Harald Kleine

An experimental investigation of the stability of converging cylindrical shock waves in air

An experimental study was made of the stability of converging cylindrical shock waves. The experiments were conducted on annular shock tubes equipped with a double exposure holographic interferometer in the Stoßwellenlabor, RWTH Aachen, and in the Institute of High Speed Mechanics, Tohoku University, Sendai, for shock Mach numbers of 1.1 to 2.1 in air. By comparing these two different shock tube experiments, it is found that in the former facility the mode-three instability is predominant at the center of convergence, whereas the mode-four instability appears in the latter setup. The instabilities are denoted in this way because the shock and the flow field behind it reveal a remarkable triangular and quadrangular symmetry, respectively. It is concluded that the converging cylindrical shock wave is always unstable and sensitive to the structure of the annular shock tube. Usefulness of holographic interferometry to this kind of shock wave research is also demonstrated.

Flow features resulting from shock wave impact on a cylindrical cavity

The complex flow features that arise from the impact of a shock wave on a concave cavity are determined by means of high-speed video photography. Besides additional information on features that have previously been encountered in specific studies, such as those relating to shock wave reflection from a cylindrical wall and those associated with shock wave focusing, a number of new features become apparent when the interaction is studied over longer times using time-resolved imaging. The most notable of these new features occurs when two strong shear layers meet that have been generated earlier in the motion. Two jets can be formed, one facing forward and the other backward, with the first one folding back on itself. The shear layers themselves develop a Kelvin–Helmholtz instability which can be triggered by interaction with weak shear layers developed earlier in the motion. Movies are available with the online version of the paper.

Color schlieren methods in shock wave research

Schlieren methods are widely known and well established to visualize refractive index variations in transparent media. The use of color allows one to obtain more data and previously inaccessible information from a picture taken with this technique. In general, a hue can be related to a certain strength or a certain direction of a refractive index gradient. While the first case essentially corresponds to the usual black- and-white system the latter correlation cannot be made adequately evident without the use of color. Two color schlieren techniques are presented here, which reach or even exceed the quality and sensitivity range of conventional black- and-white methods. Using a powerful short duration light source these methods are applied to visualize transient flow phenomena in a shock tube.

Time-resolved visualization of shock---vortex systems emitted from an open shock tube

When a shock wave leaves an open-ended shock tube, it generates a vortex ring that subsequently detaches from the shock tube and follows the expanding shock front. This classical problem of shock–vortex interaction has been visualized in unprecedented detail and temporal resolution by means of time-resolved shadow, schlieren and shearing interferometry sequences obtained with a newly developed ultrahigh-speed color video camera. This device is capable of taking 144 frames with full-frame resolution of 720 × 410 pixels at rates of up to one million frames per second. Apart from shadowgraphy, the visualization techniques used in this study were direction- and magnitude-indicating color schlieren and polychrome shearing interferometry. The process was observed both with the standard normal view of the flow field and with an oblique view, which facilitated the identification of some three-dimensional flow features. The obtained results clearly show the development of individual flow elements, including some that so far have eluded a proper description. One example is the secondary, counter-rotating vortex ring, which at a later time wraps around the main vortex ring before disintegrating upon merging with the shear layer that surrounds the gas exiting from the tube.Graphical abstract

Diode-laser-based near-resonantly enhanced flow visualization in shock tunnels

Two new near-resonantly enhanced flow visualization techniques suitable for hypersonic low-density flows in shock or arc tunnels have been developed using seeded lithium (Li) metal as the refractivity-enhancing species. Two semiconductor lasers, single-longitudinal-mode and multimode, are compared with respect to their suitability as light sources for the technique. Transient wake-flow structures around a cylinder and a model of a planetary entry vehicle are visualized to demonstrate the capabilities of this comparatively inexpensive and simple visualization system. The images show flow features which are undetectable with conventional schlieren, shadowgraph, or interferometry techniques. Furthermore, the effect of density inhomogeneities along the line-of-sight outside the region of interest can be reduced by enhancing the refractivity only in selected parts of the flowfield.

