The primary aim of this thesis is to investigate the transition between different flow regimes in porous media. The complete transition spectrum of single-phase flow, from creeping flow to inertial, unsteady laminar, and turbulent flow regimes, was examined in sphere packings. Further understanding of the fundamental fluid dynamics was derived based on the pore-scale flow visualisation using magnetic resonance velocimetry (MRV). Spiral imaging was selected as the ultrafast imaging protocol to probe the transient phenomena, and the acquisition was further accelerated by combining subsampling and compressed sensing reconstruction. In a random sphere packing column with column-to-diameter ratio of 3.44, the inertial effect and the onset of unsteady regime were examined with respect to the principal flow characteristics: the inertial core/channeling, backflow, and helical vortices. Helical vortices have been observed experimentally in a random packing for the first time, and the analogy between the swirling flow and helical vortices provides insight into the design and operation of packed bed reactors. Another new observation is that the transition to the unsteady regime is a highly heterogeneous process, where the evolution of the flow instability depends on the pore geometry. Moreover, pixelwise validation was achieved between the experimental and simulation results on three-dimensional velocity fields in the inertial regime; this is enabled by an image-based meshing pipeline, which reproduces the geometry of the random packing in MRV for the numerical simulation. The unsteady regimes were further investigated using a regular sphere packing, the simple cubic packing (SCP). The spectral analysis, in both the random and regular packing, revealed a route to chaos from the steady to periodic, quasi-periodic, and chaotic dynamics, which was only predicted numerically before. During the transition to turbulence, the coherent structures were extracted using proper orthogonal decomposition (POD), which yields a coherent picture regarding the turbulent dynamics, when combined with the skewness, flatness, and quadrant analysis. Furthermore, it was found that the macroscopic properties converged at lower Reynolds number than the microscopic features. In conclusion, the opportunity to measure flow fields at high spatial and temporal resolution will play an increasingly significant role in the advancement of fundamental fluid dynamics. In this thesis, MRV is used, which is particularly advantageous for non-invasive measurements in opaque systems. This thesis provides the experimental and analysis toolkits for such studies and has demonstrated the contribution to characterising and understanding different flow regimes in porous media.

Thesis (MTech (Chemical Engineering))--Cape Technikon, 2004 / When designing the open channels to transport the homogenous non-Newtonian slurries, the effect of channel shape is one of the parameters that should be checked and very little research has been conducted to address this matter. Open channels are commonly applied in the mining industry where mine tailings have to be transported to the disposal dams at high concentrations to save water consumption. This thesis addresses the effect of the cross-sectional shape of the channel with emphasis on laminar and transitional flow of non-Newtonian fluids. The literature review on the flow of Newtonian and non-Newtonian fluids has been presented. The most relevant one to this topic is the work done by Straub et al (1958) for Newtonian fluids and the analytical work presented by Kozicki and Tiu (1967) for non-Newtonian fluids. Authors like Coussot (1994) and Haldenwang (2003) referred to their work but did not comprehensively verified it experimentally. Three flume shapes were designed to investigate this problem namely, rectangular, semi circular, and trapezoidal flume shape. The test rig consisted of a 10 m long by 300mm wide tilting flume that can be partitioned into two sections to form a 150 mm wide channel. All three flume shapes were tested in both the 150 mm and 300 mm wide flumes. This flume is linked to the in-line tube viscometer with three tube diameters namely, 13 mm; 28 mm; and 80 mm. The experimental investigation covered a wide range of flow rates (0.1-45l/s), and flume slopes (1-5 degrees). The fluids tested were kaolin suspension (5.4 - 9% v/v), CMC solution (1 - 4% m/m), and bentonite suspension (4.6 and 6.2% mlm). The models found in the literature were evaluated with the large database compiled from the test results to predict the laminar and transitional flow of these fluids with the aim of checking the effect of the cross-sectional shape of these channels selected in these flow regimes. For all the flume shapes and non-Newtonian fluids selected in this thesis it was found that in predicting the laminar flow, the effect of shape is adequately accounted for by the use of hydraulic radius. In predicting the transitional flow, it was found that the effect of shape does not have to be included.

A dynamic hybrid Reynolds-averaged Navier-Stokes (RANS)-Large Eddy Simulation (LES) modeling framework has been investigated and further developed to improve the Computational Fluid Dynamics (CFD) prediction of turbulent flow features along with laminar-to-turbulent transitional phenomena. In recent years, the use of hybrid RANS/LES (HRL) models has become more common in CFD simulations, since HRL models offer more accuracy than RANS in regions of flow separation at a reduced cost relative to LES in attached boundary layers. The first part of this research includes evaluation and validation of a dynamic HRL (DHRL) model that aims to address issues regarding the RANS-to-LES zonal transition and explicit grid dependence, both of which are inherent to most current HRL models. Simulations of two test cases—flow over a backward facing step and flow over a wing with leading-edge ice accretion—were performed to assess the potential of the DHRL model for predicting turbulent features involved in mainly unsteady separated flow. The DHRL simulation results are compared with experimental data, along with the computational results for other HRL and RANS models. In summary, these comparisons demonstrate that the DHRL framework does address many of the weaknesses inherent in most current HRL models. Although HRL models are widely used in turbulent flow simulations, they have limitations for transitional flow predictions. Most HRL models include a fully turbulent RANS component for attached boundary layer regions. The small number of HRL models that do include transition-sensitive RANS models have issues related to the RANS model itself and to the zonal transition between RANS and LES. In order to address those issues, a new transition-sensitive HRL modeling methodology has been developed that includes the DHRL methodology and a physics-based transition-sensitive RANS model. The feasibility of the transition-sensitive dynamic HRL (TDHRL) model has been investigated by performing numerical simulations of the flows over a circular cylinder and a PAK-B airfoil. Comparisons with experimental data along with computational results from other HRL and RANS models illustrate the potential of TDHRL model for accurately capturing the physics of complex transitional flow phenomena.

