Instationäre Strömungen und die Dynamik von Partikeln in der Stoßschicht beim Aufprall eines zweiphasigen Überschallfreistrahls auf eine Platte

Partikelbeladene Überschallfreistrahlen, die senkrecht auf eine Platte auftreffen, werden untersucht. Hier wird ein Freistrahl bei Mach-Zahlen von 2,6-2,8 untersucht. Die Ruhetemperaturen liegen bei 300-500K. Die disperse Phase besteht aus festen Partikeln verschiedener Dichte (2700-7900kg/m3) und Größe (10-100 μm). Zuerst werden die Eigenschaften des Freistrahls, der durch eine extrem schlanke Düse erzeugt wird, analysiert. Daran anschließend wird das instationäre Verhalten der Strahl-Platten Wechselwirkung mittels einer neu entwickelten Methode der Mehrfachbelichtungsphotographie, die mit Druckmessungen auf der Plattenoberfläche synchronisiert wird, untersucht. Es zeigt sich, dass es drei Schwingungstypen gibt, die über eine spektroskopische Analyse der Druckvariationen in der Stoßschicht untersucht werden. Besondere Aufmerksamkeit wird dabei auf solche Strömungsfelder gelegt, in denen in der Stoßschicht ein Rezirkulationsgebiet auftritt. In einem weiteren Abschnitt wird die Dynamik der Partikel sowohl in stationären als auch instationären Strömungen theoretisch und experimentell betrachtet. Dies betrifft insbesondere die Bildung einer Partikelwolke vor der Platte. Gegenstand des abschließenden Abschnitts ist die Wechselwirkung von Partikeln mit der Platte. Die Bildung einer Schicht und der Mechanismus von durch Partikelaufprall induzierter Lumineszenz werden diskutiert

On oscillating shock layers of supersonic impinging jets

Shock layers of supersonic jets impinging on a plate are studied by means of multi-exposure photography and synchronized pressure measurements performed at the stagnation point on the plate. At certain nozzle-to-plate distances the bow shock formed ahead of the plate oscillates strongly. The pressure variations are strongly correlated with the oscillating shock. Peak frequencies are determined by a spectral analysis of the pressure signal. The spectra show a characteristic dependence of the peak frequencies on the nozzle-to-plate distance. The behavior of the primary peaks depends strongly on the characteristics of the supersonic jet (over/underexpansion). Frequencies of secondary peaks that decrease about inversely with the nozzle-to-plate distance, depend only little on the characteristics of the jet. In addition, low frequency bursts that occur for certain nozzle-to-plate distances are traced to the formation and destruction of a recirculation bubble

Spectral analysis of unsteady interactions between a supersonic jet and a plate

The flow field in the stagnation point region of a supersonic jet that impinges on a plate can be highly unsteady and impingement tones are formed. A parameter governing the unsteady behavior is the distance between the exit of the nozzle from which the jet issues and the plate. At certain nozzle-to-plate distances the bow shock formed ahead of the plate oscillates strongly. The motion of the bow shock has been studied using a multi-exposure visualization technique. Synchronized pressure measurements performed at the stagnation point on the plate have shown that the pressure variations are strongly correlated with the oscillating shock. These features are connected with the formation and destruction of a recirculation bubble in the impingement region of the supersonic jet. Main frequencies of the unsteady flow field are determined by a spectral analysis of the pressure signal obtained at the stagnation point on the plate. Spectra obtained for different nozzle-to-plate distances are plotted as multi-configuration spectra using the nozzle-to-plate distance as a parameter. This is repeated for different pressure ratios of the nozzle, i.e., for under-, ideally, and overexpanded jets

Luminescence induced by high-velocity impacts of metallic particles on metal surfaces

On impact of metallic particles on a metal surface light can be emitted from the region of impact. This phenomenon is studied experimentally for impact conditions that are typical of the cold spray deposition process, i.e., particle diameters and velocities are of the order of 10−5 m and 500 m/ s, respectively. The characteristics of impact-induced luminescence are analyzed. It is found that the intensity of luminescence depends not only on the energy of the impinging particles and on the stresses produced on impact but also on the particular combination of materials of the particles and the plate. Possible sources of luminescence are discussed

