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<p>Microfluidic devices using electroosmotic flows (EOFs) in microchannels have been developed and widely applied in chemistry, biology and medicine. Advantages of using these devices include the reduction of reagent consumption and duration for analysis. Moreover the velocity profile of EOFs, in contrast to the parabolic profile found in pressure-driven flows, has a plug-like profile which contributes significantly less to solute dispersion. It also requires no valve to control the flow, which is done with the appropriate application of electrical potentials, thus becomes one of the favourite techniques for sample separation. However, high potentials of several hundred volts are usually required to generate sufficient EOF. These high potentials are not practical for general usage and could cause electrical hazard in some applications. One of the possible solutions is the introduction of zeta potential modulation. The EOF in a microchannel can be controlled by the zeta potential at the liquid/solid interface upon the application of external gate potentials across the channel walls. Combined with AC EOF, it can rectify the oscillating flows and generate pressure that can be used for microfluidic pumping applications. Since the flow induced by the alternating electric field is unsteady and periodic, it is critical to visualize the flow with high spatial and temporal resolutions in order to understand fluid dynamics. A novel method to obtain high temporal resolution for high frequency periodic electrokinetic flows using phase sampling technique in micro particle image velocimetry (PIV) measurements are first developed in order to characterize the AC electroosmotic flow. After that, the principle of zeta potential modulation is demonstrated to transport particles, cells, and other micro organisms using rectified AC EOF in open microchannels. The rectified flow is obtained by synchronous zeta-potential modulation with the driving potential in the microchannel. Subsequently, we found that PDMS might not be the best material for some pumping and biomedical applications as its hydrophobic surface property makes the priming process more difficult in small microchannels and also causes significant protein adsorption from biological samples. A more hydrophilic and biocompatible material, polyurethane (PU), was chosen to replace PDMS. A polyurethane-based soft-lithography microfabrication including its bonding, interconnect integration and in-situ surface modification was developed providing better biocompatibility and pumping performance. Finally, an electroosmotic pumping device driven by zeta potential modulation and fabricated by PU soft lithography was presented. The problem of channel priming is solved by the capillary force induced by the hydrophilic surface. Its flow rate and pressure output were found to be controllable through several parameters such as driving potential, gate potential, applied frequency, and phase lag between the driving and gate potentials.</p> / Doctor of Philosophy (PhD)

The turbulent flow through a 90o pipe bend is complex with secondary flow that can affect pressure drop and heat/mass transfer. The mean and unsteady flow is studied using refractive index matched two-dimensional two-component (2D2C) Particle Image Velocimetry in a single 90o bend with Rc/D = 1.5 and at Re = 34800. The measurements were performed in a closed loop using a 1-inch diameter test section that was machined out of acrylic. The flow is imaged in the symmetric plane parallel to the axial flow and at different cross sectional planes including 0.25D and 1D upstream, 10o, 20o, 70o, 80o from the bend inlet and 0.25D and 1D downstream of the bend. The axial flow accelerates on the inner wall at the inlet and then moves towards the outer wall at 40o-50o. A shear layer is formed between high velocity fluid near the outer wall and the slower moving fluid at the inner wall side in the second half of the bend. The axial turbulent kinetic energy ((u^2 ) ̅+(v^2 ) ̅) is found to be high in regions corresponding to high velocity gradient regions: (i) at the outer wall near the inlet that extends up to the outlet, (ii) near the inner wall at 40o-50o, and (iii) at the shear layer formed near the inner wall. In the cross sectional planes, two vortices are formed and have a maximum strength at 80o from the bend inlet. The cross sectional turbulent kinetic energy ((v^2 ) ̅+(w^2 ) ̅) is found to be highest on the inner wall at the 80o plane. The snapshot Proper Orthogonal Decomposition (POD) technique is used to study the unsteady flow structures within the flow. There are long and short flow structures in the upstream pipe which can be related to Very Large Scale and Large Scale Motions. The secondary flow at 20o and further downstream cross sectional planes show evidence of unsteadiness as two vortices oscillate about the symmetry axis with low frequencies of St ~ 0.07, 0.13 and higher frequency at St ~ 0.3-0.6. The low frequency oscillations can be related to Very Large Scale Motions while high frequency oscillations are related to separation of the flow on the inner wall side. Evidence of swirl switching in the high frequency range (St ~ 0.3-0.5) is found at cross sectional plane 1D downstream. / Thesis / Master of Applied Science (MASc)

Yes / This paper provides results of a comprehensive investigation into the use of waste carpet fibres for reinforcement of clay soil slopes. The interaction between laboratory scale model slopes made of fibre reinforced clay soil and surface strip footing load was examined. Results for the influence of two variables namely fibre content and distance between the footing edge and the crest of the slope are presented and discussed. Particle Image Velocimetry (PIV) technique was employed to study the deformation of the slope under the surface loading. The front side of the tank was made of a thick Perspex glass to facilitate taking accurate images during the loading stage. To study the stress induced in the slope under footing pressure, excess pore-water pressure and total stress increase were measured at predetermined locations within the slope. The results showed that fibre reinforcement increased the bearing resistance of the model slope significantly. For instance, inclusion of 5% waste carpet fibre increased the bearing pressure by 145% at 10% settlement ratio. / The post-print of this article will be released for public view when the version of record has been published by ASCE.

