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<p>In this thesis, we unveiled the impact of fluid stratification and viscoelasticity on the transport of microorganisms and microparticles. The thesis is divided into four chapters. Chapters 2 and 3 focus on the transport of the swimming sheet in density and viscosity stratified fluids. Chapter 4 is devoted to analyze the motion of anisotropic particles in density stratified fluids. Chapter 5 focuses on the effect of viscoelasticity on the motion of a suspension of spherical particles.</p>

<p> A virtual impactor type dust flow concentrator was developed and an experimental investigation was performed to characterize the hydraulic and particulate matter (PM) separation performance of the device. In particular, the pressure drop characteristics, the ratio of the flow through the two branches, and the PM concentration in the minor and main branch of the flow concentrator were evaluated using experiments on a diesel exhaust rig and an air rig. Tests were performed to examine the effect of the inlet flow Reynolds number, the inlet tube lip position relative to the concentration probe, and the flow distribution between the minor and main branches. Numerical simulations were also performed for a simplified concentrator geometry to examine the flow streamlines and pressure drop. The results showed that the ratio of the concentration in the minor and main flow branches changed as the flow rate in these branches changed, reaching a maximum at a minor flow ratio that depended on the lip position. It was observed the difference of the particulate matter concentration in the minor and main branch was greater for higher inlet Reynolds number. For example, the concentration ratio increased 50% as the Reynolds number increased from 2,200 to 25,700. A similar result was observed when the inlet tube lip was moved further into the concentration body. The pressure losses seem to be mainly caused by the changes in flow directions and the change in the cross sectional areas.</p> / Thesis / Master of Applied Science (MASc)

There have been very few studies investigating the rough wall pressure spectra under fully rough flows, which are relevant to many common engineering applications operating within this regime. This investigation uses the Virginia Tech Stability Wind Tunnel to perform experiments on a series of high Reynolds number zero pressure gradient turbulent boundary layers formed over rough walls in an effort to better understand and characterize the behavior of the rough wall pressure spectrum. The boundary layers were fully rough, and the boundary layer height remained sufficiently larger than the height of the roughness elements. Two rough surfaces were tested. One consisted of an array of 1-mm ordered hemispherical elements spaced 5.5-mm apart, and the other contained 3-mm hemispherical elements randomly spaced, but with the same element density as 1/3 of the 1-mm ordered roughness. The wall pressure spectrum and its scaling were then studied in detail, and it was found that the rough wall turbulent pressure spectrum at vehicle relevant conditions is defined by three scaling regions. One of which is a newly discovered high frequency scaling defined by viscosity, but controlled by the friction velocity adjusted to exclude the pressure drag on the roughness elements. Based on these three scaling regions an empirical model describing the wall pressure spectra for hydraulically smooth, traditionally rough, and fully rough flows was explored. Two point wall pressure fluctuations were also analyzed for each surface condition, and it was found that the roughness inhibits the convective velocities within the inner portions of the boundary layer. / Master of Science

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.

Turbulence research in the laboratory over the past three decades has confirmed the existence of quasi-coherent structures amidst the chaos of a turbulent boundary layer. It has been observed that a quasi-periodic phenomena called "bursting" accounts for a major contribution to the turbulent Reynolds stress and the production of turbulent kinetic energy. Bursting is the term used for a sequence of events, where a low-speed streak of fluid from the near wall region lifts away from the wall, slowly at first, and then rapidly moves away from the wall as it convects downstream where it becomes unstable and breaks up violently upon interaction with the outer flow. This ejection of low speed fluid into the mean flow is responsible for locally high values of turbulent kinetic energy. Though a great deal is known about these structures in laboratory flows, little has been done to investigate their existence in the turbulent air flow over the ocean. It would seem, intuitively, that such structures, if present in the marine atmospheric boundary layer, would playa major role in the transfer of momentum, mass and heat across the air-sea interface. The present study is aimed at identifying the existence of burst structures in the marine atmospheric boundary layer. The standard ejection detection schemes like the quadrant, the VITA and the modified u-level techniques were applied to the turbulent wind data measured over the ocean. It was found that the proportion of contribution to the Reynolds stress from the four quadrants of the u'w' plane is in close agreement with the corresponding contributions for a laboratory flow. Ejection detection followed by the grouping of ejections into bursts yielded a mean burst period of 47 sec., at a height of 8.2 m above the water surface, where the mean wind velocity was 6.74 m/s. This burst period corresponds well with the peaks obtained from the autocorrelation of the streamwise velocity signal and the first moment of the stress spectrum. Furthermore, phase averages of these events show a structure which is similar to the structure of the events detected in the laboratory flows. / Master of Science

