Conserved discrete unified gas-kinetic scheme with unstructured discrete velocity space

Discrete unified gas-kinetic scheme (DUGKS) is a multiscale numerical method for flows from continuum limit to free molecular limit, and is especially suitable for the simulation of multiscale flows, benefiting from its multiscale property. To reduce integration error of the DUGKS and ensure the conservation property of the collision term in isothermal flow simulations, a conserved-DUGKS (C-DUGKS) is proposed. On the other hand, both DUGKS and C-DUGKS adopt Cartesian-type discrete velocity space, in which Gaussian and Newton-Cotes numerical quadrature are used for calculating the macroscopic physical variables in low-speed and high-speed flows, respectively. However, the Cartesian-type discrete velocity space leads to huge computational cost and memory demand. In this paper, the isothermal C-DUGKS is extended to the nonisothermal case by adopting coupled mass and inertial energy distribution functions. Moreover, since the unstructured mesh, such as the triangular mesh in the two-dimensional case, is more flexible than the structured Cartesian mesh, it is introduced to the discrete velocity space of C-DUGKS, such that more discrete velocity points can be arranged in the velocity regions that enclose a large number of molecules, and only a few discrete velocity points need to be arranged in the velocity regions with a small amount of molecules in it. By using the unstructured discrete velocity space, the computational efficiency of C-DUGKS is significantly increased. A series of numerical tests in a wide range of Knudsen numbers, such as the Couette flow, lid-driven cavity flow, two-dimensional rarefied Riemann problem, and the supersonic cylinder flows, are carried out to examine the validity and efficiency of the present method.