The implementation that we will use here is called ultraSPARTS and is based on a
published code called PDSC++ (Su 2013). This, in turn, was based on the PDSC
(Parallelized Direct Simulation Monte Carlo Code) program developed by Wu and
co-workers (Wu and Lian 2003; Wu et al. 2004; Wu and Tseng 2005). Other
implementations have been used by, amongst others, Combi (1996), who was the
first to use DSMC for cometary research, Skorov and Rickman (1999), Crifo et al.
(2002a), Davidsson (2008), and Tenishev et al. (2011). Other publicly available
codes that might also be used include SMILE,
6 OpenFOAM
7 (Scanlon et al. 2010;
White et al. 2018) and Bird’s codes.
8 A derivative of SMILE was used by Skorov
et al. (2006) to study cometary comae.
The expansion of the gas produced at a cometary surface implies that, even if the
gas is collisional at the surface, eventually it becomes collisionless with distance and
the time to transfer energy between the rotational and translational degrees of
freedom increases. This is, in part, one of the reasons why DSMC is a useful tool
in analysing cometary comae. On the other hand, the number of collisions needed to
equilibrate the gas (the collisional relaxation number) is not well established with
values between 1 and 8 having been used by different authors. Liao et al. (2016)
looked at the significance of this number and found that, for cometary expansion, the
exact value is of limited importance in defining the final flow field although details
close to the nucleus may be influenced.
In comparing DSMC and the fluid description, Crifo et al. (2002a) noted that
complete consistency between the approaches is by no means straightforward to
attain because the properties of the molecules must be represented consistently. In
the case of complex molecules such as H 2 O, approximations are unavoidable, and
they are formulated differently in the two approaches. In the fluid case, a kinematic
viscosity law is used whereas in DSMC a collision model is used. Consistency to
within 10% in density for any complex flow should be considered perfectly reasonable. There are standard test cases to verify DSMC implementations and new codes
should be checked against these.
Table 3.8 Basic algorithm
for DSMC calculations
(Alexander and Garcia 1997)
Initialize system with particles
Loop over time steps
Create particles at open boundaries
Move all the particles
Process any interactions of particles and boundaries
Sort particles into cells
Sample statistical values
Select and execute random collisions.
Output particle properties per cell
6 http://lnf.nsu.ru/en/smile.html
7 https://www.openfoam.com
8 http://www.gab.com.au
3.4 Gas Expansion
227
published code called PDSC++ (Su 2013). This, in turn, was based on the PDSC
(Parallelized Direct Simulation Monte Carlo Code) program developed by Wu and
co-workers (Wu and Lian 2003; Wu et al. 2004; Wu and Tseng 2005). Other
implementations have been used by, amongst others, Combi (1996), who was the
first to use DSMC for cometary research, Skorov and Rickman (1999), Crifo et al.
(2002a), Davidsson (2008), and Tenishev et al. (2011). Other publicly available
codes that might also be used include SMILE,
6 OpenFOAM
7 (Scanlon et al. 2010;
White et al. 2018) and Bird’s codes.
8 A derivative of SMILE was used by Skorov
et al. (2006) to study cometary comae.
The expansion of the gas produced at a cometary surface implies that, even if the
gas is collisional at the surface, eventually it becomes collisionless with distance and
the time to transfer energy between the rotational and translational degrees of
freedom increases. This is, in part, one of the reasons why DSMC is a useful tool
in analysing cometary comae. On the other hand, the number of collisions needed to
equilibrate the gas (the collisional relaxation number) is not well established with
values between 1 and 8 having been used by different authors. Liao et al. (2016)
looked at the significance of this number and found that, for cometary expansion, the
exact value is of limited importance in defining the final flow field although details
close to the nucleus may be influenced.
In comparing DSMC and the fluid description, Crifo et al. (2002a) noted that
complete consistency between the approaches is by no means straightforward to
attain because the properties of the molecules must be represented consistently. In
the case of complex molecules such as H 2 O, approximations are unavoidable, and
they are formulated differently in the two approaches. In the fluid case, a kinematic
viscosity law is used whereas in DSMC a collision model is used. Consistency to
within 10% in density for any complex flow should be considered perfectly reasonable. There are standard test cases to verify DSMC implementations and new codes
should be checked against these.
Table 3.8 Basic algorithm
for DSMC calculations
(Alexander and Garcia 1997)
Initialize system with particles
Loop over time steps
Create particles at open boundaries
Move all the particles
Process any interactions of particles and boundaries
Sort particles into cells
Sample statistical values
Select and execute random collisions.
Output particle properties per cell
6 http://lnf.nsu.ru/en/smile.html
7 https://www.openfoam.com
8 http://www.gab.com.au
3.4 Gas Expansion
227