Direct Simulation Monte Carlo Method
Visual Programs at GAB Consulting
Latest update January 22, 2008
The direct simulation Monte Carlo (or DSMC) method is widely
used for the modeling of gas flows through the computation of
the motion and collisions of representative molecules. Computation
at the molecular level is necessary for studies in rarefied gas
dynamics (or RGD) because the transport terms in the Navier-Stokes
equations are not valid in this flow regime. The essential characteristic
of a "rarefied" flow is that the molecular mean free
path is not negligible and many applications involve normal and
high density flows with very small physical dimensions.
2008 Roadmap for the DSn(V)
programs
The latest DS2V version 4 employs separate
computational and visual programs that are linked by binary stream
files. This has proved to have significant advantages over the
integrated graphics in the current version of DS3V, especially
in terms of program portability. A similar update was planned
for DS3V, but the work outlined in the DSMC07 Short Course notes
showed that both programs would benefit from basic changes to
the underlying DSMC procedures and geometry models.
(1) The latest CPU's are essentially
64bit and memory capacity has been increasing at a far greater
rate than processing speed. The round-off errors associated with
32 bit arithmetic have been responsible for a large proportion
of the difficulties in developing he programs and for ongoing
problems with the geometry-related procedures in the programs.
There is now an overwhelming case for a move to programs in which
all floating point variables are 64 bit.
(2) Most of the effort in recent years
has been associated with the geometry models and visual interface.
At the same time, most the "1994" basic DSMC procedures
have been superseded. This has resulted in many ad-hoc changes
to the coding and the programs would benefit from a complete re-write
of the basic subroutines, especially those related to intermolecular
collisions that involve chemical reactions (see below).
(3) While the current geometry models
are generally superior to those that have been employed in earlier
programs, the irregular nature of the cells has a deleterious
effect on the results. It is proposed to move to a "tree-structure"
of rectangular cells that can be readily adapted to a uniformly
small number of simulated molecules in each collision cell. The
ability to rapidly adapt he cells without having to restart the
programs has proved to be one of the advantages of the current
geometry models. This advantage should not only be retained, but
improved upon so that the cells can be adaped, if necessary, at
every output interval.
It has been decided that the new generation
of programs should start with a DS1 program for one-dimensional
flows and a preliminary version of this is now running. This has
confirmed that there enormous advantages in all 64 bit procedures.
Also, the effect of the number of molecules per collision cell
on the collision rate that was found with DS2V (see the DSMC07
Short Course notes) is absent with the new collision reading.
This confirms recent calculations by Michael Gallis. While the
geometry model is trivial in a one-dimensional program, the array
structure associated with the cell adaption in DS1 is designed
to be easily scaled to two and three dimensions. This means that
the DS1 program will incorporate the most advanced DSMC procedures
and a structure that can serve as the basis of new versions of
the DS2 and DS3 programs. An interactive visual interface is not
required in one dimension.
It is intended that the source code
of the completed DS1 program will be made freely available. The
future of the family of programs will be best assured if the development
of the new DS2 and DS3 programs is a collaborative effort. These
will be all Fortran programs that can be compiled for all platforms.
A collaborative approach will allow trials of many approaches
to multi-threading and possibly of other forms of parallelism.
Visual programs for data production production, interactive execution
and post-processing could be produced in many forms. It is hoped
that the DS1 source code will be available some time in the second
half of 2008.
Notes from the September 30
Short Course at DSMC07 (Sept. 30, 2007 Santa Fe)
New quantum-kinetic chemistry
model for DSMC
DS2V version 4.5.03 incorporated the first stage of the new
quantum-kinetic model for vibration related chemical reactions.
The model has now been extended to include exchange reactions
and recombinations in addition to dissociation. It will be fully
implemented in DS2Vafter the Santa Fe meeting. It will be the
default model, but the old chemistry model will be available as
an option until version 5 appears. The main advantage of the new
model is that there is no longer any reference to the essentially
"continuum" chemical rate equations. It has been shown
that the experimentally based rates can be reproduced by simple
particle-based procedures that just employ the standard gas properties.
While this may appear to be a drastic step, the aim of phenomenological
modeling is to accurately reproduce the gas physics through the
simplest possible model. The physical understanding lies as much
in what can be left out as in what must be included. Because the
procedures no longer make any reference to the macroscopic temperature,
the model is far more likely to provide a good representation
of flows with extreme nonequilibrium. The model takes full advantage
of the the ability of DSMC to cope directly with discontinuous
and event-driven physical processes. There are no adjustable parameters
and the only experimentally based data that is required is the
value of the vibrational relaxation collision number at a single
reference temperature. Finally, because the procedures are trivially
simple to implement within the code, the new model is computationally
far mor efficient.
DS2V Version 4.5.05 now
available
DS2V Version 4 has similar capabilities
to Version 3, but the DSMC calculation is made by the "all
Fortran 95" program DS2.EXE. Instead of the integrated visual
components, the DS2 program runs within the new DS2V program that
is written in REALbasic. This means that the application can be
compiled to run under Windows, Linux, or Apple. The interactive
graphics have been greatly enhanced and the data input is completely
new. The program can be run in a post-processing mode as well
as the interactive mode and there should now be little need for
external post-processing programs. The data is generated in a
"parallel" rather than a "serial" fashion
and this should eliminate the previously all-too-frequent need
to re-start the data generation process when changes were made
to the geometry. The program can run the data files produced by
Version 3, but the DS2VD.DAT file should be edited so that
there is a new line for each variable (this affects only the co-ordinates
of the end-points of surface segments). The version number
has advanced from 4.1 to 4.2 because the chemistry model now allows
ionizing reactions. This has required extensions to the data file
and the version number must change so that older data files can
be recognized and accepted. Similarly, 4.2 has advanced to 4.3
to allow for additional data relating to the moving wall boundary
condition. Note that the version numbers in DS2.EXE and DS2V.EXE
must match.
