The issue of the effects of free stream turbulence on the flow field over a surface-mounted prism is examined through experimental and numerical investigations. In the experimental studies, particle image velocimetry measurements are conducted in the ESM water tunnel at Reynolds number of $9,600$ and under two cases of turbulent inflow conditions. The results show that the mean flow separation, reattachment and parameters such as mean velocity, root mean square, Reynolds stresses and turbulent kinetic energy are affected by the turbulence characteristics of the incident flow. The instantaneous dynamics of the interactions between the separating shear layer and the solid wall and between the shear layer and the turbulence in the incident flow are detailed. In the numerical studies, large eddy simulations of the flow over a surface-mounted prism under two inflow conditions, namely, smooth inflow and isotropic homogeneous turbulence inflow, are performed. The use of a fifth-order scheme (CUD-II-5), which is a member of a family of Compact Upwind Difference schemes, in large eddy simulations of this flow is assessed. The performance of this scheme is validated by comparing the rate of temporal decay of isotropic turbulence with available experimental measurements for grid-generated turbulence. The results show that the spectra are sensitive to the method of flux vector splitting needed for the implementation of the upwind scheme. With van Leer splitting, the CUD-II-5 scheme is found to be too dissipative. On the other hand, using the Lax-Friedrichs vector splitting yields good agreement with experiments by controlling the level of artificial dissipation. This led us to recommend a new procedure, we denote by C6CUD5 scheme, that combines a compact sixth-order scheme with the CUD-II-5 scheme for large eddy simulation of complex flows. The simulation results, including flow patterns, pressure fields and turbulence statistics show that the CUD-II-5 scheme, with Lax-Friedricks flux vector splitting, provides high resolution of local flow structures. The results present new physical aspects of the flow topology over surface-mounted prisms. The effects of the incident homogeneous turbulence on the size of the separation region and suction pressures are determined by pointing out differences in the flow topologies between the two incident flow cases. / Ph. D.

Chrysopelea paradisi, also known as the flying snake, possesses one of the most unique forms of aerial locomotion found in nature, using its entire body as a dynamic lifting surface without the use of wings or membranes. Unlike other airborne creatures, this species lacks appendages to aid in controlling its flight trajectory and producing lift. The snake exhibits exception gliding and maneuvering capabilities compared with other species of gliders despite this lack of appendages. While gliding, C. paradisi morphs its body by expanding its ribs, essentially doubling its width and utilizing its entire length as a reconfigurable wing. Its cross-sectional shape transforms into a thick, airfoil shape with a concave ventral surface, outwards protruding lips at the leading and trailing edges, a somewhat triangular dorsal surface with a round apex, and fore-aft symmetry. This study investigated the aerodynamic performance of this unique shape by simulating a single, static segment of the snake's body over a wide range of Reynolds numbers (3,000 to 15,000) and angles of attack (-10 to 60o) to simulate the full range of the snake's flight kinematics. This is the first study on an anatomically accurate snake model, and few aerodynamic studies have been performed in this low Reynolds number regime. Load cell measurements and time-resolved digital particle image velocimetry (TRDPIV) were performed on a 2D anatomically accurate model to determine the lift and drag coefficients, wake dynamics, and vortex shedding characteristics. This geometry produced a maximum lift coefficient of 1.9 and maximum lift to drag ratio of 2.7, and maintained increases in lift up to 35o. Overall, this geometry demonstrated robust aerodynamic behavior by maintain significant lift production and near maximum lift to drag ratios over a wide range of test parameters. These aerodynamic characteristics may enable the flying snake to glide at steep angles and over a wide range of angles of attack, often encountered in gliding trajectories. This geometry also produced larger maximum lift coefficients than many other bluff bodies and airfoils in this low Reynolds number regime. This thesis is organized as follows. The first section contains a broad introduction on gliding flight and C. paradisi's unique mode of gliding. The following section is a manuscript that will be submitted to a journal and contains the experimental analysis on the snake's cross-sectional shape. Several appendices attached to the end of this thesis contain additional analysis and work performed throughout the duration of this project and unique Matlab algorithms developed during this research. / Master of Science

