The dynamics of a converging conical shock wave were first considered theoretically as a mechanism for generating high pressures for the initiation of nuclear fusion. However, an anomaly was identified whereby the axial jet curved away from the system axis, seemingly related to occlusions of the shock tube upstream, during some experimental tests of these dynamics. This study confirms the effect of upstream occlusion on jet deflection though the mechanism is unclear as the jet may curve either toward or away from the upstream occlusion azimuth depending on the distance from the reflection point. The primary mechanisms suggested are either the wake of the upstream occlusion and the contingent momentum deficit, or else the effect of the occlusion on the shape of the conical shock wave at focus. Further numerical work is suggested as a means of resolving this.
KEYWORDS: Reflection, Wave propagation, High speed imaging, Light sources, Visualization, Photonics, Current controlled current source, Cameras, High speed cameras, Video
Experiments were conducted in a shock tube in order to determine the increases in shock wave strength due to reductions in area. Previous work has shown that if the reduction is too sudden significant wave reflections occur and gains are limited. A variety of curved symmetrical contractions are used, made up of parabolic surfaces with different points of inflection. High-speed Schlieren imaging was used to characterize the wave patterns with particular emphasis on wave reflections. Greatest wave amplification is present when Mach reflection of the wave is not reached at all, and this was found to occur with parabolic profiles with inflection point at 60% of the profile length. Clear Mach reflection is evident with the inflection point at 40% and the post shock flow shows significant reflected waves with their associated losses. With an area reduction of 80% and a inflection point at 60% of the contraction, a typical result gives an increase in Mach number from 1.6 to 2.0, corresponding to a 61% increase in post-shock pressure. It is found that profiles with later inflection points provide a more gradual initial area change and allow weaker compression waves to develop which can significantly reduce or even avoid transition to Mach reflection.
Experiments were conducted in a shock tube to determine the effect of planar wedge inlet geometry on the shock wave reflection pattern that occurred on a wedge. High-speed schlieren imaging was used to visualize the experiments conducted in air with a nominal incident shock strength of Mach 1.31. The experimental test pieces consisted of a wedge mounted above the floor of the shock tube where the underside wedge angle was varied. The upper wedge angle was fixed at 30°, resulting in a Mach reflection. The underside wedge angle was either 30° or 90°, corresponding to a conventional and blunt wedge respectively. For the cases presented here, the reflected shock from the initial interaction reflects off of the shock tube floor and diffracts around the wedge apex. A density gradient is formed at the wedge apex due to this process and results in a vortex being shed for the 90° wedge. It was shown by simple measurements that the diffracted wave could reach the triple point of the upper Mach reflection if the wedge were of sufficient length.
Xenon arc lamps have been identified as a suitable continuous light source for high-speed imaging, specifically high-speed Schlieren and shadowgraphy. One issue when setting us such systems is the time that it takes to reduce a finite source to the approximation of a point source for z-type schlieren. A preliminary design of a compact compound lens for use with a commercial Xenon arc lamp was tested for suitability. While it was found that there is some dimming of the illumination at the spot periphery, the overall spectral and luminance distribution of the compact source is quite acceptable, especially considering the time benefit that it represents.
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