Due to stringent emission regulations, it is of practical significance to understand cycle-to-cycle variations in the combustion of fossil or renewable fuels to reach future emission regulations. The present study aims to conduct a parametric investigation to analyse the influence of the valve lift and different mass flows of an inlet valve of the test engine “Flex-OeCoS” on the flow structures. To gain a deeper understanding of the flow behaviour, an optical test bench for 2D Particle Image Velocimetry (PIV) and a Large Eddy Simulation (LES) are used. Turbulence phenomena are investigated using Proper Orthogonal Decomposition (POD) with a quadruple decomposition and the Reynolds stress transport equation. The results show good agreement between the PIV and LES. Moreover, the main flow structures are primarily affected by valve lift while being unaffected by mass flow variation. The turbulent kinetic energy within the flow field increases quadratically to the mass flow and to the decreasing valve lift, where large high-energetic flow structures are observed in the vicinity of the jet and small low-energetic structures are homogeneously distributed within the flow field. Furthermore, the convective flux, the turbulent diffusive flux, the rate of change, and the production of specific Reynolds stress are the dominant terms within the specific Reynolds stress transport equation.

Transonic planar flows around a circular cylinder are investigated numerically for laminar and turbulent flow conditions with Reynolds numbers of 50≤ReD≤300 and 8890≤ReD≤ 80,000 and free stream Mach numbers in the range of 0.2≤Ma∞≤2. A commercially available CFD tool is used and validated for this purpose. The results show that the flow phenomena occurring can be grouped into eight regimes. Compared to the incompressible flow regimes, several new phenomena can be found. In contrast, at higher Ma∞ of 0.6≤Ma∞≤0.8 vortices in the wake of the cylinder are suppressed for ReD=50. In some cases, Ma∞=0.8 and ReD≥300, λ-shocks are formed in the near cylinder wake. For supersonic Ma∞, two different phenomena are observed. Beside the well-known oblique and detached shocks, for 50≤ReD≤300 a wake with instabilities is formed downstream of the cylinder. Furthermore, the temporal mean drag coefficient C¯D, the Strouhal number Str, as well as the critical Mach number Macrit are calculated from the simulation results and are interpreted.

A new measurement method has been developed that enables an acoustic detection of individual coarse mode particles (aerodynamic particle diameter > 1 µm) by impaction on a piezo transducer. The aerosol is accelerated and each momentum transfer by a particle is measured as a characteristic pulse in the transducer signal whose amplitude is directly proportional to the particle mass. The current single particle mass detection limit is approximately 50 picograms, which corresponds to an aerodynamic particle diameter of ∼5 µm. The measurement technique allows a direct in-situ mass measurement of single coarse mode particles that is unique because it is based on first principles, is relatively simple and less prone to measurement artefacts compared to other methods. This particle mass measurement is independent of assumptions on particle properties like shape, density or refractive index. This technology is of interest for scientific, industrial and health-related monitoring applications as different sources of atmospheric aerosols can be identified via size-resolved mass measurements. Challenges to be overcome include a further lowering of the detection limit, eliminating systematic errors from bouncing phenomena, optimizing the correct mass assignment as well as improving the robustness against sensor vibrations and acoustic noise. In addition, the complete sensor should be portable and affordable. With the future goal to detect submicron particles, coincidence and a reduction of the impactor's cutoff diameter additionally become important issues.