abstract: Over the past three decades, particle image velocimetry (PIV) has been continuously growing to become an informative and robust experimental tool for fluid mechanics research. Compared to the early stage of PIV development, the dynamic range of PIV has been improved by about an order of magnitude (Adrian, 2005; Westerweel et al., 2013). Further improvement requires a breakthrough innovation, which constitutes the main motivation of this dissertation. N-pulse particle image velocimetry-accelerometry (N-pulse PIVA, where N>=3) is a promising technique to this regard. It employs bursts of N pulses to gain advantages in both spatial and temporal resolution. The performance improvement by N-pulse PIVA is studied using particle tracking (i.e. N-pulse PTVA), and it is shown that an enhancement of at least another order of magnitude is achievable. Furthermore, the capability of N-pulse PIVA to measure unsteady acceleration and force is demonstrated in the context of an oscillating cylinder interacting with surrounding fluid. The cylinder motion, the fluid velocity and acceleration, and the fluid force exerted on the cylinder are successfully measured. On the other hand, a key issue of multi-camera registration for the implementation of N-pulse PIVA is addressed with an accuracy of 0.001 pixel. Subsequently, two applications of N-pulse PTVA to complex flows and turbulence are presented. A novel 8-pulse PTVA analysis was developed and validated to accurately resolve particle unsteady drag in post-shock flows. It is found that the particle drag is substantially elevated from the standard drag due to flow unsteadiness, and a new drag correlation incorporating particle Reynolds number and unsteadiness is desired upon removal of the uncertainty arising from non-uniform particle size. Next, the estimation of turbulence statistics utilizes the ensemble average of 4-pulse PTV data within a small domain of an optimally determined size. The estimation of mean velocity, mean velocity gradient and isotropic dissipation rate are presented and discussed by means of synthetic turbulence, as well as a tomographic measurement of turbulent boundary layer. The results indicate the superior capability of the N-pulse PTV based method to extract high-spatial-resolution high-accuracy turbulence statistics. / Dissertation/Thesis / Animation of N-pulse PIVA measurement of flow-structure interaction / Doctoral Dissertation Mechanical Engineering 2018

Friction factors have been determined experimentally for longitudinal flow of water and various aqueous solutions of polyethylene glycol between regular enclosed arrays of cylinders. Two different geometries were investigated: a 4 × 4 square array enclosed by a square duct and a 19-rod equilateral triangular array enclosed by a hexagonal duct. Four different pitch-to-diameter ratios were studied in each geometry. These covered a range of 1.07 to 2.00 in the square array and 1.11 to 2.06 in the triangular array. No spacers were used in the rod bundle. The Reynolds number range investigated was approximately 2.5 to 40,000 (equivalent diameter based on total wetted perimeter). The fully developed friction factors for all eight arrays were correlated by the Nikuradse equation for smooth round tubes when the Reynolds number exceeded 10,000 and when the equivalent diameter, used in both the Reynolds number and the friction factor, was taken as four times the hydraulic raduis based on total wetted perimeter. In laminar flow no equivalent diameter was found that would consolidate the results of all eight arrays. Deviations from the laminar flow theory of Sparrow and Loeffler for infinite arrays could be consistently accounted for by the wall effect in the present study. The critical Reynolds number for each array ranged between 90 and 450 (equivalent diameter based on total perimeter) and the transition from laminar to turbulent flow appeared to extend over a large Reynolds number range. Friction factors in both laminar and turbulent flow were apparently established in a very short distance from the entrance to the rod bundle, since no entrance effect was detected, even as close as 9.6 total equivalent diameters from the entrance. Local friction factors were also measured in the entrance and fully developed region of a round smooth tube with a sharp-edged entrance. The Reynolds number range covered was 394 to 77,000. The inside tube diameter, determined by forcing the fully developed laminar flow friction factors to fit Poiseuille's equation, was in excellent agreement with the measured diameter, and the fully developed turbulent flow friction factor results, based on the computed diameter, were in good agreement with Nikuradse's equation for a smooth round tube. The Newtonian behavior of the polyethylene glycol solutions over all rates of shear encountered in the present study was thus established. Good agreement was also obtained between the local laminar flow friction factors, measured in the entrance region of the tube, and those predicted theoretically by Langhaar for a rounded entrance. In turbulent flow, the friction factor became fully developed within 50 tube diameters of the entrance. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

