By using two high-speed cameras and a slightly extended visualization setup (typically based on a Toepler system) one can generate two simultaneous time-resolved records of the same flow, where these records can be obtained with different visualization methods, different spatial and different temporal resolutions. This allows one to generate visualizations that can complement each other in various ways and thus yield a considerably increased amount of information on the observed flow.
The fundamental shock wave interaction process of a wave diffraction at a sharp 90 degree corner was investigated by means of time-resolved Mach-Zehnder interferometry. The obtained results show that the process is macroscopically self-similar as has been assumed in the past but that some elements of the flow show non-selfsimilar behavior. The use of polychrome (white light) interferometry enables one to quantify the density distribution and by how much it is altered as a result of the non-self-similar behavior.
High-speed flow visualisation has played an important role in the investigations conducted at the Stoßwellenlabor of the RWTH Aachen University for many decades. In addition to applying the techniques of high-speed imaging, this laboratory has been actively developing new or enhanced visualisation techniques and approaches such as various schlieren methods or time-resolved Mach-Zehnder interferometry. The investigated high-speed flows are inherently highly transient, with flow Mach numbers ranging from about M = 0.7 to M = 8. The availability of modern high-speed cameras has allowed us to expand the investigations into problems where reduced reproducibility had so far limited the amount of information that could be extracted from a limited number of flow visualisation records. Following a brief historical overview, some examples of recent studies are given, which represent the breadth of applications in which high-speed imaging has been an essential diagnostic tool to uncover the physics of high-speed flows. Applications include the stability of hypersonic corner flows, the establishment of shock wave systems in transonic airfoil flow, and the complexities of the interactions of shock waves with obstacles of various shapes.
Recent improvements in camera technology and the associated improved access to high-speed camera equipment have made it possible to use high-speed imaging not only in a research environment but also specifically for educational purposes. This includes high-speed sequences that are created both with and for a target audience of students in high schools and universities. The primary goal is to engage students in scientific exploration by providing them with a tool that allows them to see and measure otherwise inaccessible phenomena. High-speed imaging has the potential to stimulate students’ curiosity as the results are often surprising or may contradict initial assumptions. “Live” demonstrations in class or student- run experiments are highly suitable to have a profound influence on student learning. Another aspect is the production of high-speed images for demonstration purposes. While some of the approaches known from the application of high speed imaging in a research environment can simply be transferred, additional techniques must often be developed to make the results more easily accessible for the targeted audience. This paper describes a range of student-centered activities that can be undertaken which demonstrate how student engagement and learning can be enhanced through the use of high speed imaging using readily available technologies.
The rotational behavior of capsule-shaped models is investigated in the transonic wind tunnel of JAXA. A special support is developed to allow the model to rotate around the pitch, yaw and roll axes. This 3-DOF free rotational mounting apparatus achieves the least frictional torque from the support and the instruments. Two types of capsule models are prepared, one is drag type (SPH model) and the other is lift type (HTV-R model). The developed mounting apparatus is used in the wind tunnel tests with these capsule models. In a flow of Mach 0.9, the SPH model exhibits oscillations in pitch and yaw, and it rolls half a turn during the test. Similarly, the HTV-R model exhibits pitch and yaw oscillations in a flow of Mach 0.5. Moreover, it rolls multiple times during the test. In order to investigate the flow field around the capsule, the combined technique of color schlieren and surface tufts is applied. This visualization clearly shows the flow reattachment on the back surface of a capsule, which is suspected to induce the rapid rolling motion.
The early evolution of laser-induced plasma explosions has been investigated by means of a high-speed time-resolved
schlieren visualisation. Images were obtained with a high-speed video camera yielding frame rates of up to 1 million
frames per second at a frame resolution of 312 by 260 pixels. With this setup it was possible to resolve the temporal
development of the ionised plasma kernel and its associated shock wave. The plasma is formed by focusing a pulsed
ruby laser beam, with pulse energies of up to 4.5 J. The time-resolved visual data have been used to yield shock speeds,
from which, together with direct energy measurements, one can determine the portion of energy released by the plasma
explosion to drive the shock. Shock sphericity as well as plasma growth and emission lifetimes have also been evaluated.
