SMILE software system

The SMILE (Statistical Modeling in Low-Density Environment) software system was developed in the Laboratory of Computational Aerodynamics of Khristianovich Insitute of Theoretical and Applied Mechanics of Siberian Branch of the Russian Academy of Sciences. The laboratory is well-known in Russia and worldwide as one of leading research centers for study of various high-altitude aerothermodynamics problems using Direct Simulation Monte Carlo (DSMC) method. Highly efficient numerical algorithms for simulation of rarefied gas flows in wide range of Knudsen numbers were developed in the Laboratry along with models, descibing physicochemical processes in a rarefied gas. The results of studies, conducted at the Lab, are regularly reported on national and international conferences and published in a variety of peer-reviewed journals.

SMILE software system is designed for simulation of fundamental and applied problems of rarefied gas dynamics [1] using the Direct Simulation Monte Carlo method [2] on parallel supercomputers. The system is also used in courses on methods and tools of computational rarefied gas dynamics for under- and postgraduate students.

SMILE software system allows to conduct numerical study of rarefied gas flows in regimes ranging from transitional to free-molecular, i.e. flows in which mean free path of molecules is comparable or much more than the characteristic length of a problem. Numerical simulation of rarefied flows is performed in such fields of research as vacuum gas dynamics, molecular beams physics, physics of plasma and gas discharge, high-altitude aerodynamics and so on. It is known, that conventional approaches based on Euler and Navier-Stokes equations are inapplicable for transitional and free-molecular regimes. A kinetic approach based on the Boltzmann equation is to be used [1]. Currently DSMC method, in which the flow is represented by an ensemble of model particles (usually 10^5-10^7 molecules) interacting with each other and solid walls, is the most power tool for solving the Boltzmann equation in 2D and 3D cases. In addition, SMILE software system incorporates the most modern and efficient DSMC [collision] schemes [2,3].

System allows to simulate 2D and 3D rarefied gas flows using simplified geometric model of object under consideration. Various models of intermolecular collisions (both elastic and non-elastic) implemented in SMILE allow to take into account real gas effects (such as energy exchange between different translational and internal degrees of freedom and chemical reactions).

SMILE uses efficient parrallelization schemes of the DSMC method, implemented using MPI (Message Passing Interface) library, which enables the employment of multi-CPU computational clusters. It is particularly important for simulation of flows in near-continuum regime with allowance for real gas effect, requiring large computational resources. SMILE Software system consists of the following subsystems:

Interactive subsystem for preparation of initial data and run preprocessing. It allows to specify free stream values, dimensions of a computational domain and size of computational grid, boundary conditions and parameters of numerical method.
Interactive subsystem for geometric model preparation for creation of simplified 2D or 3D configuration. Collisions of model particles with solid boundaries are simulated using classic [1] gas-surface interations models.
The computational module itself for 1D, 2D and 3D numerical simulation.

Interactive subsystem for data postprocessing and analysis of results, which allows to visualize main gasdynamic quantities (density, pressure, velocity, temperture) and some surface quantities as 2D flowfields, isolines, streamlines and surface distributions. Also there are options for data export in native formats of some popular visualization tools, including Tecplot and VTK.

All interactive subsystems have user-friendly graphical interface, making them intuitive and easy to use. Functionality of SMILE software system is confirmed by numerous publications, in which SMILE simulation results were compared with experimental data (cf. [4-8]).

