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.
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.
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.
It is shown that with the construction of unique experimental facilities together with the use of short-duration and timeresolved
digital imaging, a number of aspects regarding shock wave reflection, which are regularly quoted in the
literature, are found to be either inexact or even incorrect. Two main issues are addressed here: the first being the
experimental resolution of the two von Neumann Paradoxes, and the other the reflection of shock waves off curved
surfaces. Neither of these could have been addressed without high-speed imaging. The paradoxes arises from the finding
that von Neumann's theory for flow across an oblique shock wave is excellent for describing regular reflection and threeshock
patterns for non-weak shocks, but fails for weak shocks and for predicting transition between regular and Mach
reflection. Specially constructed rigs, one which magnifies the process ten times and the other which removes the effect
of the wall illustrate the reasons for the paradoxes. Recent studies on shock reflection in cavities using a novel flow
visualization technique and high-speed time resolved imaging has shown that the shock wave reflection off a curved wall
is somewhat different from that described in the literature and which has been extracted from single shot imaging.
Dynamic shock wave reflection generated by a rapidly pitching wedge in a steady supersonic free stream have been studied with numerical simulation previously. This paper publishes the development and design of an experimental facility to characterize these dynamic shock wave reflection phenomena. The paper documents details of the experimental rig, flow visualization technique and the high-speed imaging system. High-speed schlieren images from tests with gradual, but dynamic wedge pitch are included. Flow field images are captured with a Photron Ultima APX-RS high-speed camera at 250 fps. Tests were conducted at Mach 1.9 and Mach 3.0 free stream conditions in a supersonic wind tunnel. The high-speed imaging enabled the accurate determination of the point of transition between regular and Mach reflection. The wedge incidence for which the shock system is swallowed and disgorged was also measured during these tests.
The visual study of unsteady shock wave dynamics has in the past predominantly been done using single-shot images.
The advent of ultra-fast, good-resolution high-speed digital cameras has changed this state of affairs and allows the true
development of the flow to be studied. It enables the detection of weaker features which are easily overlooked in singleshot
visualizations by virtue of the fact that human vision is very sensitive to detecting the motion of an object, even if it
generates only a faint optical signal. Recent application of these devices to the study of the focusing of a shock wave in a
cylindrical cavity has identified a number of previously unknown features, while other features that previously had been
inadequately reported could be clearly identified and explained The observation of deliberately generated weak
disturbances allows the quantification of which part of the flow is influenced by which part of the boundaries
encompassing it. Whilst the imaging itself is very useful it is also highly desirable to use techniques from which
quantitative data can be obtained. Color, such as in direction- and magnitude-indicating color schlieren, and polychrome
shearing interferometry, adds an additional dimension to such investigations.
The application of electronic speckle pattern interferometry (ESPI) to the visualization of a typical high-speed compressible flow is investigated. ESPI is an interferometric technique that has established itself as a reliable alternative to holographic interferometry in the measurement of small displacements and of vibrations, and is increasingly being used in flow visualization. It can instantly and in real time produce interferometric images in digital form on a video screen, with no photographic processing being required. In this paper two flows are examined, the one a low speed flow of a thermal plume arising from a hot soldering iron, for which real-time visualization is achievable; and the other single frame imaging of a shockwave emerging from a small round open- ended shock tube. ESPI is shown to be a valuable tool in the visualization of compressible flows, and a good alternative to holographic interferometry in obtaining quantitative density data about a flow field. A method for obtaining interferograms with finite fringe-width is presented. The main benefit of using ESPI for flow visualization is that the interferometric image is immediately accessible for viewing on a monitor, so avoiding the tedious photographic holographic reconstruction process. Advances in camera technology are fast overcoming its disadvantage, low image resolution.
