Simulation of the dynamics of dust-gas circumstellar discs is crucial in understanding the mechanisms of planet formation. The dynamics of small grains in the disc is stiffly coupled to the gas, while the dynamics of grown solids is decoupled. Moreover, in some parts of the disc the concentration of the dust is low (dust to gas mass ratio is about 0.01), while in other parts it can be much higher. These factors place high requirements on the numerical methods for disc simulations. In particular, when gas and dust are simulated with two different fluids, explicit methods require very small timestep (must be less than dust stopping time t stop during which the velocity of a solid particle is equalized with respect to the gas velocity) to obtain solution, while some implicit methods requires high temporal resolution to obtain acceptable accuracy. Moreover, recent studies underlined that for Smoothed particle hydrodynamics (SPH) when the gas and the dust are simulated with different sets of particles only high spatial resolution h < c s t stop guaranties suppression of numerical overdissipation due to gas and dust interaction.To address these problems, we developed a fast algorithm based on the ideas of (1) implicit integration of linear (Epstein) drag and (2) exact conservation of local linear momentum. We derived formulas for monodisperse dust-gas in two-fluid SPH and tested the new method on problems with known analytical solutions. We found that our method is a promising alternative for the previously developed two-fluid SPH scheme in case of stiff linear drag thanks to the fact that spatial resolution condition h < c s t stop is not required anymore for accurate results.
We discuss the ability of the smoothed particle hydrodynamics (SPH) method combined with a grid-based solver for the Poisson equation to model mass accretion onto protostars in gravitationally unstable protostellar discs. We scrutinize important features of coupling the SPH with grid-based solvers and numerical issues associated with (1) large number of SPH neighbours and (2) relation between gravitational softening and hydrodynamic smoothing length.We report results of our simulations of razor-thin disc prone to fragmentation and demonstrate that the algorithm being simple and homogeneous captures the target physical processes -disc gravitational fragmentation and accretion of gas onto the protostar caused by inward migration of dense clumps.In particular, we obtain two types of accretion bursts: a short-duration one caused by a quick inward migration of the clump, previously reported in the literature, and the prolonged one caused by the clump lingering at radial distances on the order of 15-25 au. The latter is culminated with a sharp accretion surge caused by the clump ultimately falling on the protostar.
Circumstellar discs, from which planetary systems are formed, consist of gas, dust and solids. Simulations of self-consistent dynamics of gas, dust and solids in circumstellar discs is a challenging problem. In the paper we present fast algorithms for computing the drag force (momentum transfer) between solid phase and gas. These algorithms (a) are universal and applicable to dust and solids with any sizes smaller than the mean free path of gas molecules, (b) can be used to calculate the momentum transfer between dust and gas instead of one-way effect, as it is done in many models, (c) can perform simulations, without a loss in accuracy, with the time step determined by gas-dynamic parameters rather than by drag force, and (d) are compatible with the widely used parallel algorithms for solving 3D equations of gas dynamics, hydrodynamic equations for dust, and the collisionless Boltzmann equation for large bodies. Preliminary results of supercomputer simulation of the gas-dust disc dynamics within the developed approach are reported.
Предложен новый параллельный метод решения задачи Дирихле для уравнения Пуассона в контексте нестационарных задач математической физики. Метод основан на декомпозиции прямоугольной декартовой области решения в одном направлении, решении уравнения Пуассона в каждой подобласти прямым методом и сопряжении подобластей с помощью быстрого вычисления потенциала выделенного слоя экранирующих зарядов.
Тестовые эксперименты, проведенные на суперкомпьютерах Межведомственного суперкомпьютерного центра и Сибирского суперкомпьютерного центра, показали хорошую масштабируемость алгоритма.
A new parallel method to solve the Dirichlet problem for Poisson's equation in the context of nonstationary problems of mathematical physics is proposed. This method is based on a decomposition of a rectangular Cartesian domain in one direction, on a direct method of solving Poisson's equation in each subdomain, and on the coupling of the subdomains using a fast procedure for evaluating a single layer potential. A number of test experiments conducted on supercomputers installed at Joint Supercomputing Center of Russian Academy of Sciences and at Siberian Supercomputing Center show a good weak and strong scalability of the parallel algorithm.
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