Abstract: The unsteady supersonic jet formed by a shock tube with a small high-pressure chamber was used as a simple alternative model for pulsed laser ablation. Understanding the vortex ring formed by the shock wave is crucial in clarifying the behavior of unsteady supersonic jet discharged from an elliptical cell. Therefore, this study investigated the behavior of vortex rings and a jet. The experiment and numerical calculation were conducted using the schlieren method and by solving the axisymmetric two-dimensional compressible Navier–Stokes equations, respectively. In both, the calculation and the experiment, laser ablation is conducted for a certain duration, followed by discharge through the exit. Moreover, a parametric study was performed to demonstrate the effect of pressure ratio on the interaction among vortex rings and the supersonic jet. The interaction between the supersonic jet and the vortex rings increased the velocity of the supersonic jet up to the magnitude of the velocity at the center of the vortex rings. The interaction between the vortex rings increased the velocity at the center of the vortex ring.
Abstract: This paper outlines the development of an
experimental technique in quantifying supersonic jet flows, in an
attempt to avoid seeding particle problems frequently associated with
particle-image velocimetry (PIV) techniques at high Mach numbers.
Based on optical flow algorithms, the idea behind the technique
involves using high speed cameras to capture Schlieren images of the
supersonic jet shear layers, before they are subjected to an adapted
optical flow algorithm based on the Horn-Schnuck method to
determine the associated flow fields. The proposed method is capable
of offering full-field unsteady flow information with potentially
higher accuracy and resolution than existing point-measurements or
PIV techniques. Preliminary study via numerical simulations of a
circular de Laval jet nozzle successfully reveals flow and shock
structures typically associated with supersonic jet flows, which serve
as useful data for subsequent validation of the optical flow based
experimental results. For experimental technique, a Z-type Schlieren
setup is proposed with supersonic jet operated in cold mode,
stagnation pressure of 4 bar and exit Mach of 1.5. High-speed singleframe
or double-frame cameras are used to capture successive
Schlieren images. As implementation of optical flow technique to
supersonic flows remains rare, the current focus revolves around
methodology validation through synthetic images. The results of
validation test offers valuable insight into how the optical flow
algorithm can be further improved to improve robustness and
accuracy. Despite these challenges however, this supersonic flow
measurement technique may potentially offer a simpler way to
identify and quantify the fine spatial structures within the shock shear
layer.
Abstract: In this work, we try to find the best setting
of Computational Fluid Dynamic solver available for the problems in
the field of supersonic internal flows. We used the supersonic air-toair
ejector to represent the typical problem in focus. There are
multiple oblique shock waves, shear layers, boundary layers
and normal shock interacting in the supersonic ejector making this
device typical in field of supersonic inner flows. Modeling of shocks
in general is demanding on the physical model of fluid, because
ordinary conservation equation does not conform to real conditions in
the near-shock region as found in many works. From these reasons,
we decided to take special care about solver setting in this article by
means of experimental approach of color Schlieren pictures and
pneumatic measurement. Fast pressure transducers were used to
measure unsteady static pressure in regimes with normal shock in
mixing chamber. Physical behavior of ejector in several regimes is
discussed. Best choice of eddy-viscosity setting is discussed on the
theoretical base. The final verification of the k-ω SST is done on the
base of comparison between experiment and numerical results.
Abstract: Supersonic open and closed cavity flows are investigated experimentally and computationally. Free stream Mach number of two is set. Schlieren imaging is used to visualise the flow behaviour showing stark differences between open and closed. Computational Fluid Dynamics (CFD) is used to simulate open cavity of flow with aspect ratio of 4. A rear wall treatment is implemented in order to pursue a simple passive control approach. Good qualitative agreement is achieved between the experimental flow visualisation and the CFD in terms of the expansion-shock waves system. The cavity oscillations are shown to be dominated by the first and third Rossister modes combining to high fluctuations of non-linear nature above the cavity rear edge. A simple rear wall treatment in terms of a hole shows mixed effect on the flow oscillations, RMS contours, and time history density fluctuations are given and analysed.
Abstract: We measured the major and trace element contents
and Rb-Sr isotopic compositions of 12 tektites from the Maoming
area, Guandong province (south China). All the samples studied are
splash-form tektites which show pitted or grooved surfaces with
schlieren structures on some surfaces. The trace element ratios Ba/Rb
(avg. 4.33), Th/Sm (avg. 2.31), Sm/Sc (avg. 0.44), Th/Sc (avg. 1.01) ,
La/Sc (avg. 2.86), Th/U (avg. 7.47), Zr/Hf (avg. 46.01) and the rare
earth elements (REE) contents of tektites of this study are similar to the
average upper continental crust. From the chemical composition, it is
suggested that tektites in this study are derived from similar parental
terrestrial sedimentary deposit which may be related to post-Archean
upper crustal rocks. The tektites from the Maoming area have high
positive εSr(0) values-ranging from 176.9~190.5 which indicate that
the parental material for these tektites have similar Sr isotopic
compositions to old terrestrial sedimentary rocks and they were not
dominantly derived from recent young sediments (such as soil or
loess). The Sr isotopic data obtained by the present study support the
conclusion proposed by Blum et al. (1992)[1] that the depositional age
of sedimentary target materials is close to 170Ma (Jurassic). Mixing
calculations based on the model proposed by Ho and Chen (1996)[2]
for various amounts and combinations of target rocks indicate that the
best fit for tektites from the Maoming area is a mixture of 40% shale,
30% greywacke, 30% quartzite.
Abstract: The mixture formation prior to the ignition process
plays as a key element in the diesel combustion. Parametric studies of
mixture formation and ignition process in various injection parameter
has received considerable attention in potential for reducing
emissions. Purpose of this study is to clarify the effects of injection
pressure on mixture formation and ignition especially during ignition
delay period, which have to be significantly influences throughout the
combustion process and exhaust emissions. This study investigated
the effects of injection pressure on diesel combustion fundamentally
using rapid compression machine. The detail behavior of mixture
formation during ignition delay period was investigated using the
schlieren photography system with a high speed camera. This method
can capture spray evaporation, spray interference, mixture formation
and flame development clearly with real images. Ignition process and
flame development were investigated by direct photography method
using a light sensitive high-speed color digital video camera. The
injection pressure and air motion are important variable that strongly
affect to the fuel evaporation, endothermic and prolysis process
during ignition delay. An increased injection pressure makes spray tip
penetration longer and promotes a greater amount of fuel-air mixing
occurs during ignition delay. A greater quantity of fuel prepared
during ignition delay period thus predominantly promotes more rapid
heat release.