This thesis describes the investigation of hydrodynamic properties of pure superfluid flow in liquid helium II by observing the motion of suspended particles in a special experimental arrangement called 'Superfluid Wind Tunnel'. The flow properties of pure superfluid in different velocity regions have been investigated by using particles made with a suitable mixture of hydrogen and deuterium gases as indicators. Two critical velocities, Vp,c and Vs,c, corresponding to 0c and 0t of the oscillating sphere experiments (Benson and Hallett (1956)) have been found. Below Vp,c, the superfluid flow is a perfect potential flow of zero viscosity. Above Vp,c quantized vortex lines are created, therefore the pure superfluid flow breaks down. On the other hand in the vicinity of Ys,c, the starting point of fully developed turbulence, the magnitude of turbulent fluctuations has a maximum which confirms Feynman's prediction (1955) about critical velocity. A rough calculation shows that the velocities of a particle, which obtains energy from a segment of quantized vortex line, are of the same order as that of experimental values. This suggests that by this way, other than Vinen's (1961) vibrating wire experiment, the quantization of superfluid circulation in units of h/m might be verified by visual observations. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Friction factors have been determined experimentally for flow of water and various aqueous solutions of polyethylene glycol in four different concentric annuli. The annuli studied covered a diameter ratio range of 0.0406 to 0.6331. The annular entrance was sharp-edged and no spacers were used within the system. The Reynolds number range investigated was approximately 200 to 26,000, based on equivalent diameter equal to four times the hydraulic radius. The fully developed friction factors for all four diameter ratios were correlated by the Nikuradse equation for a smooth pipe when the Reynolds number exceeded 3500 and by the Knudsen and Kata theory for laminar flow when the Reynolds number was less than 2200. Deviations from the theory of Hanks for laminar - turbulent transition in the well-developed flow region of concentric annuli could be tentatively accounted for by the extra turbulence arising from the sharp-edged entrance in the present investigation. For well-developed flow the critical Reynolds numbers for the various diameter ratios ranged between 2650 and 3700, and the mode of transition was sharp. However, for low, intermediate and high values of entrance length, respectively, three different modes of transition were found to exist in the annuli studied. Local friction factors were based on pressure gradient, uncorrected for changes in kinetic energy due to the developing velocity profile. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Nanofluids are a novel class of heat transfer fluids in which nanoparticles are dispersed in traditional heat transfer fluids. They offer enhanced thermophysical, rheological and radiative properties. These enhancements have resulted in recent research being centred on the application of nanofluids to various systems. An example of such systems is the solar volumetric flow receiver in which great efficiency improvements have been reported. To explain this efficiency increase, researchers have evaluated the impact of enhanced radiative properties of nanofluids while largely neglecting that of enhanced thermophysical properties. This study looks at the impact of enhanced thermophysical properties on the performance of nanofluid-based solar volumetric receivers. Particular focus is drawn to the impact of temperature dependent conductivity and volumetric specific heat capacity. Copper oxide - water nanofluid is employed as its temperature dependent properties have been characterised. [Please note: this thesis file has been deferred until June 2016]

A comprehensive eigen-mode analysis of an intermediate coupled model linearized with respect to arrays of basic states is performed to study the regimes of leading ocean-atmosphere coupled modes of relevance to the El Niño Southern Oscillation phenomenon. Different kinds of leading modes are found to coexist and to become unstable under wide ranges of basic states and parameter conditions. In particular, two main kinds of modes have periods around 4 years and 2 years. They are thus referred as to quasi-quadrennial (QQ), quasi-biennial (QB) modes, respectively. The positive coupled feedback destabilizes and quantizes the near-continuous spectrum for the low-frequency modes of the upper ocean dynamics giving rise to these leading modes with distinct periodicities. The QQ mode can be understood to a large extent by the mechanisms elucidated in the simple conceptual recharge oscillator which relays on slow oceanic dynamic adjustment of equatorial heat content, whereas anomalous advection of sea surface temperature by equatorial zonal current anomalies plays an important role in the QB mode. One of the findings of this study is that the QQ and QB mode may coalesce under realistic conditions through a codimension-2 degeneracy in the parameter space. The coexistence or multiplicity of ENSO-related coupled modes under present climate conditions may provide a plausible explanation for the observed dominating QQ and QB variability of rich ENSO behaviors. / A Dissertation Submitted to the Geophysical Fluid Dynamics Institute in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy. / Summer Semester, 2006. / April 17, 2006. / Advective Feedback, Thermocline Feedback, Heat Budget Analysis, Stability Analysis, Unstable Coupled Modes, Coexistence / Includes bibliographical references. / Fei-Fei Jin, Professor Directing Dissertation; Guosheng Liu, Outside Committee Member; Xiaolei Zou, Committee Member; Ming Cai, Committee Member; Allan Clarke, Committee Member; James J. O'Brien, Committee Member.

