Collaborative research center 1667 ATLAS
At VLEO altitudes, the atmosphere is rarefied but still dense enough to affect satellite operations. The primary atmospheric constituent at these altitudes is atomic oxygen. These particles interact with the satellite’s surface, which leads to Drag, surface erosion and thermal effects.
Direct Simulation Monte Carlo (DSMC) is a computational method used to model gas flows in rarefied conditions, such as those found in VLEO. It provides accurate models of gas-surface interactions, helping engineers design more resilient and efficient satellites.
This project is part of the Collaborative Research Centre (CRC) 1667 “Advancing Technologies of Very Low Altitude Satellites (ATLAS)”.
An experimentally well characterized ground test facility will be realised in this project based on a plasma wind tunnel adapted with a skimmer (as seen in Figure). Tests under VLEO conditions will be conducted here, providing insight into material behaviour in Very Low Earth Orbits. The associated methodology for combined experimental and numerical investigations of gas-surface interactions will be established and evaluated. This will particularly allow for an experimental validation of physical models developed and used in the numerical tools in this project.
The PIC-DSMC solver PICLas will be applied to investigate post-skimmer AO flows, further enabling a numerically guided optimisation of the facility’s interchangeable skimmer module (orifice diameter, length/diameter ratio, angles, etc.). Aspects to be considered comprise the influence of gas-surface interactions between the flow and the skimmer as well as the vacuum vessel’s walls on measurements conducted within the facility. Whereas gas-phase chemistry is negligible in free molecular flow environments, heterogeneous catalytic recombination reactions and reflections off these structures may lead to a falsification of measurement results, the effects of which will be analysed cooperatively with Project A02 (gas-surface interactions in VLEO) in order to iteratively improve the experimental setup.
Due to the working principle of turbomolecular pumps utilised to generate and maintain the secondary, low-pressure volume within the test facility, their pumping performance varies between different gas species. This will result in dissimilar suction rates of the corresponding residual partial gas pressures as well as distinct equilibrium levels of partial pressures for a given influx via the skimmer.
In particular, residual hydrogen or water, which are omnipresent in high vacuum systems may then also falsify experimental results, for example through chemical reactions with AO on surfaces or in the gas phase. To account for such effects in the numerical reconstruction, the adaptive boundary condition presently implemented in PICLas, through which a level of homogeneous backflow can be considered at the outflow boundary, will be extended further by the implementation of a species-distinguishing method. The simulation results could then provide information about the necessity and subsequent positioning of additional shielding and/or absorbing walls in the vacuum chamber to minimise these effects.
The post-skimmer core flow will be at a relatively high macroscopic velocity. However, the surrounding of the core flow will be dominated by the random thermal velocity distribution of the present species. For the simulation of flows featuring low macroscopic velocities with particle-based methods, the inherent statistical noise in the extraction of macroscopic parameters is relatively high, in particular for trace species. Obtaining accurate time-averaged values with sufficiently low uncertainty margins can thus be expensive in terms of required computation times. As this project requires the efficient numerical assessment of multiple distinct flow conditions, strategies for an effective noise reduction will be implemented and evaluated.
Kim-Sophie Ellenberger
M. Sc.Research Associate