The aim of this work was to understand the physical requirements as well as to develop methodology required to employ Time Resolved Digital Particle Image Velocimetry (TRDPIV) for measuring high speed, high magnification, near wall flow fields. Previous attempts to perform measurements such as this have been unsuccessful because of both limitations in equipment as well as proper methodology for processing of the data. This work addresses those issues and successfully demonstrates a test inside of a transonic turbine cascade as well as a high speed high magnification wall jet. From previous studies it was established that flow tracer delivery is not a trivial task in a high speed high back pressure environment. Any TRDPIV measurement requires uniform spatial seeding density, but time-resolved measurements require uniform temporal seeding density as well. To this end, a high pressure particle generator was developed. This advancement enhanced current capability beyond what was previously attainable. Unfortunately, this was not sufficient to resolve the issue of seeding all together, and an advanced data reconstruction methodology was developed to reconstruct areas of the flow field that where lost do to inhomogeneous seeding. This reconstruction methodology, based on Proper Orthogonal Decomposition (POD), has been shown to produce errors in corrected velocities below tradition spatial techniques alone. The combination of both particle generator and reconstruction methodology was instrumental for successfully acquiring TRDPIV measurements in a high speed high pressure environment such as a transonic wind tunnel facility. This work also investigates the development of a turbulent wall jet. This experiment helped in demonstrating the capability of taking high speed high magnification TRDPIV measurements. This experiment was very unique in that it is one of only a few experiments that studied the developing region of these jets. The Reynolds number ranged for this experiment from 150 – 10,000 which corresponded to velocities of 1 - 80 m/s. The results from this experiment showed good agreement with currently published time averaged data. Using scaling laws for fully developed jets a new scaling law was found for the developing region of the jet that could be applied to all Reynolds numbers in this study. A temporal investigation was also carried out using the temporal coefficients from POD. A vortex identification scheme was also applied to all of the Reynolds numbers showing clear trends as Reynolds number increased. / Master of Science

A stator cascade was developed to simulate the flow conditions within a close-stage-spacing transonic axial compressor. Experiments were conducted in a linear transonic blowdown cascade wind tunnel with an inlet Mach number of 0.65. The bow shock from the downstream rotor was simulated by a single moving normal shock generated with a shock tube. First, steady pressure data were gathered to ensure that the stator cascade operated properly without the presence of the shock. Next, the effects of the passing shock on the stator flow field were investigated using shadowgraph photography and Digital Particle Image Velocimetry (DPIV). Measurements were taken for three different shock strengths. In every case studied, a vortex formed near the stator trailing edge as the shock impacted the blade. The size of this vortex was shown to be directly related to the shock strength, and the vortex remained present in the trailing edge flow field throughout the cycle duration. Analysis of the DPIV data showed that the vortex acts as a flow blockage, with the extent of this blockage ranging from 2.9% of the passage for the weakest shock, to 14.3% of the passage for the strongest shock. The vortex was also shown to cause flow deviation up to 75° for the case with the strongest shock. Further analysis estimated that the total pressure losses due to shock-induced vorticity ranged from 46% to 113% of the steady wake losses. Finally, the total pressure loss purely due to the upstream-propagating normal shock was estimated to be roughly 0.22%. / Master of Science

Noise induced hearing loss is a large concern for the Department of Defense. Personnel on aircraft carriers are exposed to dangerous noise levels of noise from tactical aircraft, causing hearing damage which results in significant costs for medical care and treatment. Additionally, NASA and the FAA have begun to investigate the viability of reintroducing supersonic commercial transport in the United States and one of the largest problems to address is reducing the noise impact of these aircraft on communities. The overarching goal of jet noise research is to optimize noise reduction techniques for supersonic jets. In order to achieve this, a more complete theoretical framework which links the jet boundary conditions to the turbulence production in the jet plume and the far-field radiated noise must be established. The research presented herein was conducted on the hypothesis that introducing thermal non-uniformities into a heated supersonic jet flow can favorably alter the turbulence structure in the jet shear layer, leading to reductions in radiated noise. To investigate the impact of temperature on the turbulence development in the jet, spatially resolved three-component velocity vectors were acquired using particle image velocimetry (PIV) performed on two small-scale perfectly expanded Mach 1.5 jet flows, one with a uniform temperature profile and another containing a geometrically offset temperature non-uniformity. Using the PIV data, the mean velocities, Reynolds stresses, and correlation coefficients were obtained from both jet flows and compared to analyze changes in the mean turbulence field. Small but significant reductions in the shear layer turbulence were observed in the near nozzle region of the thermally offset jet when compared to the uniform jet case. The changes result in a thickening of the shear layer nearest the location of the cold plume which alters the integral length scales of the coherent turbulent structures in the offset jet in a manner consistent with other techniques presented in the literature that reduce jet noise. Applying quadrant analysis, a conditional averaging technique, to the jet turbulence plume revealed changes in the statistical flow field of Reynolds shear stress structures. The changes provide strong evidence of the presence of intermittent stream-wise vortical structures which serve to reduce the spatial correlation levels of turbulence in the thermally offset jet flow when compared to the uniform baseline jet. / Doctor of Philosophy / Increasingly large and powerful engines are required as the mission requirements for tactical aircraft become more advanced. These demands come at the cost of an increased production of noise which is particularly hazardous to crewpersons operating on Navy aircraft carriers during take-off and landing. Noise-induced hearing loss from extended exposure to high noise levels has become a major medical expenditure for the Navy. To address this issue in tactical aircraft engines, the sources of jet plume noise must be reduced, but doing so requires improved understanding of the connections between nozzle boundary conditions, the jet turbulence plume, and the radiated noise while keeping in consideration system constraints and performance requirements. The current study introduces a novel method for controlling supersonic jet noise induced by turbulence through the introduction of an offset non-uniform temperature perturbation at the nozzle mouth. Non-invasive flow measurements were conducted using stereoscopic particle image velocimetry to obtain high-resolution velocity and turbulence data. Analysis of the flow data indicate that an offset reduced temperature plume introduced at the nozzle exit has a first-order effect on the turbulence evolution which result in small, but significant reductions in jet noise levels. The reductions observed are attributed to a disruption in the coherence of the primary noise generating turbulence structures in the jet plume which are associated with the formation of stream-wise vortical structures induced by the cold plume.