The DS2.EXE program from version 4.4.10
on has been compiled by Intel Visual Fortran Verion 10 and, for
the sample flow, is 33% faster than the earlier versions that
were compiled by Compaq Visual Fortran Version 6. Versions 4.5.02
and up have reverted to Compaq Visual Fortran because the Intel
compiler options that were used for 4.5.01 introduced errors in
the compiled code.
Version 4.5 provides a choice between
the new particle-based chemistry model and the continuum or temperature
based model. The new model integrates dissociation with vibrational
excitation, but is currently implemented only for dissociation
reactions:- see CYL2501.PDF and the data file CYL2501.DAT.
This program is a compressed executable
and should be placed in an empty directory or folder. When run,
it produces DS2V.EXE, DS2.EXE and the sample data file DS2VD.DAT.
The latter is the first demonstration case from the earlier versions.
DS2V is best run first, with the DS2 calculation started from
within DS2V. The new (April 2) introductory window is shown above.
A similar DS3V Version 3 is in an early
stage of preparation. This will not have the two-core parallel
option of Version 2, but will be followed by n-core versions
of both programs. Any comments would be appreciated.
The option to produce QuickTime
movies from of unsteady flows has now been implemented.
The movies are produced in a post-processing mode from files
that are produced whenever the flow is specified as a "continuing
unsteady flow". The earlier versions saved bitmap files for
only one flow flow property and a new run was required if a movie
was to be made for a different flow property. Movies that show
a wide selection of flow properties can now be made from the files
produced by a single run.
A sample movie SHOCK.EXE
that shows an argon shock wave moving into a wedge shaped cavity
can be downloaded. This is a self-extracting compressed file that
produces SHOCK.QT. This can be viewed with the free QuickTime
viewer. The compressed file is 10.5MB and this expands to
92.7MB, but CLICK HERE TO DIRECTLY
VIEW A LOW QUALITY MPEG THAT IS ONLY 0.22MB!
The data file that was used for the calculation (about 20
hours on an average PC) can also be downloaded as DS2VD.SHOCK.
This should be renamed to DS2VD.DAT and the run can then be duplicated.
Alternatively the data file can be modified to make shock diffraction
calculations for other geometries, gas compositions, and surface
conditions. The surfaces in this calculation were specularly reflecting,
but the data is readily modified to make the reflection diffuse.
The free QuickTime viewer can be upgraded to PRO at a low cost
to edit and combine movies, add titles and soundtracks, or produce
an MPEG as above.
Version 2.6 of the General 3D
"Visual DSMC" Program
The Visual DSMC program DS3V is a general program
for three-dimensional flows. Like the DS2V program, it can be
applied to a very wide range of flows. It employs the same password
as the DS2V program. A demonstration program with three built-in
demonstation cases may downloaded and run without any requirement
for a password.
Version 2 features ease of use
in that all computatioal parameters, other than the initial number
of simulated molecules, are set automatically by the program.
It has other enhancements that make it consistent with Version
3 of the DS2V program. It comes with a support program that facilitates
the production of the surface definition file.
There is now provision for a
molecule file from DS2V to be used as a flow input file to DS3V.
This allows the efficient calculation of flows that are part two-dimensional
or axially stmmetric and part three-dimensional.
The program has an option for
parallel operation on dual-core CPUs. This employs domain
decomposition and is the first step in extensions to the programs
that will make full use of the capabilities of the new generation
of multi-core CPUs. The parallel option can be run for demonstration
purposes on a conventional PC.
Version 3.8 of the General 2D/Axi
"Visual DSMC" Program
The Visual DSMC program DS2V version 3.8 has now been
superseded by version 4.
Version 3 features ease of use
in that all computatioal parameters, other than the initial number
of simulated molecules, are set automatically by the program.
It has many other enhancements and there are eight integrated
tutorial/demonstration examples that can be run without any need
for a password. Version 3.6 now includes molecule input/output
files that allow linked DS2V runs with a change in timescale or
allow DS2V output as input to DS3V. In addition, time limits may
be placed on specified input flows so that, for example, pulsed
jets may now be studied. Version 3.8 allows combined DS2V/3V calculations
that are linked by molecule files.
View or download a description
of the examples DS2VEX.PPT
(855 KB)
or an extended file with movies of the permanently
unsteady flow cases DS2VEXM.PPT
(6.45 MB)
The following Windows 95/98/NT programs do not require
expertise in fluid mechanics or experience with the method. All
computational parameters are set automatically and the programs
employ advanced graphical user interfaces.
The "real time" interactive "VISUAL
WIND TUNNEL" programs DSMCS and DSMCX promote an understanding
of the DSMC method and free-molecule to near-continuum gas flows.
This is an 800x600 image and is recommended for 2-400
MHz. computers.
This is a 1024x768 image and is recommended for 600+
MHz. computers.
The program DSWT is for quantitative studies of two-dimensional
or axially symmetric "wind tunnel type" flows. The program
is interactive with a progressive display of user-selected surface
and flow properties. Modern PC's allow computations well into
the continuum regime and students in introductory fluids courses
are able to perform realistic and instructive CFD calculations
on subsonic through to hypersonic flows.
The now obsolete general DSMC program DS2G can be
freely downloaded from this site.
The general
two-dimensional/axially symmetric program DS2G.
The following file is associated with the book "Molecular
Gas Dynamics and the Direct Simulation of Gas Flows"
Corrigendum
and source code of demonstration programs for book.
Further enquiries should be addressed to Graeme Bird
at gab@gab.com.au