The location of longest emission lifetime was found to change as a function of laser pulse energy: for high energy pulses,
the longest-living plasma luminosity was located ahead of the focal spot, i.e. closer to the laser source, while with lower
energy pulses the longest-living plasma luminosity was located behind the focal spot. This behaviour was also observed
for double-pulsed plasma explosions, when a second laser pulse was generated with a delay time of 50 μs. The
experiments show that for single pulses, more than 50 percent of the laser energy is expended in generating the shock
wave.
Three different applications of high-speed near-resonantly enhanced shearing interferometry to visualise and investigate
hypersonic wake flows are described. In the present application, two axisymmetric objects, a sphere and a model of a
planetary entry vehicle, are placed in a Mach 10 shock tunnel flow. The influence of different mounting structures on the
wake flow of the entry vehicle is demonstrated. Planar laser induced fluorescence (PLIF) thermometry is used as an
additional tool to monitor base flow temperatures. The unsteadiness of the wake flow of the sphere is compared to the
flow unsteadiness around the entry-probe. The velocity in selected parts of the wake flow field is also determined with
the help of a time-resolved time-of-flight method.
In order to investigate the unsteady flow field around a spiked body in supersonic flow, time-resolved color schlieren
visualization was applied using a high-speed video camera which could take up to 1 000 000 frames per second at full
frame resolution. Conically and spherically tipped spikes of six different lengths could be attached at the center of the
model and their effect on the flow unsteadiness was visually observed. The obtained images revealed in great detail the
interaction between the incoming free stream flow and the high-pressure region near the model base, which could make
its presence known upstream at the tip of the spike by means of displacing the boundary layer on the spike and
subsequently inducing a large-scale instability of the flow.
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.
This paper presents further results of an ongoing experimental and numerical investigation into the unsteady process of
blast wave reflection from straight smooth surfaces. It is shown that basic blast wave phenomena such as the transition
from regular to irregular wave reflection can be adequately and conveniently studied in a laboratory environment by
using small charges with masses in the milligram range. While the laboratory scale generally provides greater
accessibility, it also imposes more stringent conditions on the diagnostics than the large-scale environment. The paper
reviews the previously found considerable discrepancies between numerical and experimental results for the location xtr
of the transition from regular to irregular wave reflection. These are caused by the initially minuscule size and gradual
growth of the Mach stem and the limited resolution of the recording material. Different techniques are used to improve
the accuracy of the experimental determination of the transition point, and a new combination of modern high-speed
photography with the traditional soot technique is shown to be the most promising tool for this purpose.
Solid obstacles close to (but not in) the flight path of supersonic projectiles influence the flow field and thus possibly the
projectile trajectory, even if no contact between projectile and obstacle occurs. In order to qualitatively clarify the basic
characteristics of such projectile/obstacle interactions, an initial investigation comprised of two different sets of
experiments has been conducted: live rounds were fired from a rifle so that they passed over a flat plate at distances
between 0.8 and 2.5 projectile diameters in the first set of tests, followed by a second set where a stationary projectile
model was placed in a supersonic wind tunnel to replicate the flow fields of the live range experiments. Time-resolved
monochrome schlieren visualization was used as the primary diagnostic tool in the first set of experiments, while singleshot
color schlieren images were obtained in the wind tunnel tests.
High-speed photography has been a primary tool for the study of blast wave phenomena, dating from the work of Toepler, even before the invention of the camera! High-speed photography was used extensively for the study of blast waves produced by nuclear explosions for which, because of the large scale, cameras running at a few hundred frames per second were adequate to obtain sharp images of the supersonic shock fronts. For the study of the blast waves produced by smaller explosive sources, ever-increasing framing rates were required. As a rough guide, for every three orders of magnitude decrease in charge size a ten-fold increase of framing rate was needed. This severely limited the use of photography for the study of blast waves from laboratory-scale charges. There are many techniques for taking single photographs of explosive phenomena, but the strongly time-dependent development of a blast wave, requires the ability to record a high-speed sequence of photographs of a single event.
At ICHSPP25, Kondo et al of Shimadzu Corporation demonstrated a 1 M fps video camera that provides a sequence of up to 100 high-resolution frames. This was subsequently used at the Shock Wave Research Center of Tohoku University to record the blast waves generated by an extensive series of silver azide charges ranging in size from 10 to 0.5mg. The resulting images were measured to provide radius-time histories of the primary and secondary shocks. These were analyzed with techniques similar to those used for the study of explosions from charges with masses ranging from 500 kg to 5 kt. The analyses showed the cube-root scaling laws to be valid for the very small charges, and provided a detailed record of the peak hydrostatic pressure as a function of radius for a unit charge of silver azide, over a wide range of scaled distances. The pressure-radius variation was compared to that from a unit charge of TNT and this permitted a detailed determination of the TNT equivalence of silver azide as a function of peak pressure and radius.
