed to generate cylindrical converging shock waves. The shock dynamics is employed to design a curved wall profile of the test section in a shock tube. When a planar shock wave propagates forward along the curved wall, the disturbances produced by the curved wall would continuously propagate along the shock surface and bend the shock wave. As an example, the wall profile for an incident shock Mach number of M0=1.2 and a converging angle of 15° is tested numerically and experimentally. Both numerical and experimental results show a perfect circular shock front, which validates our method.

Perturbation analysis on gas flow in a straight microchannel

In the present paper, a steady subsonic gas flow either in a circular micropipe or in a planar microchannel driven by pressure within the slip flow regime is studied theoretically by using a perturbation expansion method to solve compressible Navier-Stokes equations. The isothermal flow assumption used in previous theoretical studies is given up. High-order boundary conditions of velocity slip and temperature jump are adopted at the wall. The set of dimensionless governing equations with two small similarity parameters, namely, the ratio of height to length e, and the Knudsen number Kn, is approximated successively by using the perturbation expansions. The various cases such as e≪Kn2, e∼Kn2, and e∼Kn1.5 are studied in detail. Explicit analytical solutions for pressure, density, velocity, temperature, and mass flow rate are obtained up to order of O(Kn2). It is shown that the solution formulas for long channels (e≪Kn2) in lower order are in exact agreement with previous theoretical results. In particular, ...

GPU accelerated CESE method for 1D shock tube problems

In the present study, a GPU accelerated 1D space-time CESE method is developed and applied to shock tube problems with and without condensation. We have demonstrated how to implement the CESE algorithm to solve 1D shock tube problems using an older generation GPU (the NVIDIA 9800 GT) with relatively limited memory. To optimize the code performance, we used Shared Memory and solved the inter-Block boundary problem in two ways, namely the branch scheme and the overlapping scheme. The implementations of these schemes are discussed in detail and their performances are compared for the Sod shock tube problems. For the Sod problem without condensation, the speedup over an Intel CPU E7300 is 23 for the branch scheme and 41 for the overlapping scheme, respectively. While for problems with condensation, both schemes achieve higher acceleration ratios, 53 and 71, respectively. The higher speedup of the condensation case can be ascribed to the source term calculation which has a local dependence on the mesh point and the SOURCE kernel has a higher acceleration ratio.

A kinetic model for heterogeneous condensation of vapor on an insoluble spherical particle

A kinetic model is developed to describe the heterogeneous condensation of vapor on an insoluble spherical particle. This new model considers two mechanisms of cluster growth: direct addition of water molecules from the vapor and surface diffusion of adsorbed water molecules on the particle. The effect of line tension is also included in the model. For the first time, the exact expression of evaporation coefficient is derived for heterogeneous condensation of vapor on an insoluble spherical particle by using the detailed balance. The obtained expression of evaporation coefficient is proved to be also correct in the homogeneous condensation and the heterogeneous condensation on a planar solid surface. The contributions of the two mechanisms to heterogeneous condensation including the effect of line tension are evaluated and analysed. It is found that the cluster growth via surface diffusion of adsorbed water molecules on the particle is more important than the direct addition from the vapor. As an example of our model applications, the growth rate of the cap shaped droplet on the insoluble spherical particle is derived. Our evaluation shows that the growth rate of droplet in heterogeneous condensation is larger than that in homogeneous condensation. These results indicate that an explicit kinetic model is benefit to the study of heterogeneous condensation on an insoluble spherical particle.

Discussion about the Similarity of Micro-scale Gas Flows

A discussion about the issue of similarity of micro-scale gas flows is present. The Couette flow as a typical example is investigated by direct simulation Monte Carlo (DSMC) method with same Mach and Knudsen numbers but different density and distance between two parallel plates. The profiles of density, temperature and velocity are given and the similarity is discussed.

GPU accelerated cell-based adaptive mesh refinement on unstructured quadrilateral grid

Abstract A GPU accelerated inviscid flow solver is developed on an unstructured quadrilateral grid in the present work. For the first time, the cell-based adaptive mesh refinement (AMR) is fully implemented on GPU for the unstructured quadrilateral grid, which greatly reduces the frequency of data exchange between GPU and CPU. Specifically, the AMR is processed with atomic operations to parallelize list operations, and null memory recycling is realized to improve the efficiency of memory utilization. It is found that results obtained by GPUs agree very well with the exact or experimental results in literature. An acceleration ratio of 4 is obtained between the parallel code running on the old GPU GT9800 and the serial code running on E3-1230 V2. With the optimization of configuring a larger L1 cache and adopting Shared Memory based atomic operations on the newer GPU C2050, an acceleration ratio of 20 is achieved. The parallelized cell-based AMR processes have achieved 2x speedup on GT9800 and 18x on Tesla C2050, which demonstrates that parallel running of the cell-based AMR method on GPU is feasible and efficient. Our results also indicate that the new development of GPU architecture benefits the fluid dynamics computing significantly.

Simulation of 1D Condensing Flows with CESE Method on GPU Cluster

We realized the space-time Conservation Element and Solution Element method (CESE) on GPU and applied it to condensation problem in a 1D infinite length shock tube. In the present work, the CESE Method has been implemented on a graphics card 9800GT successfully with the overlapping scheme. Then the condensation problem in 1D infinite shock tube was investigated using the scheme. The speedup of the condensation problem with the overlapping schemes is \(71 \times \) (9800GT to E7300). The influence of different meshes on the asymptotic solution in an infinite shock tube with condensation was studied by using the single GPU and GPU cluster. It is found that the asymptotic solution is trustable and is mesh-insensitive when the grid size is fine enough to resolve the condensation process. It is worth to mention that the peak value of computing reaches 0.88 TFLOPS when the GPU cluster with 8 GPUs is employed.