Bei einer Reihe bedeutsamer industrieller Prozesse, wie dem Stahl-Strangguss oder der Kristallzucht für die Photovoltaik, ist die Strömung flüssiger Metalle oder Halbleiter entscheidend für den Energieaufwand bei der Herstellung und die Qualität des Endproduktes. Eine gezielte, berührungslose Einwirkung von Lorentzkräften auf die heißen Schmelzen kann dabei die Ressourceneffizienz eines Prozesses signifikant steigern. Die komplexe Interaktion von elektrisch leitfähigen Fluiden und magnetischen Wechselfeldern wird dazu in der Magnetohydrodynamik (MHD) durch Experimente im Labormaßstab an niedrigschmelzenden Metallen untersucht. Die dabei auftretenden instationären, dreidimensionalen Strömungsfelder erfordern eine nicht-invasive, bildgebende Strömungsmesstechnik für opake Fluide mit hoher Orts- und Zeitauflösung, welche derzeitig nicht für die MHD zur Verfügung steht. Im Rahmen dieser Arbeit soll mit den Mitteln der Elektrotechnik eine für MHD-Modellexperimente geeignete Messtechnik basierend auf dem Ultraschall-Doppler-Prinzip geschaffen werden. Dabei wird der Ansatz verfolgt, die Komplexität eines Messsystems vom mechanischen Aufbau hin zur Rechentechnik zu verlagern, um durch die dort in jüngster Zeit verfügbaren Ressourcen neuartige Signalverarbeitungsmethoden und eine höhere Flexibilität zu ermöglichen. Mit dem Ultrasound Array Doppler Velocimeter (UADV) wurde ein flexibles Messsystem für MHD-Modellexperimente geschaffen, welches eine mehrkomponentige Mehrebenenmessung durch Sensordatenfusion von bis zu neun linearen Wandlerarrays im kombinierten Zeit- und Ortsmultiplex erreicht. Die Signalverarbeitung ist durch eine auf einem Field Programmable Gate Array implementierte Datenkompression onlinefähig. Sie reicht trotz geringer rechentechnischer Komplexität bis auf Faktor 3 an die fundamentale Grenze der Messunsicherheit, die Cramér-Rao-Schranke, heran. Das UADV wurde über ein Kalibrierexperiment mit interferometrischer Referenzmessung auf die SI-Einheiten zurückgeführt. Das UADV wurde an einer magnetfeldgetriebenen Strömung in einem kubischen Gefäß angewandt. Numerische Simulationen sagen dort nicht-deterministisch einsetzende Instabilitäten im Übergangsbereich des laminaren zum turbulenten Strömungsregimes vorher. Durch eine simultane Zweiebenenmessung mit hoher örtlicher (3 ... 5 mm) und zeitlicher Auflösung (Bildrate 11,2 Hz) bei gleichzeitig langer Aufnahmedauer (> 1000 s) konnten die Instabilitäten erstmals experimentell charakterisiert werden. Eine Hauptkomponentenanalyse identifizierte ein gekoppeltes Paar von Strömungsmoden, welche eine spontan anfachende harmonische Oszillation mit der Frequenz f = 0,072 Hz beschreiben und durch komplexe Wirbel gekennzeichnet sind. Die Analyse der Messunsicherheit für das gegebene Experiment ergab, dass diese mit σ v,rel = 13,9 % hauptsächlich durch das räumliche Auflösungsvermögen bestimmt wird. Das Schallfeld ist bei ultraschallbasierten Messverfahren ausschlaggebend für die Eigenschaften der Bildgebung. Mit dem Phased Array Doppler Velocimeter (PAUDV) wurde ein modulares Messsystem mit adaptiven Schallfeld aufgebaut, wobei durch digitale Strahlformung die örtliche und zeitliche Auflösung signifikant erhöht werden kann. Eine aktive Kontrolle des Schallfeldes ermöglicht zudem die Messung durch Objekte mit komplexen, unbekannten Ausbreitungseigenschaften. Mit dem Time Reversal Virtual Array (TRVA) wird dabei eine effiziente Methode zur Bildgebung vorgestellt und auf die Strömungsmessung durch einen Multimode-Wellenleiter angewandt. Damit kann die Beschränkung bildgebender Ultraschallmesstechnik hinsichtlich der Betriebstemperatur der Wandler umgangen und heiße Schmelzen industrieller und technischer Prozesse für nichtinvasive In-Prozess-Bildgebung zugänglich gemacht werden. / In many important industrial processes, such as continuous steel casting or crystal growth for photovoltaic silicon, the flow of liquid metals or semiconductors determines the energy consumption of the process and the quality of the product. Influencing the hot melts contactlessly through Lorentz forces for a targeted flow control can significantly improve the resource-efficiency of a process. The complex interaction of electrically conductive fluids and alternating magnetic fields is investigated in the field of magnetohydrodynamics (MHD) through laboratory-scale experiments in low melting metals. The emerging instationary, three-dimensional flows require a temporally and spatially high-resolved non-invasive flow imaging system, which currently is not available for MHD research. In this work, a flow instrumentation for MHD experiments based on the ultrasound Doppler principle is created through means of electrical engineering. The general approach is to shift the complexity of a system from mechanics over electronics to an algorithmic implementation in order to exploit the recent computational advances, enabling novel signal processing methods and increasing the flexibility. The ultrasound array Doppler velocimeter (UADV) has been created as a flexible instrumentation system for MHD experiments. It supports multicomponent, multiplane velocity measurements through sensor fusion of up to nine linear transducer arrays with spatiotemporal multiplexing. An online signal processing is realized through data compression on a field-programmable gate array (FPGA). It achieves an uncertainty as low as a factor 3 of the Cramér-Rao lower bound despite a low computational complexity of the algorithm. The UADV has been applied to a magnetically-driven flow in a cubic vessel. Numerical simulations predicted a non-deterministic instability in the transitory region between laminar and turbulent flow regimes. A simultaneous two-dimensional two-component flow measurement with high spatial (3 ... 5 mm) and temporal resolution (frame rate 11,2 Hz) over long durations (> 1000 s) allowed to characterize those instabilities experimentally for the first time. A principal component analysis identified a pair of coupled modes with a complex vortex structure that performs a spontaneously onsetting oscillation at f = 0,072 Hz. The measurement uncertainty for the experiment has been evaluated to be σ v,rel = 13,9 % and is primarily caused by the spatial resolution of the system. The properties of ultrasound-based imaging are primarily determined by the sound field. The phased array Doppler velocimeter (PAUDV) has been developed as a modular flow instrumentation system with an adaptive sound field, which allows to increase the spatial and temporal resolution. Furthermore, an active control of the sound field enables measurements despite a complex, unknown sound propagation. A method to image through strong aberrations efficiently has been proposed with the time reversal virtual array (TRVA). It has been applied to flow imaging through a multimode waveguide, thus allowing to circumvent the limitation of common ultrasound imaging systems regarding their maximum operating temperature. This paves the way for in-process flow imaging of hot, opaque liquids in technical and industrial processes.

