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

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.

Ce travail s’intéresse à l’analyse expérimentale du développement de la zone de mélange turbulente (ZMT) produite par une instabilité de Richtmyer-Meshkov (IRM). Les expériences sont réalisées au sein d’un tube à chocs vertical, et l’analyse s’appuie sur des mesures simultanées mettant en œuvre des techniques expérimentales de type capteurs de pression pariétaux, visualisations strioscopiques résolues en temps et mesures de vitesse par Vélocimétrie Laser Doppler (LDV). Une caractérisation de l’installation expérimentale est tout d’abord effectuée en situation homogène (air pur, sans mélange), afin de déterminer la qualité de l’écoulement de base et connaître le niveau de turbulence de fond du tube à chocs. Les configurations de mélange, principalement entre de l’air et de l’hexafluorure de soufre (SF6), sont ensuite abordées. On s’intéresse dans un premier temps aux caractéristiques globales de la zone de mélange : en particulier à l’évolution de son épaisseur et à son taux de croissance. Plusieurs configurations de mélange sont étudiées en faisant varier différents paramètres expérimentaux tels que la hauteur de la veine d’essais du tube à chocs, la forme de la perturbation initiale de l’interface entre les deux gaz et le nombre d’Atwood, dans le but de déterminer leur influence sur le développement de la ZMT. On montre ainsi une sensibilité du taux de croissance post-rechoc à plusieurs de ces paramètres. Des comparaisons avec des simulations numériques réalisées par nos partenaires du Commissariat à l’Énergie Atomique (CEA) montrent des tendances similaires entre expériences et simulations sur ce point. L’étude est ensuite complétée par une caractérisation plus locale de la ZMT, en mesurant les niveaux de turbulence en différents points de la veine d’essais à l’aide de la LDV. Après avoir quantifié les contraintes de convergence statistique imposées par l’expérience pour ce type de mesures, on donne une estimation des intensités turbulentes produites par l’écoulement de mélange à différents stades de son développement. / This experimental study sheds some light on the development of the turbulent mixing zone (TMZ) arising from a Richtmyer-Meshkov instability (RMI). The experiments are conducted in a vertical shock tube, and the analysis relies on simultaneous measurements involving pressuretransducers, time-resolved Schlieren visualizations and Laser Doppler Velocimetry (LDV). In a first step, a thorough characterization of the experimental apparatus is conducted in order to qualify the basic flow configuration corresponding to homogeneous situations (pure air withoutmixing), and to evaluate the « background » turbulence level of the shock tube. Mixing configurations (mainly between air and sulfur hexafluoride, SF6) are then investigated. We first focus on a global description of the mixing zone such as the time evolution of its thickness and the corresponding growth rate. We consider several mixing configurations, varying the length of the test section, the shape of the initial interface between the two gases and the Atwood number. A clear influence of some of these parameters is shown on the the post-reshock increasing rate of the mixing zone, in good accordance with numerical results obtained from the Commissariat à l’Energie Atomique (CEA, french atomic energy commission). A more local description of the flow is then obtained in a second step by measuring the turbulence levels at different locations inside the test section thanks to the LDV technique. After quantifying the issues linked to the statistical convergence of the turbulent quantities in such specific configurations, we provide an estimation of the turbulent intensities produced by the mixing at various stages of its development.

To improve optical measurements, which are degraded by optical distortions, wavefront correction systems can be used. Generally, these systems evaluate a guide star in transmission. The guide star emits wellknown wavefronts, which sample the distortion by propagating through it. The system is able to directly measure the distortion and correct it. There are setups, where it is not possible to generate a guide star behind the distortion. Here, we consider a liquid jet with a radially open surface. A Mach–Zehnder interferometer is presented where both beams are stabilized through a fluctuating liquid jet surface with the Fresnel guide star (FGS) technique. The wavefront correction system estimates the beam path behind the surface by evaluating the incident beam angle and reflected beam angle of the Fresnel reflex with an observer to control the incident angle for the desired beam path. With this approach, only one optical access through the phase boundary is needed for the measurement, which can be traversed over a range of 250 μm with a significantly increased rate of valid signals. The experiment demonstrates the potential of the FGS technique for measurements through fluctuating phase boundaries, such as film flows or jets.

