Very low Earth orbits (VLEOs) have an altitude of 100-450 km and have become a popular topic in space technology owing to their high application potential. However, spacecraft experience stronger aerothermodynamic effects at VLEOs. In this study, we used a parallel direct simulation Monte Carlo (DSMC) code called PDSC++ and compared it with a traditional DSMC code. The results are in good agreement with the benchmarks. We also simulated the aerothermodynamic factors of a CubeSat, including the operating attitude, orbit altitude, size configuration, and solar panel deployment. The simulations indicate that CubeSats experience minimum drag at an angle of attack of 45 degrees. The orbit altitude strongly affects the particle flow around and stress on a CubeSat. Moreover, a large CubeSat with solar panels experiences high drag and stress. These results can be used as a guide for further research on the service length and other configurational factors of CubeSats.

A massively parallelized unstructured-grid plasma simulation code based on a multifluid plasma model has been developed and validated. The code uses a collocated cell-centered finite-volume method and is designed to simulate intermediate low-pressure and atmospheric-pressure (AP) plasma sources with complex machine geometries. One of the novel features of this code is that it is implemented in a highly flexible computational platform named ultrafast massively parallel processing (ultraMPP), which allows straightforward addition and integration of different partial differential equation (PDE) solvers in a self-consistent manner. As to the numerical methods, the Scharfetter-Gummel scheme is adopted to handle the drift-diffusion flux of electrons, whereas the Harten-Lax-van Leer (HLL)-type approximate Riemann solver is used to handle the convection terms of ion momentum equations. For discretization of the diffusion terms in an unstructured grid, the Taylor expansion is used to deal with the effects of nonorthogonality of the cells, and the cell-center gradient is calculated using a least-squares method. The simulation code with a local-field approximation (LFA) was validated for two cases of AP plasmas as well as a case of an argon capacitively coupled plasma generated in a Gaseous Electronic Conference (GEC) reference cell with local mean energy approximation (LMEA) at intermediate low pressure. The simulation results were found to be in good agreement with the previously published experimental and simulation data.

This study proposes an application of the variable time step (VTS) method to solve the non-conservation of fluxes encountered in the conventional direct simulation Monte Carlo (DSMC) method in axisymmetric simulations. A legacy parallel DSMC code with an unstructured cell using C(++)language (PDSC++) was employed by adjusting the reference scale of the original VTS to fulfill the requirements of axisymmetric simulation. Three test cases were simulated to verify the proposed axisymmetric DSMC method. First, the hypersonic flow past a cylinder was simulated using different wall boundary conditions. Second, the hypersonic flow past a 1.6-m sphere was simulated considering complicated air chemistry. Third, the flow field of the hypersonic flow past a double-cone blunt body was simulated and compared with the experimental data. The results showed that the newly proposed axisymmetric DSMC method can meet the simulation accuracy without an inconvenience through particle cloning and destruction.

With a combination of the Direct Simulation Monte Carlo (DSMC) calculation and test particle computation, the ballistic transport process of the hydroxyl radicals and oxygen atoms produced by photodissociation of water molecules in the coma of comet 67P/Churyumov–Gerasimenko is modelled. We discuss the key elements and essential features of such simulations which results can be compared with the remote-sensing and in situ measurements of cometary gas coma from the Rosetta mission at different orbital phases of this comet.

The expansion of sublimating gas from cometary nucleus surface is a complex physical process. It involves the diurnal temperature effect of the outgassing rate, the gas drag to the dust, the irregular shape of the nucleus at different scale lengths, transition from the collisional flow regime to the free-molecular flow regime, and the direct gas flow over or into regions in the shadow. Most of these effects which have been discussed before can now be tested by imaging observations and in-situ measurements at comet 67P/Churyumov-Gerasimenko (67P/C-G). We produce the surface temperature distribution and its diurnal variation by a geometrical thermal model of comet 67P/C-G. And we use a parallel 3D Direct Simulation Monte Carlo (DSMC) code, named PDSC++ [1, 2], from Wu’s group at NCTU to calculate the gas flow near the cometary nucleus. In the presentation, we will show the results and basic characteristics of the gas coma pattern of comet 67P by including non-uniform gas composition (i.e. H2O-rich vs. CO2/CO) from different regions (i.e. neck vs. head/body). In addition, preliminary results on the photochemical effects of a distributed source will be described.

This paper presents the implementation, validation and application of TCE (total collision energy) model for simulating hypersonic reactive flows in a parallel direct simulation Monte Carlo code, named PDSC++, using an unstructured grid. A series of benchmarking test cases, which include reproduction of theoretical rate constants in a single cell, 2D hypersonic flow past a cylinder and 2D-axisymmetric hypersonic flow past a sphere, were performed to validate the implementation. Finally, detailed aerothermodynamics of the flown reentry Apollo 6 Command Module at 105 km is simulated to demonstrate the powerful capability of the PDSC++ in treating realistic hypersonic reactive flow at high altitude.