No description available.

This dissertation presents results from a thorough study of the tip-gap turbulent flow structure in a low-speed linear compressor cascade wind tunnel at Virginia Tech that includes a moving belt system to simulate the relative motion between the tip and the casing. The endwall pressure measurements and the surface oil flow visualizations were made on a stationary endwall to obtain the flow features and to determine the measurement profiles of interest. A custom-made miniature 3-orthogonal-velocity-component fiber-optic laser-Doppler velocimetry (LDV) system was used to measure all three components of velocity within a 50 mm spherical measurement volume within the gap between the endwall and the blade tip, mainly for the stationary wall with 1.65% and 3.30% tip gaps as well as some initial experiments for the moving wall. Since all of the vorticity in a flow originates from the surfaces under the action of strong pressure gradient, it was very important to measure the nearest-wall flow on the endwall and around the blade tip. The surface skin friction velocity was measured by using viscous sublayer velocity profiles, which verified the presence of an intense lateral shear layer that was observed from surface oil flow visualizations. All second- and third-order turbulence quantities were measured to provide detailed data for any parallel CFD efforts. The most complete data sets were acquired for 1.65% and 3.30% tip gap/chord ratios in a low-speed linear compressor cascade. This study found that tip gap flows are complex pressure-driven, unsteady three-dimensional turbulent flows. The crossflow velocity normal to the blade chord is nearly uniform in the mid tip-gap and changes substantially from the pressure to suction side. The crossflow velocity relies on the local tip pressure loading that is different from the mid-span pressure loading because of tip leakage vortex influence. The tip gap flow is highly skewed three-dimensional flow throughout the full gap. Normalized circulation within the tip gap is independent of the gap size. The tip gap flow interacts with the primary flow, separates from the endwall, and rolls up on the suction side to form the tip leakage vortex. The tip leakage vortex is unsteady from the observation of the TKE transport vector and oil flow visualizations. The reattachment of tip separation vortex on the pressure side strongly depends on the blade thickness-to-gap height ratio after the origin of tip leakage vortex but is weakly related to it before the origin of tip leakage vortex for a moderate tip gap. Other than the nearest endwall and blade tip regions, the TKE does not vary much in tip gap. The tip leakage vortex produces high turbulence intensities. The tip gap flow correlations of streamwise and wall normal velocity fluctuations decrease significantly from the leading edge to the trailing edge of the blade due to flow skewing. The tip gap flow is a strongly anisotropic turbulent flow. Rapid distortion ideas can not apply to it. A turbulence model based on stress transport equations and experimental data is necessary to reflect the tip gap flow physics. For the moving endwall, relative motion skews the inner region flow and is decorrelated with the outer layer flow. Hence, the TKE and correlations of streamwise and wall normal velocity fluctuations decrease. / Ph. D.

