Drag and flow pattern around the truss structure of the International Space Station using molecular dynamics

Rarefied gas flow over a long solid hexagonal cylinder has been simulated. Each molecule is modeled as a sphere of equal diameter and mass undergoing elastic collisions. Conditions for the simulation closely follow those that would be experienced by the International Space Station in orbit. These conditions are Mach number 8 and 0.167 for the temperature ratio between the solid and gas. The actual Knudsen number for the orbital condition is of order 1,000 and spacing ratio 58,000 (the spacing ratio is the ratio of mean spacing between two particles and to the diameter.) Though the whole flow regime can be considered as being free molecular flow where intermolecular collisions can be neglected, accumulation of particles on the leading surface will force a lower Knudsen number and spacing ratio. A Knudsen number of order 1 already displays the free molecular flow regime behavior. Therefore the simulation used Knudsen numbers of 0.75 to 1.09. The overall spacing ratio of 3.5 is used throughout the simulation as it was shown by Greber and Wachman to be sufficiently large for gaseous behavior. The computational strategy used is molecular dynamics where all particles are tracked at all times. Due to the shape of the hexagon, triangular data-collecting grids were used where the relative position of the particles with respect to the vertices of the triangle determined the quantities of interest such as density, the 3 components of the velocity and the temperature