Paper VT-MoA11
Moving Surfaces in DSMC: Implementation, Validation and Applications

Monday, October 20, 2008, 5:20 pm, Room 205

Applications in vacuum technology exist where moving surfaces play a role. Under rarefied conditions, moving surfaces influence heat and momentum transfer and surface stress. Several methods have been used to model moving surfaces in Test Particle and Direct Simulation Monte Carlo methods. In simple situations, such as plane Couette flow, one can add an extra velocity vector to molecules hitting a moving surface (the modeled surface stays at rest in an inertial domain). In other examples the flow field is simulated by using a moving calculation domain and the molecule trajectory is corrected for this. Both methods are limited to only one moving surface. But in cases where the domain contains both moving and non-moving surfaces these methods cannot be applied or separate domains need to be defined and the interaction at the interface needs to be prescribed or iteratively determined. We have developed an algorithm to simulate an arbitrary number of moving and non-moving planes in one domain for DSMC methods. The planes can have any velocity vector but should not change the geometry. The method uses periodic boundary conditions (especially implemented for this) and is grid independent. In the DSMC scheme molecule trajectories are determined in two steps: particle movement between collisions and particle collisions. Boundary interaction is taken into account during the particle movement phase. In our method the moving surface is a special boundary with all properties of a normal boundary (temperature, accommodation coefficient, reflection velocity distribution). The difference is that the trajectory of each surface is determined and the exact time and place of interaction between the boundary and a molecule is determined. This way, the exact molecule trajectory is determined during its movement phase taking into account the plane movements during that time. Multiple collisions in one time step between moving and non-moving surfaces are taken into account. The current algorithm is limited to 2D with planes moving in a straight line, but the principle is valid for 3D and planes moving with arbitrary and changing velocities (although the calculation of collisions between plane and molecules becomes more tedious). Results will be shown of validation simulations and possible applications, such as simulations of displacement pumps, moving stages or surfaces in lithography, CVD, PVD and ALD systems, sample manipulation, vacuum conveyor belts etc.