Modeling the effects of surface roughness is an area of concern in many practical engineering applications. Many current roughness models to this point have involved the use of empirical 'constants' and equivalent sand grain roughness. These underdeveloped concepts have little direct relationship to realistic roughness and cannot predict accurately and consistently the flow characteristics for different roughness shapes. In order to aid in the development of turbulence models, the present research is centered around the experimental investigation of seven various shaped single roughness elements and their effects on turbulence quantities in a two-dimensional turbulent boundary layer. The elements under scrutiny are as follows: cone, cone with spatial variations equal to the smallest sublayer structure length scale, cone with spatial variations equal to 2.5 times the smallest sublayer structure length scale, Gaussian-shaped element, hemisphere, cube aligned perpendicular to the flow (cube at 90°), and a cube rotated 45° relative to the flow. The roughness element heights, k+, non-dimensionalized by the friction velocity (U_tau) of the approaching turbulent boundary layer, are 145, 145, 145, 145, 80, 98, and 98 respectively. Analysis of a three-dimensional fetch of the same Gaussian-shaped elements described previously was also undertaken. In order to analyze the complex flow fields, detailed measurements were obtained using a fine-measurement-volume (50 micron diameter) three-velocity component laser-Doppler velocimetry (LDV) system. The data reveals the formation of a horseshoe vortex in front of the element, which induces the downwash of higher momentum fluid toward the wall. This 'sweep' motion not only creates high Reynolds stresses (v^2, w^2, -uv) downstream of the element, but also leads to higher skin-friction drag. Triple products were also found to be very significant near the height of the element. These parameters are important in regards to the contribution of the production and diffusion of the turbulent kinetic energy in the flow. The 'peakiness' of the roughness element was found to have a direct correlation to the production of circulation, whereas the spatial smoothing does not have an immense effect on this parameter. The peaked elements were found to have a similar trend in the decay of circulation in the streamwise direction. These elements tend to show a decay proportional to (x/d)^-1.12, whereas the cube elements and the hemisphere do not have a common trend. A model equation is proposed for a drag correlation common to all roughness elements. This equation takes into account the viscous drag and pressure drag terms in the calculation of the actual drag due to the roughness elements presence in the boundary layer. The size, shape, frontal and wetted surface areas of the roughness elements are related to one another via this model equation. Flow drawings related to each element are presented which gives rise to a deeper understanding of the physics of the flow associated with each roughness element. / Master of Science