The oil-film surface flow visualization technique was applied to circular arc compressor blades in a solid wall, high aspect ratio cascade for the purpose of describing the transition from corner stall to full blade stall, and the blade surface flow under fully stalled conditions. Photos of the visualizations for three stagger angles are presented and analyzed. A map quantitatively describing the observed boundary layer development at midspan is presented. The most interesting discovery of the work showed the suction surface flow to be essentially two-dimensional, in the geometric sense, preceding and following the transition to a fully separated flow at the leading edge. Corner stall was the observed three-dimensional mechanism prior to full stall. For fully-stalled conditions, the three-dimensional mechanism took the form of recirculating flow regions at the blade ends. Complete separation at the leading edge occurred at lower angles of attack for the higher stagger angles. Special blade oil-flow tests were conducted to evaluate Reynolds number and tip clearance effects on boundary layer development. The experimental work was done as part of a larger research program aimed at measuring and predicting the stalled performance of a compressor cascade. / Master of Science

The impressive kinematic capabilities and structural adaptations presented by bio-locomotion continue to inspire some of the advancements in today's small-scaled flying and swimming vehicles. These vehicles operate in a low Reynolds number flow regime where viscous effects dominate flow interactions, which makes it challenging to generate lift and thrust. Overcoming these challenges means utilizing non-conventional lifting and flow control mechanisms generated by unsteady flapping body motion. Understanding and characterizing the aerodynamic phenomena associated with the unsteady motion is vital to predict the unsteady fluid loads generated, to implement control methodologies, and to assess the dynamic stability and control authority of airborne and underwater vehicles. This dissertation presents experimental results for forced oscillations on multi-element airfoils and hydrofoils for Reynolds numbers between Re=104 and Re=106. The document divides the work into four main sections: The first topic presents wind tunnel measurements of lift forces generated by an oscillating trailing edge flap on a NACA-0012 airfoil to illustrate the effects that frequency and pitching amplitude have on lift enhancement. The results suggest that this dynamic trailing edge flap enhances the mean lift by up to 20% in the stalled flow regime. Using frequency response approach, it is determined that the maximum enhancement in circulatory lift amplitude occurs at stalled angles of attack for lower pitching amplitudes. The second topic presents wind tunnel measurements for lift and drag generated by a sinusoidal and non-sinusoidal oscillations of a NACA-0012 airfoil. The results show that 'trapezoidal' pitching enhances the mean lift and the RMS lift by up to 50% and 35% in the pre-stall flow regime, respectively, whereas the 'reverse sawtooth' and sinusoidal pitching generate the most substantial increase of the lift-to-drag ratio in stall and post-stall flow regimes, respectively. The third topic involves a study on the role of fish-tail flexibility on thrust and propulsive efficiency. Flexible tails enhance thrust production in comparison to a rigid ones of the same size and under the same operating conditions. Further analysis indicates that varying the tail's aspect ratio has a more significant effect on propulsive efficiency and the thrust-to-power ratio at zero freestream flow. On the other hand, changing the material's property has the strongest impact on propulsive efficiency at non-zero freestream flow. The results also show that the maximum thrust peaks correspond to the maximum passive tail amplitudes only for the most flexible case. The final topic aims to assess the unsteady hydrodynamic forces and moments generated by a three-link swimming prototype performing different swimming gaits, swimming speeds, and oscillatory frequencies. We conclude that the active actuation of the tail's first mode bending produces the most significant thrust force in the presence of freestream flow. In contrast, the second mode bending kinematics provides the most significant thrust force in a zero-freestream flow. / Doctor of Philosophy / It is by no surprise that animal locomotion continues to inspire the design of flying and swimming vehicles. Although nature produces complex kinematics and highly unsteady flow characteristics, simplified approximations to model bio-inspired locomotion in fluid flows are experimentally achievable using low degrees of freedom motion, such as pitching airfoils and trailing edge flaps. The contributions of this dissertation are divided into four primary foci: (a) wind tunnel force measurements on a flapped NACA-0012 airfoil undergoing forced pitching, (b) wind tunnel measurements of aerodynamic forces generated by sinusoidal and non-sinusoidal pitching of a NACA-0012 airfoil, (c) towing tank measurements of thrust forces and torques generated by a one-link swimming prototype with varying tail flexibilities, and (d) towing tank measurements of hydrodynamic forces and moments generated by active tail actuation of a multi-link swimming prototype. From our wind tunnel measurements, we determine that lift enhancement by a trailing edge flap is achieved under certain flow regimes and oscillating conditions. Additionally, we assess the aerodynamic forces for a sinusoidal and non-sinusoidal pitching of an airfoil and show that 'trapezoidal' pitching produces the largest lift coefficient amplitude whereas the sinusoidal and 'reverse sawtooth' pitching achieve the best lift to drag ratios. From our towing tank experiments, we note that the role of tail flexibility enhances thrust generation on a swimming device. Finally, we conclude that different kinematics on an articulating body strongly affect the hydrodynamic forces and moments. The results of the towing tank measurements are accessible from an online public database to encourage research and contribution in underwater vehicle design through physics-based low-order models that can accommodate hydrodynamic principles and geometric control concepts.