Articles techniques et présentations

The aim of this investigation is to apply computational fluid dynamics simulations to support the design development of an impaction-based probe collector. This collector should allow submicron aerosol impactions in the supersonic free stream radially aside a sounding rocket body in the mesosphere (altitude of 85 km). The overall goal of the development process is to collect mesospheric aerosols and to physico-chemically analyze them to start closing the research gap in the upper atmosphere and to gain further insight into high atmosphere processes. However, sampling and analysis is only possible by considerable costs and great effort by using sounding rockets, which is why only a sparse database is available so far.

The development process and efficiency analyses are based on numerical simulations achieved by the software COMSOL Multiphysics®, where the simulation workflow is divided into two independent studies: First, with the CAD Import Module in COMSOL®, the detailed rocket geometry is implemented into the model. Then, the supersonic flow field around the rocket under varying flight attitudes is simulated by the High Mach Number Laminar Flow interface in COMSOL Multiphysics® by solving the Navier-Stokes equations for compressible fluids, where a steady state solution is obtained. Sharp flow field discontinuities (e.g., at the location of the shockwave) are adequately resolved by a performed mesh refinement. Of particular interest for the design and arrangement of the probe collector is the evaluation of the evolving flow field around the sounding rocket at free stream Mach numbers Ma1 = 1.31 and Ma2 = 1.75 (at 85 km) and the localization of the occurring shockwave. Furthermore, the thickness of the boundary layer is investigated to prevent it from influencing the aerosol impactions as an artifact.

Secondly, for modeling the particle trajectories and thus the aerosol impactions on the probe collector, Particle Tracing for Fluid Flow in COMSOL® is utilized, where Newton's second law is applied. For this purpose, a preliminary investigation of possible particle forces and their magnitude is performed and corresponding forces (drag and Brownian force) are considered in the model. Various design options for the collector surfaces are investigated with respect to the best possible collection performance. With the final collector design, the number of impacted particles (by particle counter application in COMSOL Multiphysics®) on collector surfaces are analyzed in a parameter study (Parametric Sweep) for different aerosol number concentrations. Furthermore, the sampling efficiency of the probe collector is estimated at different angles of attack and aerosol number concentrations. In conclusion, impactions onto designated collector surfaces are highly probable according to simulation results. Moreover, our COMSOL® model can be validated by measurement results of the future planned rocket flight.