The work presented in this thesis was on nominally two-dimensional turbulent boundary layers at zero pressure gradient subjected to high free-stream turbulent intensities of up to 7.9% in preparations for high free-stream turbulence studies on three-dimensional boundary layers, which will be done in the future in the Aerospace and Ocean Engineering Boundary Layer Wind Tunnel at Virginia Tech. The two-dimensional turbulent flow that will impinge three-dimensional bodies needed to be characterized, before the three-dimensional studies can be made. An active turbulence generator designed to create high free-stream turbulence intensities in the wind tunnel was tested and modified in order to obtain the lowest possible mean flow non-uniformities. A seven-hole pressure probe was used to obtain planes of mean velocity measurements. A three-component state of the art laser-Doppler velocimeter (LDV) was used to obtain mean and fluctuating velocities. Previous high free-stream turbulence studies have been reviewed and are discussed, and some of the previously published data of other authors have been corrected. Based on the measurements obtained with the LDV, it was also determined that the semi-log law of the wall is valid for high free-stream turbulence cases, but with different constants than the ones proposed by Coles, where the constants for the high free-stream cases may be dependent on the turbulence intensity. For the first time, the skin friction coefficient (Cf) was deduced from the viscous sublayer. The difference between the U_tau obtained in the viscous sublayer mean velocity profile and the U_tau obtained in the semi-log layer was 1.5%. The skin friction coefficient was determined to increase by 10.5% when the two-dimensional turbulent boundary layer was subjected to high free-stream turbulence effects. Spectral data obtained with the LDV, were compared to the von Kármán model spectrum and to the Pope's model spectrum, where the von Kármán spectrum was proven to fit the spectral data slightly better than the Pope's spectrum. Finally, the Hancock-Bradshaw-Blair parameter obtained for this experiment agreed very well with previously published data. / Master of Science

<p>  </p> <p>This work explored the dynamic behavior and failure mechanisms of an additively manufactured high-entropy alloy (HEA) when subjected to high-strain rate shock impacts. A laser-induced projectile impact testing (LIPIT) setup was used to study the dynamic behavior of the Cantor alloy CoCrFeMnNi samples manufactured using a directed-energy deposition technique. HEA flyers were accelerated by a pulse laser to velocities up to 1 km/s prior to impact with lithium fluoride glass windows. A photon Doppler velocimetry (PDV) system recorded the velocity of the flyer during the acceleration and subsequent impact. From this velocity profile, the Hugoniot coefficient and sound speed of the HEA samples were determined.</p> <p><br></p> <p>Upon determination of key shock parameters, spallation occurring due to shock was analyzed. Using the same LIPIT and PDV systems as the earlier testing, aluminum flyers of various thicknesses were accelerated into HEA samples. The back-surface velocity profiles of the HEA samples showed a characteristic “pullback” caused by the interaction of the tensile stress waves indicative of spall occurrence in the material. The magnitude of this pullback and the material properties determined in the first experiments allow for the measurement of spall strength at various strain-rates. This data is compared to previous data looking at similar HEAs manufactured using traditional methods. A comparison of this data showed that the spall strength of the HEA samples was equivalent to that of similar alloys but at significantly higher strain rates. As an increased strain-rate tends to result in increased spall strengths, further examination was needed to determine the reasons for this decreased spall strength in the AM samples.</p> <p><br></p> <p>Post-shock specimen recovery allowed for the failure mechanisms behind the spallation to be observed. Scanning electron microscope (SEM) images of the cross-section of the samples showed ductile fracture and void growth outside of the predicted spall region. Further imaging using energy dispersive spectroscopy (EDS) showed the presence of potentially chromium-oxide deposits in regions outside of the predicted spall plane. It is hypothesized that these regions created nucleation points about which spallation occurred. Thus, to achieve spall strength in AM HEAs equivalent to strengths in traditionally-casted alloys, the AM sample must be refined to reduce the occurrence of these deposits and voids.  </p>

No description available.