An advanced laser-Doppler velocimeter is designed to acquire fully-resolved turbulence structural measurements in high Reynolds number two- and three-dimensional turbulent boundary layers. The new instrument combines, for the first time, new techniques allowing for the direct measurement of particle acceleration and sub-measurement-volume-scale position resolution so that second-order 3D particle trajectories may be measured at high repetitions. Using these measurements, several terms in the Reynolds stress transport equations may be directly estimated, giving new data for modeling and understanding the processes leading to the transport of turbulence in boundary layer flows. Due to the unique performance of the probe, many aspects of LDV instrumentation development were addressed. The LDV configuration was optimized for lowest uncertainties by considering the demanding applications of particle position and acceleration measurements. Low noise light detection and signal conditioning was specified for the three electronic channels. A high-throughput data acquisition system allows for exceptional burst rate acquisition. Signal detection and processing algorithms have been implemented which draw from previous techniques but also address distinctive problems with the current system. In short, the instrument was designed to advance the state-of-the-art in LDV systems. Measurements presented include turbulence dissipation rate and fluctuating velocity-pressure gradient correlations that have been measured in 2D and 3D turbulent boundary layers using the unique capabilities of the CompLDV--many of these measurements are the first of their kind ever acquired in high Reynolds number turbulent flows. The flat-plate turbulent boundary layer is studied at several momentum thickness Reynolds numbers up to 7500 to examine Reynolds numbers effects on terms such as the velocity-pressure gradient correlation and the dissipation rate in the Reynolds transport equations. Measurements are also presented in a pressure-driven three-dimensional turbulent boundary layer created upstream from a wing-body junction. The current results complement the extensive data from previous studies and provide even richer depth of knowledge on the most-completely-documented 3D boundary layer flow in existence. Further measurements include the wakes of three circular-cylinder protuberances submerged in a constant pressure turbulent boundary layer. / Ph. D.

L’étude présentée dans cette thèse relève de la mécanique des fluides expérimentale et numérique appliquée aux écoulements pariétaux de refroidissement de chambres de combustion aéronautiques. En présence de phénomènes thermo-acoustiques, comme les instabilités de combustion, il est important d’évaluer si les capacités de l’écoulement pariétal à protéger les parois de chambre restent suffisantes. C’est ainsi que nous nous sommes intéressés aux écoulements de paroi multiperforée soumis à une excitation acoustique. Dans ce but, le banc d’essais MAVERIC a été amélioré grâce à l’installation d’un système qui permet de forcer acoustiquement l’écoulement transverse dans lequel les jets pariétaux débouchent. Nous avons pu alors mettre en évidence la forte sensibilité de ce type d’écoulements à l’excitation acoustique. Le bon accord entre les résultats expérimentaux et les simulations numériques aux grandes échelles (LES) effectuées est très encourageant dans le cas d’un forçage par onde stationnaire. Le forçage par onde progressive, étudié uniquement par simulations numériques, s’est révélé être capable de modifier significativement la topologie de l’écoulement. Enfin, à partir de l’outil numérique AVBP-AVTP qui permet le couplage de calculs fluide-solide, nous avons réalisé une étude de l’influence de la présence d’une excitation acoustique sur le comportement thermique de l’écoulement autour d’une paroi multiperforée de chambre de combustion. / This experimental and numerical study in the field of fluid mechanics deals with jets-in cross flow configurations that are relevant for the cooling of aero engine combustion chambers. Indeed, in presence of instabilities it is important to determine to which extent the film cooling is able to do its job of preserving the combustion chamber walls from the thermal load. The test facility MAVERIC has been upgraded in order to acoustically force the crossflow in which the jets are discharging. The strong sensitivity of the overall flow unsteady properties to the presence of the acoustic forcing has been clearly evidenced. The agreement between the experimental results and large-eddy simulations proved to be quite encouraging for a stationary acoustic wave whereas the case of a propagating acoustic wave investigated only numerically reveals also quite a significant change of the flow topology. In this context, the effect of the acoustic forcing on the wall thermal behavior has been analyzed thanks to the use of the fluid-solid coupled AVBP-AVTP solver.