Spectral analysis of sound generation by supersonic impinging jets

The flow field in the stagnation point region of a supersonic jet impinging on a plate can be highly unsteady. This can lead to a lift loss in the case of hovering STOVL planes, and to increased noise levels in engineering processes in which supersonic jets are applied as, for example, in hypervelocity oxygen fuel spraying. A parameter governing the unsteady behavior is the distance between the exit of the nozzle from which the jet issues and the plate. At certain nozzle-to-plate distances the bow shock formed ahead of the plate oscillates strongly. The motion of the bow shock has been studied using a multi-exposure visualization technique (Klinkov et al., 2006). Synchronized pressure measurements performed at the stagnation point on the plate have shown that the pressure variations are strongly correlated with the oscillating shock. Under certain conditions these features are connected with the formation and destruction of a recirculation bubble in the impingement region of the supersonic jet (Klinkov Rein, 2004). Subject of the present contribution are the properties of the main frequencies of the unsteady flow field. Peak frequencies are determined by a spectral analysis of the pressure signal obtained at the stagnation point on the plate. Spectra obtained for different nozzle-to-plate distances are analyzed by forming multi-configuration spectra using the nozzle-to-plate distance as a parameter. This is repeated for different pressure ratios of the nozzle, i.e., for under-, ideally, and overexpanded jets. Multi-configuration spectra obtained as described above show a characteristic dependence of peak frequencies on the nozzle-to-plate distance. Frequencies related to primary peaks decrease with nozzle-to-plate distance then jump to a higher value and decrease again. The behavior of the primary peaks depends strongly on the characteristics of the supersonic jet (over/underexpansion). The frequency of primary peaks remains always limited to a small range near 30 kHz. The frequencies of further peaks, called secondary peaks hereafter, decrease about inversely with the nozzle-to-plate distance covering a much broader frequency range then that of primary peaks. The dependence of frequencies of secondary peaks on the characteristics of the jet is small. Considering multi-configuration spectra it will be shown that there exists a distinct difference in the dependence on the nozzle-to-plate distance of primary and secondary peaks. The latter ones are well-known to be caused by a feedback loop (Neuwerth, 1974). Disturbances in the shear layer of the jet are convected to the plate. On impingement sound is generated that propagates to the nozzle exit through the ambient gas. At the nozzle exit new disturbances are excited in the shear layer, and so on. In most publications on acoustic waves produced by impinging jets spectra of oscillations were traced to this mechanism. However, the approach of multi-configuration spectra shows that the behavior of primary peaks is influenced by an additional mechanism. In a model the behavior of primary peaks may be explained by adding the shock layer into the feedback loop or by considering entropy variations caused by an oscillating bow shock (Kuo Dowling, 1996). In addition to the features described so far low frequency bursts are present in the spectra for certain nozzle to plate distances. Then, the corresponding spectrum does no longer exhibit characteristic peaks. This is likely related to so-called zones of silence observed by other authors (Henderson, 2002)

Multi-exposure method for studying compressible multiphase flows

In unsteady compressible multiphase flows it is of interest to simultaneously visualize the development in time both, of the dispersed phase and of discontinuities. In order to do so a multi-exposure method has been developed. The method is based on a combination of shadowgraphy and particle tracking. A modulated laser diode is applied as a short-pulse light source. Shadowgram images are received by a CCD-camera. Control signals for laser, camera etc are formed by a signal sequencer. The whole system is controlled by a personal computer. Both, pulse shapes and sequences of light pulses can be varied within a wide range, the duration of single pulses being as low as 20 ns and the interval between different pulses being greater than 20 ns. The exposure time of the CCD-camera is as low as 10 μs. The time delay of a signal between the signal sequencer and any device is smaller than 150 ns. The time accuracy of the signal sequencer is 1 ns. The present method is particularly well suited for investigating the behavior of small particles at high velocities and small concentrations. Furthermore, taking shadowgraphs of the gas flow in addition to particle images and thus obtaining information on gas flow and particle motion at the same time enables an easy interpretation of unsteady processes occurring in the flow. The wide variability of sequences of light pulses allows for determining not only the velocity and acceleration of particles but provides also additional information as, for example, on concentration, shape and size of particles or the shape of shock waves. The diagnostic method of multi-exposure visualization has been used for studying processes in the thin shock layer ahead of a plate in supersonic jet impingement flows. A nearly ideally expanded jet of Mach number M = 2.8, laden with metallic particles (radii: 5-50 μm, velocity: ~400 m/s), was directed against a plate. At certain nozzle-to-plate distances a recirculation bubble is repeatedly formed and destructed in the shock layer. Then, the flow ahead of the plate and the position of the bow shock oscillate strongly. The behavior of reflected particles in the cloud ahead of the plate also changes essentially. The comparatively slow motion of the shock ahead of the plate has been detected by using a sequence of 5 light pulses (duration: 100 ns) with time intervals between the pulses ranging from 2 to 10 μs (Fig. 1a). Different intervals were chosen in order to determine the direction of the shock motion. Fig. 1b shows an image of particles in the shock layer ahead of the plate. Here, a sequence of three light pulses (duration: 50 ns, time intervals between the pulses: 200 ns and 400 ns) was applied. High-velocity particles ahead of the bow shock are represented by a track of three images (marked by white circles in Fig. 1b). In the particle cloud ahead of the plate the velocity of most particles is so small that the short time intervals between the pulses do not suffice to separate the three images of individual particles. Particles having just one image are also present upstream of the shock. Typically, a bow shock can be seen ahead of such particles. These particles have been reflected off the plate. Particle velocities in the shock layer were determined by increasing the time intervals up to 4 μs. Variations in time of location of the shock and of the axial distribution of reflected particles in the unsteady shock layer are shown in Fig. 2. It can be seen that particles normally stay within the shock layer close to the plate. However, when the shock approaches the plate, reflected particles are also present upstream of the shock. A comparison of characteristic times of the oscillating shock and relaxation times of particles shows that the former is at least an order of magnitude greater than the latter. Therefore, the particle cloud has enough time to expand and fill the whole space between the upstream moving shock and the plate. When the shock moves back towards the plate the shock is very fast. Measured velocities of the shock reach up to values of uSL ≈ 100 m/s. Hence, it takes the shock about 10 μs to change its position. This time is much shorter than the relaxation time of particles. Thus, particles remain upstream of the plate shock in the supersonic flow for some time until they reach the shock layer again

Behavior of Supersonic Overexpanded Jets Impinging on Plates

Steady and unsteady interactions of supersonic overexpanded free jets with plates are visualized by standard shadowgraphy and modeled numerically using the DLR Tau Code. Unsteady behaviors of jet-plate interactions are studied by the method of multi-exposure photography that has been combined with a synchronized pressure measurement on the plate. It is shown that there exists a distinct correlation between shock motions and pressure fluctuations on the plate