Flow physics of the gasdynamically induced particle production

Im Fokus der Arbeit stehen Simulationen zur Klärung strömungsmechanischer und partikeldynamischer Detailfragen bei der gasdynamisch initiierten Partikelerzeugung (GiP). Insbesondere werden die Struktur des Pseudo-Stoßsystems, die experimentellen und numerischen Einflussfaktoren auf den Druckverlauf, die Stoßoszillation und das Symmetrieverhalten untersucht und erklärt. Durch die gekoppelte Simulation der Strömung und des Partikelwachstums im Partikelreaktor wird der Prozess der Partikelbildung, die Partikelgröße und -größenverteilung in sehr guter Übereinstimmung zum Experiment vorhergesagt.This thesis reports on numerical simulations that provide a better understanding of the gasdynamically induced particle production (GiP). In particular, the structure of the pseudo-shock system as well as the effect of experimental and numerical parameters on the observed pressure distribution, shock oscillations and symmetry breaking are analyzed and explained. Predictions of particle formation, particle size and particle size distribution are in excellent agreement with experimental results due to the coupled simulation of particle dynamics and fluid flow

Impact of bypass mass flow on shock motion in pseudo-shock systems

During previous investigations on pseudo-shock systems we have observed reproducible differences between measurement and simulations for the pressure distribution as well as for size and shape of the pseudo-shock system. A systematic analysis of the deviations leads to the conclusion that small gaps of delta z = O(10^−4) m between quartz-glass side walls and the metal contour of the test section are responsible for this mismatch. The focus of the present study is put on a numerical analysis of the interaction between gap-flow and shock oscillation. Furthermore, the impact of gaps on the occurrence of shock asymmetry is discussed

Numerical and experimental investigation of the effect of bypass mass flow due to small gaps in a transonic channel flow

In order to perform Schlieren visualizations of transonic nozzle flows, quartz glass windows are mounted on both side walls of the test section. However, to avoid strong stresses within the glass, these windows cannot be flush mounted to the facility.Thus, small gaps of delta x = O(10-4) m occur between the glass side walls and the metal contour. During a previous investigation we observed that these gaps result in a small bypass mass flow that affects the pressure distribution within the supersonic part of the Laval nozzle as well as the shock location. The focus of the present study is put on detailed numerical analysis of the flow within the gaps and its feedback on the transonic base flow within a slender nozzle

Analysis of gap influence on pseudo-shock system asymmetry in rectangular Laval nozzles

A pseudo-shock system in a rectangular nozzle with parallel side walls is investigated at different pre-shock Mach numbers by means of wall pressure measurements and high-speed schlieren visualization. In detail, we analyze the effect of small streamwise gaps (Δz = O(10^-4) m) between constructional elements. An asymmetrical pseudo-shock system is observed for a gap-free configuration whereas configurations including small gaps show a symmetric behavior. Furthermore, the internal structure of the entire pseudo-shock system visibly changes by increasing the gap width. A detailed analysis of pressure measurements as well as high speed schlieren images is shown and possible explanations are discussed

Pseudo-shock system structure in rectangular Laval nozzles with gaps

A detailed experimental investigation of the influence of small gaps on a pseudo-shock system has been performed for different gap sizes and pressure ratios p02/p01. In addition to schlieren images that only provide integral information of the total light deflection through the test section, 3-D numerical data was additionally used to investigate 3-D phenomena of the shock system. The combination of experimental and numerical techniques considerably increased the understanding of the underlying mechanisms that cause the change in structure and position of the pseudo-shock system when increasing the gap size

Numerical and experimental investigations of pseudo-shock systems in a planar nozzle: impact of bypass mass flow due to narrow gaps

During previous investigations on pseudo-shock systems, we have observed reproducible differences between measurement and simulations for the pressure distribution as well as for size and shape of the pseudo-shock system. A systematic analysis of the deviations leads to the conclusion that small gaps of Δz=O(10 −4 ) m between quartz glass side walls and metal contour of the test section are responsible for this mismatch. This paper describes a targeted experimental and numerical study of the bypass mass flow within these gaps and its interaction with the main flow. In detail, we analyze how the pressure distribution within the channel as well as the size, shape and oscillation of the pseudo-shock system are affected by the gap size. Numerical simulations are performed to display the flow inside the gaps and to reproduce and explain the experimental results. Numerical and experimental schlieren images of the pseudo-shock system are in good agreement and show that especially the structure of the primary shock is significantly altered by the presence of small gaps. Extensive unsteady flow simulations of the geometry with gaps reveal that the shear layer between subsonic gap flow and supersonic core flow is subject to a Kelvin–Helmholtz instability resulting in small pressure fluctuations. This leads to a shock oscillation with a frequency of f=O(10 5 )s −1 . The corresponding time scale τ (s) is 16 times higher than the characteristic time scale τ δ =δ/U ∞ of the boundary layer given by the ratio of the boundary layer thickness δ directly ahead of the shock and the undisturbed free stream velocity U ∞ . To assess the reliability of our numerical investigations, the paper includes a grid study as well as an extensive comparison of several RANS turbulence models and their impact on the predicted shape of pseudo-shock systems