Shock wave interaction with convex circular cylindrical surfaces

The reflection of shock waves off cylindrical surfaces of different radii is examined with the help of time-resolved flow visualization. The primary diagnostic is a newly developed technique based on the tracking of deliberately introduced small perturbations in the flow. The main focus of the investigation is to determine at which position the shock wave receives information about the shape of the wall that it reflects off. In the pseudo-steady shock reflection off a plane wall, it is commonly accepted that the reflection changes from regular to irregular as soon as sonic signals generated behind the reflection point catch up with the reflection point, and the common interpretation is that this corresponds to the transition from regular to Mach reflection (the so-called sonic criterion). The results obtained here for convex circular surfaces show that this ‘catch-up’ condition occurs at wall angles considerably higher than in the plane wall case, while a visible Mach stem occurs only further along the surface at wall angles distinctively lower than those for plane walls. Tests are also conducted on a surface where a cylindrical portion is followed by a fixed angle plane section. The Mach numbers are chosen to be on either side of the plane wall transition condition so as to examine the adjustment from reflection on the cylindrical portion to that on the plane wall. These tests confirm that the wall angle at which sonic ‘catch-up’ to the reflection point occurs on the cylindrical surface is much higher than the corresponding wall angle predicted by the sonic criterion for a plane wall. While the transition from regular to irregular reflection is not the main concern of this contribution, the present results show that the transition criteria developed for steady and pseudo-steady flows are only of limited use in the fully unsteady flows such as the one investigated here.

Supersonic Flow over a Shallow Open Rectangular Cavity

An experimental investigation was conducted on a supersonic flow at a freestream Mach number of 2 over a shallow open cavity, including the effects of adding streamwise serrated edges. These flows have relevance to weapons bays and airframe gaps on high-speed aircraft. The measurements consisted of single-shot and time-resolved schlieren visualization, as well as unsteady pressure spectra. The length-to-depth ratio of the cavity was 8. The tests conducted at different Reynolds numbers with the baseline cavity (straight leading and trailing edges) showed that increasing the Reynolds number increases the root-mean-square pressure inside the cavity. The addition of serrations to the cavity leading or trailing edge did not show any significant effect on the separating shear layer nor in controlling the oscillations of the shear layer. There was also no noticeable effect on the overall sound pressure levels inside the cavity. A new expression for calculating shallow-cavity resonant frequencies applicable at su...

Pressure measurements in laboratory-scale blast wave flow fields.

The present study examines the effects that temporal and spatial averagings due to finite size and finite response time of pressure transducers have on the pressure measurements in blast wave flow fields generated by milligram charges of silver azide. In such applications, the characteristic time and length scales of the physical process are of the same order of magnitude as the temporal and spatial characteristics of the transducer. The measured pressure values will then be spatially and temporally averaged, and important parameters for the assessment of blast effects may not be properly represented in the measured trace. In this study, face-on and side-on pressure transducer setups are considered. In the experiments, face-on and side-on readings at the same distance from the charge as well as time-resolved optical visualization of the whole flow field are obtained simultaneously for the same explosive event. The procedure of data extraction from the experimental pressure traces is revisited and discussed in detail. In the numerical modeling part of the study, numerical blast flow fields are generated using an Euler flow solver. A numerical pressure transducer model is developed to qualitatively simulate the averaging effects. The experimental and numerical data show that the results of pressure measurements in experiments with small charges must be used with great caution. The effective averaging of the pressure signal may lead to a significant underestimation of blast wave intensities. The side-on setup is especially prone to this effect. The face-on setup provides results close to those obtained from optical records only if the pressure transducer is sufficiently remote from the charge.

Optical imaging techniques for hypersonic impulse facilities

The application of optical imaging techniques to hypersonic facilities is discussed and examples of experimental measurements are provided. Traditional Schlieren and shadowgraph techniques still remain as inexpensive and easy to use flow visualisation techniques. With the advent of faster cameras, these methods are becoming increasingly important for time-resolved high-speed imaging. Interferometrys quantitative nature is regularly used to obtain density information about hypersonic flows. Recent developments have seen an extension of the types of flows that can be imaged and the measurement of other flow parameters such as ionisation level. Planar laser induced fluorescence has been used to visualise complex flows and to measure such quantities as temperature and velocity. Future directions for optical imaging are discussed.

Aerodynamics of a Supersonic Projectile in Proximity to a Solid Surface

Flow around a Mach 2.4 NATO 5.56 mm projectile in close proximity to a ground plane was investigated using computational fluid dynamics for a direct numerical reproduction of live-range experiments. The numerical approach was validated against both the live-range tests and subsequent wind-tunnel experiments. A nonspinning half-model and a full, spinning projectile were examined to clarify the influence of rotation. Multiple ground clearances were tested to obtain clear trends in changes to the aerodynamic coefficients, and the three-dimensional propagation and reflection of the shock waves were considered in detail. The behavior of the flow in the near wake was also studied as ground clearance was reduced. Ground proximity was found to significantly increase the drag force acting on the projectile, as well as generate a force normal to the ground and an increased side force, when ground clearance was less than one diameter. For clearances between approximately 0.4 and 1 diameter, the pitching moment produced was nose-down. For lower clearances, a more distinct nose-up trend was produced. The generated side force was orders of magnitude lower than the normal and drag forces.