Junction flows originate from the interaction between a fluid moving over a wall with an obstacle mounted on the same surface. Understanding the physics of such flows is of great interest to engineers responsible for the design of systems consisting of wall-body junctions. From aerodynamics to turbomachinery and electronics to bridge hydraulics, a number of phenomena (drag, heat transfer, scouring) are driven by the behavior of the most prominent feature of junction flows: the horseshoe vortex system (HVS). Focusing on turbulent flows, the complex dynamics of the HVS is established through its unsteadiness and non-uniformity. The fundamentals of this dynamically-rich phenomenon have been described within the body of a rapidly-expanding literature. Nevertheless, important aspects remain inadequately understood and call for further scrutiny. This study emphasized three of them, by investigating the effects of: model scale, wall roughness, and bed geometry. High-resolution experiments were carried out using Particle Image Velocimetry (PIV). Statistical analyses, vortex identification schemes, and Proper Orthogonal decomposition were employed to extract additional information from the large PIV datasets. The time-averaged topology of junction flows developing over a smooth and impermeable wall was independent of the flow Reynolds number, Re (parameter that expresses the effects of scale). On the contrary, time-resolved analysis revealed a trend of increasing vorticity, momentum, and eruptions of near-wall fluid with Re. New insights on the modal dynamics of the HVS were also documented in a modified flow mechanism. Wall roughness (modeled with a permeable layer of crushed stones) diffused turbulence and vorticity throughout the domain. This effect manifested with high levels of intermittency and spatial irregularity for the HVS. Energetic flow structures were also identified away from the typical footprint of the HVS. Finally, a novel implementation of PIV allowed for unique velocity measurements over an erodible bed. It was demonstrated that, during the initial stages of scouring, the downflow at the face of the obstacle becomes the dominant flow characteristic in the absence of the HVS. Notwithstanding modeling limitations, the physical insight contributed here could be used to enhance the design of systems with similar flow and geometrical characteristics. / Ph. D.

The development of aerodynamic boundary layers on wind turbine blades is an important consideration in their performance. It can be quite challenging to replicate full scale conditions in laboratory experiments, and advanced diagnostics become valuable in providing data not available from traditional means. A new variant of Doppler global velocimetry (DGV) known as cross-correlation DGV is developed to measure boundary layer profiles on a wind turbine blade airfoil in the large scale Virginia Tech Stability Wind Tunnel. The instrument provides mean velocity vectors with reduced sensitivity to external conditions, a velocity measurement range from 0ms^-1 to over 3000ms^-1, and an absolute uncertainty. Monte Carlo simulations with synthetic signals reveal that the processing routine approaches the Cramér-Rao lower bound in optimized conditions. A custom probe-beam technique is implanted to eliminate laser flare for measuring boundary layer profiles on a DU96-W-180 wind turbine airfoil model. Agreement is seen with laser Doppler velocimetry data within the uncertainty estimated for the DGV profile. Lessons learned from the near-wall flow diagnostics development were applied to a novel benchmark model problem incorporating the relevant physical mechanisms of the high amplitude periodic turbulent flow experienced by turbine blades in the field. The model problem is developed for experimentally motivated computational model development. A circular cylinder generates a periodic turbulent wake, in which a NACA 63215b airfoil with a chord Reynolds number Re_c = 170, 000 is embedded for a reduced frequency k = (pi)fc/V = 1.53. Measurements are performed with particle image velocimetry on the airfoil suction side and in highly magnified planes within the boundary layer. Outside of the viscous region, the Reynolds stress profile is consistent with the prediction of Rapid Distortion Theory (RDT), confirming that the redistribution of normal stresses is an inviscid effect. The fluctuating component of the phase- averaged turbulent boundary layer profiles is described using the exact solution to laminar Stokes flow. A phase lag similar to that in laminar flow is observed with an additional constant phase layer in the buffer region. The phase lag is relevant for modeling the intermittent transition and separation expected at full scale. / Ph. D.