No description available.

The purpose of this research was to improve the smoothing operation in smoothed particle hydrodynamics, SPH, when the flow of matter is not smooth. Our main focuses are on the kernel selection, identifying the discontinuities in the sequences to be smoothed, and use of the Laplacian as opposed to artificial viscosity for improved physical accuracy. The results show that alternative kernels result in differences in how matter flows. These effects are explained by the kernels' gradient and Laplacian properties. Five alternative kernels were included in our analysis and our SPH-based simulation cases. Further, the sequences to be smoothed by the kernel function were found to contain numerous discontinuities. As it is well known from multiple areas of science, such discontinuities lead to degraded accuracy if the smoothing is performed without taking discontinuities into consideration. Several methods are introduced to detect discontinuities and perform smoothing by individually and independently smoothing the segments between discontinuities. We analyzed results from sloshing tank SPH simulations and found such segmentwise smoothing impacts the flow. Discontinuities were identified by first-generation wavelets. We found that in about 24 to 27 percent of the fluid particles have sequences containing discontinuities, independent of time step. A second-generation wavelet analysis showed coherent vorticity structure in the flow, and the fluid particles with discontinuous sequences combined with coherent vorticity were the focus of our quantification of effects on particle movement. The research work presented here serves as a tool for further improvement of the SPH method, and is substantiated by the results obtained herein.

The movement of fluids has a significant impact on the biological world, from the transport of critical medications, to the shaping of cellular life. The presence of a fluid-fluid interface gives rise to regions where a fluid---and its contents---can be selectively transported or trapped, and where stresses from the rearranging interface can lead to damage or even death of nearby microorganisms. First, we examine the role of local displacement on network level transport. Multiphase fluid flow through small length-scale networks---such as porous rock or tumor vasculature---can be described by examining local interactions of two adjacent channels (pores) using a pore doublet model. However, the traditional pore doublet model does not take into account the region at the interface of the two fluids, and thus the applicability of this model for low aspect ratio pores is unknown. Here we show using computational fluid dynamics (CFD) that traditional pore doublet models begin to break down for lower channel aspect ratios due to increased energy dissipation in the fluid interface region. We also show that our pore doublet model is able to extend previous models, elucidating network level behavior from a local response. Second, we focus on the generation of highly uniform droplets. When air is blown in a straw near an air-liquid interface, typically one of two behaviors is observed: a dimple in the liquid's surface, or a frenzy of sputtering bubbles, waves, and spray. Here we report and characterize an intermediate oscillatory regime that can create monodisperse aerosols from periodic angled jets. The underlying mechanisms responsible for this highly periodic regime are not well understood. We present experimentally validated scaling arguments to rationalize the fundamental frequencies driving this system, as well as the conditions that bound the periodic regime. This mechanism has the potential to aerosolize microorganisms in the bulk fluid. Third, we look at the role of fluid stresses on nearby biological life. In the biotechnology industry mammalian cells are grown in aerated tanks where locally elevated stresses---created by bubbles rapidly changing shape---can be high enough to kill nearby cells; however the effect of elevated stresses on cells at the timescales of these bubble events is unclear. Here we investigate the effect on cell viability from fluid stresses created by a bubble undergoing topological change, using a combination of CFD, numerical particle tracking, and experimental microfluidics. Using this approach we elicit an overall bubble-induced effect on a cell population's viability. Finally, we examine the role stresses can have on bacterial aerosolization. A key component of the airborne infection pathway is the survival of the pathogen during aerosolization pinch-off processes. Due to a rapidly rearranging interface, pinch-off processes have the potential to generate an enormous amount of hydrodynamic stress in the surrounding fluid. However, the magnitude and duration of the hydrodynamic stresses in these droplets is unknown. We show using numerical simulations the spatial and temporal hydrodynamic stress history of microscale aerosol droplets produced by the central jet of collapsing bubbles. This stress history can then be linked to the stress tolerance of various bacteria allowing for the creation of a new stress-based metric for bacterial survival during aerosolization. / 2021-01-28T00:00:00Z

We present a detailed method for proving shear-free perfect fluid theorems in General Relativity. This method uses the (1+3)-covariant formalism to establish the consistency of the Einstein gravitational field equations under the barotropic shear-free perfect fluid condition. Using a Mathematica package xTensor, we were able to prove the following cases: the case where the pressure is constant, the acceleration vector is parallel to the vorticity, the components of a rescaled acceleration vector field orthogonal to the vorticity are basic and the case where the dot product of the rescaled acceleration vector field and the unit vorticity vector is basic, leading to the existence of a Killing vector along the vorticity