Experimental and numerical analysis of the structure of pseudo-shock systems in laval nozzles with parallel side walls

Detailed numerical and experimental investigations of pseudo-shock systems in a Laval nozzle with parallel side walls are carried out. The location of the pseudo-shock system is defined in this system of two choked Laval nozzles by the ratio of the critical cross sections A2*/A1*, the stagnation pressure loss across the shock system and viscous losses. The wall pressure distributions and high-speed schlieren videos recorded in the experiments are compared to the results of a steady and an unsteady numerical simulation. For the steady case, good agreement is found between the calculated and measured shock structure and pressure distribution along the primary nozzle wall, except for a remaining slight deviation in the shock position. For the unsteady case, in which asymmetric shock configurations are observed, deviations of the results with respect to the stochastic wall attachment of the shock system are given which indicate the necessity of further investigations on that topic

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

Analysis of Pseudo-Shock System Structure and Asymmetry in Laval Nozzles with Parallel Side Walls

The scope of this work is to investigate the structure and stochastic asymmetry of pseudo-shock systems in Laval nozzles with parallel side walls in detail by numerical and experimental means. The location of the pseudo-shock system is defined in an adiabatic system of two successive chocked Laval nozzles by the ratio of the critical cross sections A2*/A1*, the stagnation pressure loss across the shock system and viscous losses. Depending on the geometry and the test conditions, the pseudo-shock system attaches stochastically to the upper and lower nozzle wall where the driving mechanism is analysed by means of high speed and short pulse Schlieren visualisation and numerical simulations

Gas-Phase Synthesis of Non-Agglomerated Nanoparticles by Fast Gasdynamic Heating and Cooling

A concept for a new shock-wave reactor for producing nanoparticles starting from the gas-phase is presented. Preliminary studies of the gasdynamic behavior of the shock system, the precursor mixing and the water injection have been carried out numerically and in experiment. The gained knowledge is utilized to design and build up a pilot facility for experimental studies, including the generation of nanoparticles