The Development and Validation of SINATRA: A Three-dimensional Direct Simulation Monte Carlo (DSMC) Code Written in Object-oriented C++ and Performed on Cartesian Grids

The field of Computational Fluid Dynamics (CFD) primarily involves the approximation of the Navier-Stokes equations. However, these equations are only valid when the flow is considered continuous such that molecular interactions are abundant and predictable. The Knudsen number, $Kn$, which is defined as the ratio of the flow\u27s mean free path, $\lambda$, to some characteristic length, $L$, quantifies the continuity of any flow, and when this parameter is large enough, alternative methods must be employed to simulate gases. The Direct Simulation Monte Carlo (DSMC) method is one which simulates rarefied gas flows by directly simulating the particles that compose the flow and using probabilistic methods to determine their collisions and properties. This thesis discusses the development of a new DSMC simulation code, named SINATRA, which was written in object-oriented C++ and validated on Cartesian grids. The code demonstrates the ability to perform standard simulation code tasks which include reading-in a user-made input file, performing the specified simulation, and generating visualization files compatible with Tecplot 360\texttrademark, a commercial post-processing software. SINATRA strategically uses an octree data structure as a storage scheme for computational grid data and uses this a backbone for particle interactions. The discussed validation cases include comparisons of initial particle properties to theoretical data, convergence studies for the sampling of macroscopic properties, and validation of transport properties through natural diffusion and Couette flow simulations. The results show successful implementation of simple DSMC procedures, and a path for future development of the code is thoroughly discussed

Knudsen pump inspired by Crookes radiometer with a specular wall

A rarefied gas is considered in a channel consisting of two infinite parallel plates between which an evenly spaced array of smaller plates is arranged normal to the channel direction. Each of these smaller plates is assumed to possess one ideally specularly reflective and one ideally diffusively reflective side. When the temperature of the small plates differs from the temperature of the sidewalls of the channel, these boundary conditions result in a temperature profile around the edges of each small plate which breaks the reflection symmetry along the channel direction. This in turn results in a force on each plate and a net gas flow along the channel. The situation is analysed numerically using the direct simulation Monte Carlo (DSMC) method and compared with analytical results where available. The influence of the ideally specularly reflective wall is assessed by comparing with simulations using a finite accommodation coefficient at the corresponding wall. The configuration bears some similarity with a Crookes radiometer, where a non-symmetric temperature profile at the radiometer vanes is generated by different temperatures on each side of the vane, resulting in a motion of the rotor. The described principle may find applications in pumping gas on small scales driven by temperature gradients

A Homegrown DSMC-PIC Model for Electric Propulsion

Powering spacecraft with electric propulsion is becoming more common, especially in CubeSat-class satellites. On account of the risk of spacecraft interactions, it is important to have robust analysis and modeling tools of electric propulsion engines, particularly of the plasma plume. The Navier-Stokes equations used in classic continuum computational fluid dynamics do not apply to the rarefied plasma, and therefore another method must be used to model the flow. A good solution is to use the DSMC method, which uses a combination of particle modeling and statistical methods for modeling the simulated molecules. A DSMC simulation known as SINATRA has been developed with the goal to model electric propulsion plumes. SINATRA uses an octree mesh, is written in C++, and is designed to be expanded by further research. SINATRA has been initially validated through several tests and comparisons to theoretical data and other DSMC models. This thesis examines expanding the functionality of SINATRA to simulate charged particles and make SINATRA a DSMC-PIC hybrid. The electric potential is calculated through a 7-point 3D stencil on the mesh nodes and solved with a Gauss-Seidel solver. It is validated through test cases of charged particles to demonstrate the accuracy and capabilities of the model. An ambipolar diffusion test case is compared to a neutral diffusion case and the electric field is shown to stabilize the diffusion rate. A steady state flow test case shows the simulation is able to stabilize and solve the electric potential for a plume-like scenario. It includes additional features to simplify further research including a comprehensive user manual, industry-standard version control, text file inputs, GUI control, and simple parallelism of the simulation. Compilation and execution are standardized to be simple and platform independent to allow longevity of the code base. Finally, the execution bottlenecks of linking particles to cells and particle moving were removed to reduce the simulation time by 95%

An open source, parallel DSMC code for rarefied gas flows in arbitrary geometries

This paper presents the results of validation of an open source Direct Simulation Monte Carlo (DSMC) code for general application to rarefied gas flows. The new DSMC code, called dsmcFoam, has been written within the framework of the open source C++ CFD toolbox OpenFOAM. The main features of dsmcFoam code include the capability to perform both steady and transient solutions, to model arbitrary 2D/3D geometries, and unlimited parallel processing. Test cases have been selected to cover a wide range of benchmark examples from 1D to 3D. These include relaxation to equilibrium, 2D flow over a flat plate and a cylinder, and 3D supersonic flows over complex geometries. In all cases, dsmcFoam shows very good agreement with data provided by both analytical solutions and other contemporary DSMC codes

FOSTRAD : An advanced open source tool for re-entry analysis

This work responds to the need of modeling the atmospheric re-entry of space debris, satellites, and spacecraft quickly, efficiently and with a reasonable reliability. The Free Open Source Tool for Re-entry of Asteroids and Debris (FOSTRAD) is a simulation suite that allows for the estimation of aerodynamics and aerothermodynamics of an entry object in a continuum or rarefied hypersonic flow by employing the local panel formulation. In this paper, the work done to integrate the tool within a comprehensive framework allowing the simulation of complex geometries using a mesh handler module, a 3DOF trajectory propagator, and a surrogate model generation function, is presented. In addition, a synchronous coupling with a 1D thermal ablation code has been implemented and tested. The mesh module allows operations such as surface local radius computation, surface facets visibility identification, and objects geometrical evolution due to the burn-up during the re-entry. In the continuum regime, the simplified aerothermodynamics are computed using a local radius formulation, while the tool employs a flat-plate based approach in the free molecular regime. A generalized nose radius-based bridging model has been introduced for the rarefied transitional regime. The tests have demonstrated that applying a local radius formulation along with the radius-based bridging model greatly improves the accuracy of re-entry heat-flux estimations. The integrated framework has been tested on two different examples of atmospheric re-entries: the ESA Intermediate Experimental Vehicle (IXV) trajectory optimization and the Stardust sample return capsule Thermal Protection System (TPS) burn-up recession; and the coupling between FOSTRAD and the thermal ablation code allowed to study a step-by-step trajectory evolution of Stardust TPS. The obtained results show good agreement with the literature

Gaseous plume flows in space propulsion

AbstractThis paper presents a gaskinetic study on high-speed, highly rarefied jets expanding into a vacuum from a cluster of planar or annular exits. Based on the corresponding exact expressions for a planar or annular jet, it is convenient to derive the combined multiple jet flowfield solutions of density and velocity components. For the combined temperature and pressure solutions, extra attention is needed. Several direct simulation Monte Carlo simulation results are provided to validate these analytical solutions. The analytical and numerical solutions are essentially identical for these high Knudsen number jet flows