This paper describes the use of high-speed photography, and videography, in the study of material distortion and movement when a shock wave traverses a highly deformable porous structure, such as a blob of foam or a porous bed of particles. The effects of surface porosity can be significant in determining the nature of reflection of shock waves from surfaces. Not only are wave geometries substantially modified but the resulting wall pressures are also strongly affected. It, in addition, the surface is highly deformable by being made up of an elastic matrix or a collection of discrete particles, then the reflection geometry and loading can be even more complex. It is known, for example, that shock wave impact on open-cell polyurethane foam attached to a wall can cause a significant increase in pressure on the wall compared to reflection off a plane rigid wall without covering. The motion of the interface is an essential consideration in understanding the dynamics of these interactions. These studies could have application to the effects of blast wave propagation over complex surfaces such as forests, grasslands, and snow; as well as in establishing the efficacy of safety padding and attenuation materials under shock and impact loading conditions. Studies on an assortment of materials are presented, using a variety of visualization techniques. Recording methods used range from short duration flash photography (both shadow and schlieren), through multi-frame videography; to single frame, multi-exposure video capture with a camera capable of rates up to 1 million pictures per second. In the case of shock wave impact on specimens of polyurethane foam, the results clearly show the expulsion and reingestion of shock heated gas from within the foam body as the material collapses and then recovers, coupled with longitudinal and transverse oscillations of the body of the foam material. For blast wave propagation over porous beds, occurrence of particle lift off, bed fluidization, and the generation of surface dunes are evident. The recordings allow the calculation of the velocities and accelerations of the various interfaces and particles to be made, using suitable image processing techniques. Thus, estimates may be made of the unsteady drag forces acting on the individual particles.
It has recently been established that the porosity of surfaces can have a significant effect on the reflection geometry of shock waves, and thus on the loads that are generated. This paper describes a comprehensive series of tests on the patterns of shock wave reflection from a surface covered with a series of slits, over the full range of angles of incidence from glancing to normal. The use of double-pulse interferometry is shown to be ideally suited to the study of complex compressible flow fields of this type, not only because of the high resolution but also because the tracking of fringes gives a very clear indication of both the general flow field as well as the fine structure, and thus helps clarify the mechanisms whereby the interaction process is modified from the case of reflection off a plane impervious wall. These features of the method allow a number of effects to be established which have not previously been evident, or are in conflict with the assumptions of previous studies. Specifically it is shown that the inflow angle relative to the plate is almost constant (at about 17 degrees) for shock incidence angles from zero to about 50 degrees and that the flow leaves the plate almost normal to the surface; although there is a slight drift in a direction opposite to that of the shock induced flow. Furthermore it is shown that many of the flow variables, and specifically the inflow velocity, exhibit a maximum in the vicinity of transition from regular to Mach reflection. An analysis of the motion of the acoustic waves generated from the lips of the perforations allows estimates to be made of the constancy of the inflow along the surface. These waves are convected with the flow and as they do not meet the wall at a right angle show that the convection is towards the wall. Using this method it is found that the inflow is constant behind the reflected wave in regular reflection but variable for the case of Mach reflection. Photographs of the region under the plate clearly show how the wavelets emerging from the perforations coalesce to form a transmitted wave. Measurements of this wave enable the pressure below the plate to be assessed and thus to obtain an estimate of the pressure drop across the plate. A striking feature on the underside of the plate is that the contact surface separating flow that was initially above the plate from that engulfed by the transmitted shock is made up of a string of vortices arising from the wave diffraction into the perforations.
Double exposure holographic interferometry and high speed laser shadowgraph photography and videography are used to investigate the mutual reflection of two plane shock waves. Normally research on the transition from regular to Mach reflection is undertaken by allowing a plane shock wave to impinge on a wedge. However due to the boundary layer growth on the wedge, regular reflection persists at wedge angles higher than that allowed for by inviscid shock wave theory. Several bifurcated shock tubes have been constructed, wherein an initially planar shock wave is split symmetrically into two and then recombined at the trailing edge of a wedge. The plane of symmetry acts as an ideal rigid wall eliminating thermal and viscous boundary layer effects. The flow visualization system used needs to provide high resolution information on the shockwave, slipstream, triple point and vortex positions and angles. Initially shadowgraph and schlieren methods, with a Xenon light source, were used. These results, while proving useful, are not of a sufficient resolution to measure the Mach stem and slipstream lengths accurately enough in order to determine the transition point between regular and Mach reflection. To obtain the required image resolution a 2 joule double pulse ruby laser, with a 30 ns pulse duration, was used to make holographic interferograms. The combined advantages of holographic interferometry and the 30 ns pulse laser allows one to obtain much sharper definition, and more qualitative as well as quantitative information on the flow field. The disadvantages of this system are: the long time taken to develop holograms, the difficulty of aligning the pulse laser and the fact that only one image per test is obtained. Direct contact shadowgraphs were also obtained using the pulse ruby laser to help determine triple point trajectory angles. In order to provide further information a one million frames per second CCD camera, which can take up to 10 superimposed images, was used to obtain multiple focussed shadowgraphs. Although limited resolution is obtained, due to the low resolution of the camera, information is obtained about the time evolution, and validity of the self similar assumption, of the shock wave structure. This paper highlights the practical implementation of, and the results obtained, using the above mentioned techniques in order to further explain the transition from regular to Mach reflection, as well as to describe the interaction of unsynchronized shock waves at the apex of a wedge. The advantages and disadvantages of each system are discussed as well as the benefits of using these different optical systems in conjunction with each other, to obtain a more complete description of the shock wave interaction.