References

Kogan M.N. Rarefied Gas Dynamics. Plenum, New York: 1969.
Bird G.A. Molecular gas dynamics and direct simulation of gas flows. Clarendon Press, Oxford, 1994.
Иванов М.С., Кашковский А.В., Гимельшейн С.Ф., Маркелов Г.Н. Статистическое моделирование разреженных гиперзвуковых течений от свободно-молекулярного до околоконтинуального режимов течения // Теплофизика и аэромеханика. 1997. Т. 4. № 3. С. 251-268.
Ivanov M.S., Kashkovsky A.V., Gimelshein S.F., Markelov G.N., Alexeenko A.A., Bondar Ye.A., Zhukova G.A., Nikiforov S.B., Vashenkov P.V. SMILE System for 2D/3D DSMC computations, Proc. of 25th Int. Symp. On RGD., Saint-Petersburg (Russia, July 21-28, 2006), Publishing House of the Siberian Branch of the Russian Academy of Sciences, 2007, pp. 539-544.
Bondar Ye.A., Markelov G.N., Gimelshein S.F., Ivanov M.S. Numerical modeling of near-continuum flow over a wedge with real gas effects // Journal of Thermophysics and Heat Transfer, 2006, V. 20, No. 4, Р. 699-709.
Markelov G.N., Ivanov M.S. Kinetic analysis of hypersonic laminar separated flows for hollow cylinder flare configurations // Journal of Spacecraft and Rockets, 2001, V. 38, No. 6, Р. 875-881.
Markelov G.N., Kashkovsky A.V., Ivanov M.S. Space station MIR aerodynamics along the descent trajectory // Journal of Spacecraft and Rockets, 2001, V. 38, №. 1, Р. 43-50.
Ivanov M.S., Markelov G.N., Gimelshein S.F. Statistical Simulation of Reactive Rarefied Flows: Numerical Approach and Applications // AIAA Paper, 1998 № 98-2669.

Brief description of SMILE software system

SMILE software system consists of pre-processing (initial data preparation) subsystem, processing (simulation using the DSMC method) and post-processing (data analysis and visualization), some auxillary utilities and built-in help. SMILE software system is based on:

numerical schemes of the Direct Simulation Monte Carlo (DSMC) method;
collisional models, describing real gas effects;
parallel algorithms of DSMC method and special accelerating techniques;
pre- and processing subsystems, which greatly simplify whole work process from creation of geometric model to visualization of computation results.

Models of intermolecular collisions:

DSMC method implementation is based on the majorant frequency scheme;
collisions are simulated using Variable Hard Sphere (VHS) and Variable Soft Sphere (VSS) models;
rotational and vibrational energies of molecules, rotational/translational and vibrational/translational energy exchange are modelled using either continuous model, discrete model of Larsen-Borgnakke with constant or temperature-dependent relaxation numbers.
chemical reactions of dissociation and exchange in a gas phase are performed using Total Collision Energy (TCE) or Vibration-Dissociation Coupling (VDC) models, each of them can be used with continuous or discrete internal energy models.
chemical database contains required parameters of all mentioned above models for components of Earth and Mars atmospheres.
Molecule interaction with solid surfaces uses either Maxwell model, or multiparameter Nocilla model.

Computational grids:

2-level rectangular grid;
dynamic grid adaptation to local mean free path value;
grids can be used in two-dimensional, axisymmetrical and three-dimensional cases.

Other numerical techniques:

a) in axisymmerical cases radial weight functions can be employed;
b) adaptive partitioning of computational domain with different time step values in each domain.

Test cases:

List of computation examples includes

aerothermodynamics of external flows of chemically reacting air around simple bodies (disc, sphere, blunted cone, double cone) at angle of attack;
flow around concave bodies in 2D and 3D cases;
shock/boundary layer interaction in flow past a hollow cylinder or double cone;
plumes exhausting into a vacuum, plume/plume interactions, plume impingement and interaction of plume with a free stream
microhannel flow.

Graphical user interface allows to simplify and quicken the process of initial data preparation, monitor running computation and view obtained results. All subsystems and utilities can be launched from GUI, including some external third-party software such as Tecplot visualization tool. Some basic operation system functions, such as creation and removal of directories, compression of data files, can also be performed from SMILE GUI. Built-in help system is included as well.

Pre-processing system perfroms interactive preparation of initial data for computations. It consists of:

Prompting subsystem, which shows recommended values for some parameters of the DSMC method;
database of chemical elements, binary collisions and chemical reactions;
subsystems allowing to create, edit and save geometric model, surface properties and initial conditions.
DSMC computation (processing) system in based on algorithms of DSMC method ann employs efficient parrallelization algorithms. After pre-processing run restart files are created and then are updated during the computation. It allows to monitor computational process and continue interrupted run.

Interactive graphical post-processing system is used to:

convert restart files to visualization tools formats;
visualize surface quantites, flowfields and distribution functions.