DS2V version 3

The current release is 3.8.04 dated February 5, 2007. When run this produces the executable program DS2V.EXE that can be freely used to run eight integrated demonstration examples.

The setting of new data or the modification of existing requires a password for the first run in a new directory. The password is e-mailed to purchasers of the DS2Vprogram who then become license holders. The same password is used for all versions.

Description of the DS2V (Version 3.7) Program

The flowfield limits are specified by the minimum and maximum values of the x and y co-ordinates. These four boundaries may be stream boundaries, planes if symmetry, vacuum boundaries or the axis of an axially symmetric flow, although the minimum y coordinate may be at any radius. This basically rectangular flowfield may be modified by the definition of one or more separate "surfaces". Each of these must be either a closed surface or an open surface that starts and finishes on a boundary of the basic rectangle. Each surface is specified either by a combination of straight line and circular arc segments . Most importantly with regard to the flexibility of the program, each of the surfaces may be a combination of solid surface segments, stream entry boundaries and specified flow entry boundaries. The surfaces may be set to move in the plane of the flow or normal to it. The latter boundary in the axially symmetric case permits the study of rotating flows.

The flow input boundaries permit the study of a wide range of problems that involve jets and plumes. A secondary stream may be set to occupy part of the initial flowfield and this permits the study of unsteady shock tube type flows and shear flows. Alternatively, shock waves may be generated by a moving "piston" type boundary for diffraction studies. Special "constant pressure" boundaries are available for the generation of steady internal flows that are driven by a pressure differential. Periodic boundary conditions are also available.

The flowfield grid consists of a background rectangular grid that is uniformly spaced in each direction. Each division is further subdivided into a large number of elements. The total number of elements is of the order of the number of simulated molecules. There is a restriction on the geometry in that the size of the elements sets the minimum thickness of bodies within the flow. The computational cells comprise those elements that are nearer to a particular cell node than to any other node. The nodes for the sampling cells are initially comprised of the centers of the divisions that are within the flow plus the points that define the sampling intervals along any surfaces. Separate cell systems are employed for collisions and the sampling of flow properties, the former being much smaller. At any time during the calculation, the cells may be adapted to the flow densities and density gradients that exist at that time. The user selects the desired number of simulated molecules within each adapted cell.

The gas may be chosen from a menu that includes ideal air, real air with vibration and chemical reactions, nitrogen, argon and a hard sphere gas. Alternatively, a custom non-reacting or reacting gas may be specified. Surface reactions are included.

The program employs the physical gas models that have been described and validated in Bird (1994). The gas is a mixture of the VHS or VSS models and the cross-sections, the viscosity-temperature index (which determines the way in which the cross-section changes with the relative velocity), are set separately for every molecular species. The VSS is to be preferred for gas mixtures because it allows the correct Schmidt number to be set. A classical Larsen-Borgnakke model is employed for the rotational degrees of freedom, while a quantum model is used for the vibrational modes. The chemical reaction model calculates reactive cross-sections that are consistent with the measured rate constants.

The classical diffuse reflection model with complete accommodation of the gas to the surface temperature is appropriate to "engineering surfaces" that have not been exposed for a long period to ultra-high vacuum. The CLL model is now included for a better representation of ultra-clean surfaces. A fraction of specular reflection may still be specified, but the specular option is recommended only for the setting of symmetry surfaces. A set temperature distribution may be specified for the surface or it may be specified as an adiabatic surface with zero heat transfer. The temperature distribution on the surface is one of the output quantities for adiabatic surfaces. The adiabatic surface may be either insulated with zero thermal conductivity or perfectly conducting with infinite thermal conductivity. Thermal radiation at a specified surface emissivity is included.

Files of the molecules crossing specified lines may be generated and these may be used as part of the molecules entry flux to the DS3V program. This facilitates the computation of floews that are part two-dimensional or axially symmetric and part three-dimensional.