The entrainment process in two-phase buoyant jets differs significantly from their singlephase counterparts, and is not well understood. Entrainment models developed for singlephase flow are often used in two-phase jetting simulations, albeit with limited success. In this work, Particle Image Velocimetry (PIV) and shadowgraph flow visualization experiments have been conducted on submerged round gas jets of varying speeds and nozzle diameters with the goal of improving our understanding of the entrainment process in a two-phase (gas-liquid) jet. The total entrainment estimated using the PIV measurements is higher than the respective values suggested by a common empirical model developed for singlephase buoyant jets. A two-phase theoretical entrainment model used for comparison shows an overestimation of entrainment, but predicts the increase in the rate of entrainment with axial distance from the jet nozzle seen in the PIV results. This thesis also presents advances in PIV processing methodology that were developed concurrently with the entrainment research. The novel Spectral Phase Correlation (SPC) allows for particle displacement to be determined directly from phase information in the Fourier domain. Some of the potential benefits of the SPC explored here include (1) avoidance of errors introduced by spatial peak-finding routines; (2) use of a modal analysis that can be used to provide information such as correlation quality; and (3) introduction of a means of incorporating information from multiple image windows. At low image noise levels, the method performs as well as an advanced CC-based method. However, difficulties unwrapping the aliased phase information cause the SPC's performance to degrade at high noise levels. / Master of Science

Heart disease is one of the primary causes of morbidity and mortality for the adult population over the age of 65. Furthermore, ailments such as hypertension can affect as many as 50% of the adult population over the age of 45. If left untreated, these ailments eventually precipitate the onset of diastolic dysfunction and heart failure. Diastolic dysfunction is the alteration or impairment of performance in either the left or right ventricle of the heart. Although there has been a marked increase in study of this disease, there is still an apparent difficulty to diagnose patients. Flow visualization techniques have been commonly employed to study the development of these diseases as they relate to the filling process of the ventricles. One method, Echo Particle Image Velocimetry (Echo-PIV) is a relatively new method for cardiac flow chamber visualization, with the potential to provide physicians with a cost-effective and safe method for obtaining high temporal resolution recordings for extending knowledge on the filling processes in cardiac chamber flow. This work presents a new approach to extending the capabilities of Echo-PIV for more accurate measurement of cardiac flows for patients with poor quality recordings. Currently, much of the literature notes that temporal resolution and poor acoustic windows results in exclusion from study. These recordings are more representative of the contrast-enhancement studies used by physicians to better identify chamber walls. When applying standard PIV cross-correlation techniques, measurements tend to fail due to image noise and artifacts. By implementing a Moving Ensemble (MWE) with Product of Correlation (PoC) processing scheme, measurement accuracy, reliability, and robustness can be obtained for measurement in left ventricular filling assessment. / Master of Science

Aerodynamic measurements were taken over models of the Boeing 777 high fidelity isolated landing gear in the 6- by 6-foot Virginia Tech Stability Wind Tunnel (VT-SWT) at a free-stream Mach number of 0.16. Noise control devices (NCD) were developed at Virginia Tech [9] to reduce noise by shielding gear components, reducing wake interactions and by streamlining the flow around certain landing gear components. Aerodynamic measurements were performed to understand the flow over the landing gear and also changes in the flow between "Baseline" and "NCD" configurations (without and with Noise Control Devices respectively). Hot-film, Pitot-static measurements and flow visualization using tufts were performed over an isolated 26% scale-model high fidelity landing gear for the "Baseline" and "NCD" configurations. Contours of turbulence intensity, normalized wake velocity and normalized total pressure loss for both configurations are compared. The "Baseline" configuration was also compared with the NASA Ames study conducted by Horne et al [7]. Hot-film measurements are also compared to Microphone Phased Array results which were acquired at Virginia Tech by Ravetta [8]. A novel technique for processing hot-film measurements by breaking turbulence into octave bands as acoustic measurements is presented. Particle Image Velocimetry (PIV) measurements were taken at six different locations over the 13% scale-model landing gear with no door and at a truck angle of zero degrees. Results are compared to PIV measurements taken over the wheels of a four-wheel landing gear by Lazos [10-12]. PIV results such as average velocity contours and vectors, streamlines and instantaneous velocity contours and vectors are presented. Results presented from PIV and flow visualization are in good agreement with results from Lazos [10-12]. / Master of Science