Fluid dynamicists are always in need of innovative instruments for flow velocity measurements. An ideal instrument would be non-intrusive, have a very fine spatial resolution as well as a very fine temporal resolution, be able to measure three-components of velocity, and be compact. Through recent advancements, laser Doppler velocimetry can now meet all of those requirements making it an important part of aerodynamicist's research toolbox. The first paper presented in this manuscript style thesis explains the development of an advanced three-velocity component, spatially-resolving laser-Doppler velocimetry (LDV) system for highly resolved velocity measurements in situations with limited optical access. The new instrument, a next generation version of the previously developed 'comprehensive' LDV technology, enables measurements of three components of velocity and particle position in the axial direction all through a single transceiving lens. Described here is the design process and the final design for the 'compact, comprehensive' LDV (Comp²LDV). The probe was designed to achieve ± 10 micron root-mean-square uncertainties in axial particle position, which combined with the long measurement volume, allow researchers to obtain a three-velocity-component velocity statistics profiles over a span of approximately 1.5mm without the need for traversing. Results from measurements in a flat plate turbulent boundary layer very near the wall have compared favorably to data from previous studies. The second paper focuses on the motion and evolution of coherent structures in supersonic jet flows and how that relates to the intense noise the flows generate. As a preliminary study to experimentally address these relationships, novel non-intrusive measurements using two-component laser Doppler velocimetry (LDV) have been conducted at exceptionally high data rates to lend insight into the statistical behavior of noise-generating flow structures. A new heated supersonic jet facility has been constructed to provide supersonic flow at total temperatures ratios (T₀/Tₐ) up to 3. In the present work, the instrumentation is validated via comparison of LDV measurements along the centerline of a screeching cold jet with microphone and high-speed shadowgraph results. Reynolds stress spectra are presented for an over-expanded case (nozzle pressure ratio of 3.2) of a design Mach number 1.65 nozzle operated cold (T₀/Tₐ = 1). A preliminary study was then conducted in the near-nozzle shear layer, up to x/d = 4.0, at design nozzle pressure ratio (4.58) and total temperature ratio of 2.0. Results are presented for Reynolds stress time-delay correlations and power spectra at Re_d = 1.1M for this case. The stream-wise Reynolds normal stress spectra are compared with published spectral behavior reported by other researchers, indicating a similar spectral shape in the downstream stations as previously measured with LDV and hot wire anemometry for cold jets, but which differ in shape from density-based techniques. / Master of Science

Approved for public release, distribution is unlimited / Resonant excitation tests of rotor blades in vacuum spin pits using discrete oil jets showed that impact erosion of the blades could limit test times, but lower excitation amplitudes were produced using mist nozzles. Smaller diameter discrete jets might extend test times, but to fully prevent erosion, oil mist droplet size needed to be 30 microns or less. The present study examined both approaches. Prototype nozzles were developed to create 0.005 inch diameter multiple discrete jets using first alumina, then stainless steel tubing, laser and micro-machine drilling. The latter technique was selected and 50 were manufactured for evaluation in HCF spin tests. A vacuum test chamber was built to observe and photograph spray patterns from the prototype nozzles and from commercially available mist nozzles. An LDV system was used successfully to determine the velocity of the oil droplets within the mist. A complete mapping of mist nozzle sprays is required to allow routine design of blade excitation systems. / Lieutenant, United States Navy

Calcific aortic valve disease is highly prevalent, especially in the elderly. Currently, the exact mechanism of the calcification process is not completely understood, limiting our ability to prevent or cure the disease. Ex vivo investigations, however, have provided evidence that the aortic valve's biological response is sensitive to mechanical forces, including fluid shear stresses, leading to the hypothesis that adverse fluid shear stress environment play a role in leading to valve calcification. This thesis seeks to investigate this hypothesis. A method for performing experimental measurement of time-varying shear stress on aortic valve leaflets under physiologic flow conditions was first developed, based on the Laser Doppler Velocimetry technique, and was systematically validated. This method was then applied to both the aortic surface and the ventricular surface of a normal tricuspid the aortic valve, and then on a congenital bicuspid aortic valve, using suitable in vitro valve models and an in vitro pulsatile flow loop. It was found that in the tricuspid valve, the peak shear stress on the aortic surface under adult resting condition was approximately 15-19 dyn/cm². Aortic surface shear stresses were elevated during mid- to late-systole, with the development of the sinus vortex, and were low during all other instances. Aortic surface shear stresses were observed to increase with increasing stroke volume and with decreasing heart rate. On the ventricular surface, shear stresses had a systolic peak of approximately 64-71 dyn/cm² under adult resting conditions. During late systole, due to the Womersley effect, shear stresses were observed to reverse in direction to a substantial magnitude for a substantial period of time. Further, it was found that a moderately stenotic bicuspid aortic valve can experience excessive unsteadiness in shear stress experienced by its leaflets, most likely due to the turbulent forward flow resulting from the stenosis, and due to the skewed forward flow. To demonstrate that the measured shear stresses can have an effect on the aortic valve biology, ex vivo experiments were performed in specific to determine the effects of these various shear stress characteristics on the biological response of porcine aortic valve leaflets, using the cone and plate bioreactor. It was found that unsteady shear stress measured in the bicuspid valve resulted in increased calcium accumulation. Further, it was found that low shear stresses and high frequency shear stresses resulted in increased calcium accumulation. Thus, shear stress was found to affect aortic valve pathology, and low and unsteady fluid shear stresses can enhance pathology.