The mechanism by which convective heat transfer is augmented by freestream turbulence in the stagnation region was studied experimentally. Previous work has suggested that the primary mechanism for the observed augmentation is the amplification of vorticity into strong vortices which dominate the flow field near the surface. Therefore, two separate experimental investigations were performed to further study this phenomenon. In the first, the spatiotemporal convection from a heated surface was measured during the normal collision of a vortex ring. The convection was observed to increase dramatically in areas where vortices forced outer fluid through the natural convection boundary layer to the surface. Regions where fluid was swept along the surface experienced much smaller increases in convection. These observations led to the development of a mechanistic model which predicted the heat transfer based on the amount of time that fluid remained within the thermal boundary layer prior to reaching the surface. In subsequent testing, the model was able to accurately predict the time-resolved convection based solely on the transient properties of the vortex present. In the second investigation, the model was applied to the vortices which form in a stagnating turbulent flow. Three turbulence conditions were tested which changed the properties of the vortices produced. Again, the model was successful in predicting the time-resolved convection over much of the experimental measurement time. The work of designing and calibrating the heat flux sensor used is also reported. A new sensor was developed specifically for the convection research performed herein as no existing sensor possessed the required spatiotemporal resolution and underwater capabilities. Utilizing spot-welded foils of thermoelectric alloys resulted in a very robust and sensitive sensing array which was thoroughly analyzed and calibrated. In the final section, the hybrid heat flux (HHF) method is presented which significantly increases the performance of existing heat flux sensors. It is shown (both numerically and experimentally) that by combining the spatial and temporal temperature measurements from a standard sensor, the time response increases by up to a factor of 28. Also, this method causes the sensor to be insensitive to the material to which it is mounted. / Ph. D.

The mechanism of heat transfer augmentation due to freestream turbulence in classic Hiemenz stagnation flow was studied experimentally for the first time using time-resolved digital particle image velocimetry (TRDPIV) and a new thin film heat flux sensor called the Heat Flux Array (HFA). Unique measurements of simultaneous, time-resolved velocity and surface heat flux data were obtained along the stagnation line on a simple, rectangular flat plate model mounted in a water tunnel facility. Identification and tracking of coherent structures in the stagnation region lends support to the theory that coherent structures experience stretching and amplification of vorticity by the mean flow strain rate upon approaching the stagnation surface. The resulting flow field in the near-wall region is comprised primarily of high strength, counter-rotating vortex pairs with decreased integral length scale relative to the imposed freestream turbulence. It is hypothesized that the primary mechanism of heat transfer augmentation is the movement of cooler freestream fluid into the heated near-wall region by these coherent structures. Furthermore, the level of heat transfer augmentation is dictated by the integral length scale, circulation strength, and core-to-surface distance of the coherent structures. To test this hypothesis, these properties were incorporated into a mechanistic model for predicting the transient, turbulent heat transfer coefficient. The model was successful in predicting the shape and magnitude of the measured heat transfer coefficient over much of the experimental measurement time. In a separate yet related set of studies, heat flux sensors and calibration methods were examined. The High Temperature Heat Flux Sensor (HTHFS) was designed and developed to become one of the most durable heat flux sensors ever devised for long duration use in high temperature, extreme environments. Extensive calibrations in both conduction and convection were performed to validate the performance of the sensor near room temperature. The measured sensitivities in conduction and convection were both very close to the predicted sensitivity using a thermal resistance model of the HTHFS. The sensor performance was unaffected by repeated thermal cycling using kiln and torch firing. Finally, the performance of Schmidt-Boelter heat flux sensors were examined in both shear and stagnation flow using two custom designed convection calibration facilities. Calibration results were evaluated using an analytical sensitivity model based on an overall sensor thermal resistance from the sensor to the heat sink or mounting surface. In the case of convection the model included a term for surface temperature differences along the boundary layer. In stagnation flow the apparent sensitivity of the Schmidt-Boelter sensors decreased non-linearly with increasing heat transfer coefficient. Estimations of the sensor's internal thermal resistance were obtained by fitting the model to the stagnation calibration data. This resistance was then used with the model to evaluate the effects of non-uniform surface temperature on the shear flow sensitivity. A more pronounced non-linear sensitivity dependence on heat transfer coefficient was observed. In both cases the main result is that convection sensitivity varies a great deal from standard radiation calibrations. / Ph. D.

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.

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.