Τα ολοκληρωμένα ηλιακά συστήματα συλλέκτη αποθήκης αποτελούνται από μία δεξαμενή αποθήκευσης, της οποίας τμήμα της επιφάνειας της χρησιμοποιείται σαν ηλιακός συλλέκτης. Συνήθως το ρευστό της αποθήκης είναι το νερό χρήσης. Στο υπό εξέταση σύστημα το νερό χρήσης θερμαίνεται έμμεσα, διερχόμενο μέσα από σωληνώσεις εναλλάκτη θερμότητας που τοποθετείται στο εσωτερικό της παραλληλεπίπεδης δεξαμενής. Για την εντατικοποίηση της μετάδοσης θερμότητας προς το νερό χρήσης, δημιουργείται ανάδευση του ρευστού του δοχείου μέσω κυκλοφορητή, ο οποίος τίθεται σε λειτουργία μόνο όταν υπάρχει ζήτηση ζεστού νερού. Προς αποφυγή παραμορφώσεων τοποθετούνται πτερύγια συγκράτησης που ενώνουν τις δύο μεγάλες επιφάνειες της δεξαμενής. Στην παρούσα διδακτορική διατριβή διερευνήθηκαν τα ροϊκά φαινόμενα στο εσωτερικό του ICS που έχει περιγραφεί προηγουμένως, με στόχο τη μεγιστοποίηση της θερμανταλλαγής μεταξύ των δύο κυκλωμάτων νερού. Για την παρατήρηση του ροϊκού πεδίου καθώς και την λήψη μετρήσεων ταχυτήτων κατασκευάστηκε πειραματική συσκευή με διαφανή τοιχώματα από Plexiglas. Ελήφθησαν μετρήσεις ταχυτήτων και διακυμάνσεων με χρήση συστήματος Laser Doppler διπλής ακτίνας. Για την οπτικοποίηση του ροϊκού πεδίου τοποθετήθηκαν σωματίδια πολυστερίνης στο εσωτερικό της συσκευής. Ελήφθησαν ψηφιακές φωτογραφίες και βιντεοσκοπήσεις του ροϊκού πεδίου. Για την υπολογιστική προσομοίωση χρησιμοποιήθηκε το εμπορικό λογισμικό FLUENT. Αναπτύχθηκε υπολογιστικό μοντέλο και επιλύθηκε με όλα τα διαθέσιμα μοντέλα τύρβης. Στη συνέχεια πραγματοποιήθηκε σειρά υπολογιστικών προσομοιώσεων, στις οποίες διερευνήθηκε η βέλτιστη θέση και το μέγεθος των στομίων ανακυκλοφορίας, η βέλτιστη διάταξη των πτερυγίων συγκράτησης και η βέλτιστη θέση του εναλλάκτη. Επιπλέον προσδιορίστηκε υπολογιστικά και πειραματικά ο χρόνος αποκατάστασης του ροϊκού πεδίου. Τέλος, τα αποτελέσματα συγκρίθηκαν και με πειραματικά αποτελέσματα άλλων εργασιών. Τα συμπεράσματα που εξάγονται έχουν ως ακολούθως: Το μοντέλο τύρβης standard k-ω δίνει τα πιο αξιόπιστα αποτελέσματα. Το υπολογιστικό μοντέλο θεωρείται πιστοποιημένο μετά από πειραματική επιβεβαίωση ταχυτήτων και θερμοκρασιών. Το στόμιο ανακυκλοφορίας δεν πρέπει να τοποθετείται κάθετα στις μεγάλες επιφάνειες της δεξαμενής, ενώ η διάμετρος του πρέπει να είναι 1/2" ή και μικρότερη. Ο χρόνος αποκατάστασης του ροϊκού πεδίου είναι περίπου 35 s. Τα πτερύγια συγκράτησης πρέπει να ακολουθούν τις ροικές γραμμές. Ως βέλτιστη θέση του εναλλάκτη θεωρείται όταν τοποθετείται σε επαφή με το τοίχωμα. / Integrated Collector Storage (ICS) solar systems use part of the hot water storage as collector, i.e. half of the storage surface is used as absorber. Usually, the storage medium serves also as the energy transfer medium (service hot water). In the examined ICS, the service water is heated indirectly, passing through a serpentine heat exchanger placed inside the tank. The heat transfer from the stored water to the service water is intensified by the agitation of the stored water. A simple solution is the recirculation of the stored water by a small pump, which is functioning whenever a request for hot water exists. Fins in suitable positions, connect the front and back surface of the ICS, to withstand the deformation due to pressures by the tank water. In the present PhD thesis, the flow phenomena inside the ICS previous mentioned, are investigated. The aim is the maximization of the heat transfer between the two water circuits. An experimental device was constructed by transparent Plexiglas, for flow visualization and velocity measurements. A dual beam Laser Doppler Velocimetry (LDV) system was used to measure velocities. Polystyrene particles were added in the comprised water, for the visualization of the path lines. Photographs and video films were also taken. The commercial code FLUENT is used for the Computational Fluid Dynamics (CFD) simulations. A CFD model is developed and solutions are obtained using all the available turbulence models. Three main factors that influence the performance are optimized: the position and size of the recirculation ports, the arrangement and size of the interconnecting fins and the heat exchanger placement. The settling time, i.e., the time required for the flow field to be fully developed, is computed both numerically and experimentally. The previous analysis leads to the following conclusions: The standard k–ω model is selected as the most appropriate. The model is validated, with good agreement, against experimental measurements of velocities and temperatures. The placement of the inlet recirculation port perpendicular to the main surfaces of the ICS should be avoided, while its diameter should be 1/2" or less. The settling time is computed about 35s. The interconnecting fins, of the two main ICS surfaces should follow the flow filed path lines. The optimal placement of the tube heat exchanger is in contact with the two major surfaces of the storage tank.