The availability of the Shimadzu high-speed framing camera has made it possible to perform experiments at the laboratory scale that previously could be done only on large-scale field trials. At the laboratory scale, many experiments can be performed on the same day, as compared to the months or even years required for the preparation of large-scale field experiments. The economic savings are even greater.
By applying two-reference beam interferometry together with digital data processing, a correspondingly modified double exposure holographic interferometer has been used to measure the density distribution in flow fields generated by very weak shock waves. The weak shock waves were generated by the explosion of milligram charges of silver azide, which were ignited by a pulsed Nd:YAG laser. With the help of this technique, density variations in a flow field associated with a shock Mach number Ms = 1.0007 were visualized and quantified. Phenomena that generated a sound intensity of 135dB could be resolved.
This paper describes the development of an optical system that can simultaneously generate an interferogram and a shadowgraph of a refractive index field (in this case, a compressible fluid flow). Such a combination allows one to record both the density and its second derivative OF THE SAME OBJECT AT THE SAME TIME. This can be achieved by splitting the object beam of a holographic interferometer before superimposing it with the reference beam. One half of this beam forms the hologram together with the reference beam, while the other is led into an imaging unit that generates an image of a plane that is located either before or behind the test object (the so-called shadow plane). This arrangement allows one to generate simultaneously two pictures, which provide more reliable quantitative and qualitative data about the investigated flow than each of the visualizations alone.
For detailed investigations of processes and phenomena in the flow of compressible fluids, it is sometimes necessary to apply more than just one flow visualization technique as each method has its own characteristic strengths and weaknesses. In the case of flows with a low degree of repeatability, it may become mandatory to perform these multiple visualizations simultaneously, i.e., within the same experiment at identical or at least almost identical instants. This contribution describes how two interferometry methods can be coupled with a color schlieren technique in order to obtain simultaneously the density distribution and the density gradients within a flow field. The resulting systems are slightly more complex than a single visualization apparatus, but they can provide an unprecedented wealth of information about the flow field.
In this study, the method of digital phase shift holographic interferometry is applied to accurately measure the density distribution behind a weak shock wave. The goal of this investigation is to visualize and quantitatively evaluate flow fields behind weak disturbances, which are only marginally stronger than sound waves. In the present experiment, the shock Mach number was approximately Ms approximately equals 1.01. The wave was generated by the explosion of a small silver-azide (micro- charge) pellet, which was ignited by an Nd:YAG laser. Using phase shift interferometry, the phase distribution of the region far behind the incident shock could be detected.
In order to meet the demands of increasingly detailed investigations of processes and phenomena in the flow of compressible fluids, flow visualization techniques have to be brought to a level that allows one to extract both qualitative and quantitative information about a flow field. This contribution presents a brief description of two density- sensitive techniques, holographic interferometry and colour schlieren, which have been shown to be very suitable for this task. Some applications demonstrate the diagnostic potential of these techniques.
Low-jitter reliable short-duration spark sources have been developed for application in color schlieren methods. With an Xe-filling of a previously used nanosecond spark source (NANOSPARK 1400), the light quantity could be increased up to tenfold while the pulse width rose up to 500 ns depending on gas pressure. When filled with air, the new extended version NANOSPARK 4000, which is about three times the size of the NANOSPARK 1400, exhibits pulse widths of 50 - 80 ns (FWHM) and BCP five times as large as the smaller version. With an Xe-filling, the light quantity is again magnified by a factor of ten, but also the pulse width goes up to 900 ns depending on gas pressure. These sources have adequately exposed customary ASA 400 film in various color schlieren experiments. Moreover they have been used in a shearing interferometer and in several monochrome visualization setups.
For a given flow field obtained in a shock tube several visualization techniques are applied to reveal both the weak and the strong points of each individual method. Particular emphasis is put on two newly developed schlieren techniques, which prove to be most suitable for a detailed optical investigation of a large number of problems in fluid mechanics.
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