A bifurcated shock tube is used to create two synchronized waves of equal strength. Essentially a single shock wave is split symmetrically in two, the two waves then are later brought back together at a trailing edge of a wedge to interact, the plane of symmetry acting as an ideal rigid wall. The normal method of studying Mach reflections is to allow a plane shock wave to impinge on a wedge, however the boundary layer growth on the wedge surface effectively ensures that the flow direction behind the Mach stem does not have to satisfy the boundary condition of being parallel to the surface of the wedge. Thus the transition from regular to Mach reflection occurs at higher angles of incidence than theory allows. The present experiment was initiated to generate data on the 'ideal' case of reflection off a plane wall. The advantage of the new system is that like classical theory and computational solutions of the inviscid Euler equations, the boundary layer no slip condition is not imposed at the plane of reflection. Optical methods ar used to investigate the post-shock flows, as well as to help explain the complex interactions which occur when the two shock waves are not synchronized. These interactions show many very interesting features and clearly indicate the need for higher resolution measurements such as are obtained using holographic interferometry, and also to extend the work to different wedge angles and Mach numbers.
A versatile light source producing three independent flashes in each of the three primary colors has been developed. The system uses an arrangement of Xenon flash tubes and dichroic filters, with the optical axis of the three emerging beams being coincident. This feature enables the three independently triggered beams to be used in a monochromatic schlieren system with a single common knife-edge and film plane. The usefulness of measurements taken from the composite image in the study of shock wave dynamics is illustrated. The three separate monochromatic images may be recovered by digital color separation, so that the evolution of the wave system may be studied.With minor adjustment of the alignment,the three effective sources can be arranged at 120 degrees to each other, so that when flashed simultaneously, and used with a suitable cut-off, the system may be converted for full 2D color schlieren photography. In this application the colors on the image give information on the density gradient directions in the flow field. This system is very much more light efficient than color schlieren methods using a diffusing screen as none of the source light is scattered.
A single-axis, three-color light source system with independent triggers, suitable for multiflash schlieren and other photography is developed. The system enables three primary color exposures to be taken on a common film plane, with each exposure, when separated into the primary color components, representing a monochromatic photograph that can be converted to a gray-scale image. The three primary color light sources on a common optical axis are achieved using a specially designed beamsplitter consisting of dichroic filters mounted against a penta-prism shaped holder. Using this penta arrangement and three independent white light flash sources (e.g., xenon lamps), three images can be recorded at independent times on a single image plane. For schlieren applications, this arrangement ensures that all three images have equal sensitivity because they pass through a common schlieren cutoff, and also have the same axis angle (or perspective) relative to the test section. The system has proven to be both low cost and versatile.
Color schlieren has come to mean, not only schlieren images in color, but images where the color can be quantitatively related to the directions of the density gradients in the flow field. A system essentially consisting of three conventional schlieren systems using knife-edge cutoffs, rotated 120 deg with respect to each other, where the light source for each system is in one of the three primary colors is described. It is shown that combinations of these colors (creating a unique color) on the schlieren image resulting from the angled cutoff configuration is a function of the direction of the density gradient only. A variety of relatively inexpensive methods of producing the primary color beams are tested and it is shown that a color beamsplitter using dichroic filters with three standard commercial xenon flash tubes is suitable to obtain good results for high-speed applications. For steady flow applications, a single incandescent lamp used in conjunction with color filters and a diffusing screen is shown to be adequate for qualitative results.
A color schlieren system, which accurately indicates density gradient directions, has been developed. This system differs from the classical schlieren system by replacing the dark/bright regions on a uniform background with colored regions, each color representing a gradient direction. A component description of the system is given, with the formation of the color schlieren image being explained. The new system is compared to other color schlieren techniques, and photographs of schlieren tests are included.