As a result of recent advances in micro and nanotechnology, the tiny movements of nanoscale active and passive objects in fluids can be probed with ultrahigh sensitivity and time resolution. The overarching theme of this dissertation is to harness these movements in fluids in order to study fundamental fluid dynamics and develop novel biomedical devices. First, we use the oscillatory movements of nanocantilevers to investigate the scaling behavior of unsteady fluid flow. Here, our expansive experimental data and rigorous theoretical analysis suggest that a generalized scaling parameter combining the length and time scales of the flow governs the scaling. Second, we turn our attention to nanoscale movements of bacteria in a buffer. We develop a simple, robust and sensitive experimental method to detect and track the random movements of bacteria. Using this method, we show evidence that these random movements of bacteria correlate with their antibiotic susceptibility. In the first part of this thesis, we explore, through experimental and theoretical work, the breakdown of the Navier-Stokes equations in oscillatory fluid flows. The Navier-Stokes equations of hydrodynamics are based on two crucial assumptions. First, the fluid is approximated as a continuum, with a well-defined ``fluid particle." Second, the stress in the fluid is assumed to be a linear function of the rate-of-strain, resulting in a so-called Newtonian fluid. If a fluid such as an ideal gas is gradually rarefied, the Navier-Stokes equations begin to fail and a kinetic description of the flow becomes appropriate. The failure of the Navier-Stokes equations can be thought to take place via two different physical mechanisms: either the continuum hypothesis breaks down as a result of a finite size effect; or the local equilibrium is violated due to the high rate of strain. Our experimental approach is to create an unsteady flow by oscillating a finite-sized body in a gas and to measure the dissipation (or the drag force) acting on the body. By using micro and nanofabrication techniques, we independently tune the relevant linear dimensions and the frequencies of the oscillating bodies. We then measure the pressure-dependent dissipation of these micro/nano oscillators in three different gases, Helium, Nitrogen, and Argon. We observe that the scaling of the fluidic dissipation is governed by a subtle interplay between the length scale and the frequency, embodied respectively in the dimensionless Knudsen (Kn) and the Weissenberg ( Wi) numbers. We collapse all the experimental data using a single scaling parameter: Wi + Kn. This new dimensionless parameter, which can be regarded as a generalized Knudsen number, combines the relevant linear dimension and the frequency of the body; it is rooted in Galilean invariance and can be obtained rigorously from the Chapman-Enskog expansion of the Boltzmann equation. In the second part of the thesis, we turn to the movements of active micro-swimmers in a buffer. This portion of the work is motivated by a serious global public health problem: the rise of multi-drug resistant bacteria. One way to prevent this threat from growing is to treat bacterial infections with effective antibiotics using the minimum dosage. However, currently-used antibiotic susceptibility tests (ASTs), which determine whether or not bacterial isolates from a patient are susceptible to administered antibiotics, take too long. Here, we aim to develop a robust and rapid AST by exploiting a recently-observed microbiological phenomenon: various nanomechanical movements of bacteria subside promptly (within minutes) when the bacteria are exposed to an effective antibiotic. Our approach is to transduce bacterial movements into electrical voltage fluctuations in a microchannel filled with a buffer solution. When a small but constant current is driven through the microchannel, bacterial movements are converted into strong voltage fluctuations due to the fact that they modulate the effective microchannel diameter. Our experiments with E. coli show that the proposed detection method can provide antibiotic susceptibility results in ~1 hour, making it a promising rapid AST. Because this approach is based on a simple electrical measurement and does not require delicate process steps and instrumentation, it may eventually be used at the point of care. / 2019-03-09T00:00:00Z

A eficiência de motores a combustão está diretamente relacionada a mistura dos reagentes. O que é muito desejado em todos os sistemas, inclusive em sistemas aeroespaciais onde a combustão ocorre de forma contínua como nos motores a jato, motores de foguete, ramjet e scramjet. No caso do scramjet a combustão ocorre em regime supersônico e conhecer como os dois fluidos se misturam na câmera de combustão é muito importante pois o tempo de residência na câmera é muito reduzido, aumentando a importância de uma mistura homogênea para a eficiência da combustão. Em determinados casos pequenas pertubações em um jato vão se amplificar podendo ocasionar a transição do escoamento laminar para turbulento. O que é desejado visto que escoamentos turbulentos são caracterizados pela grande capacidade de mistura. No presente trabalho estudamos a estabilidade de jatos coaxiais composto por dois fluídos com pequenas pertubações na base para escoamentos supersônicos para casos bidimensionais e tridimensionais. O escoamento foi investigado utilizando simulação numérica e a teoria de estabilidade linear, os resultados de ambos os métodos foram comparados, casos com pertubações bidimencionais e tridimencionais foram analizados. As simulações numéricas foram realizadas utilizando diferenças finitas de alta ordem de precisão para a discretização espacial. A integração temporal foi feita utilizando o método de Runge-Kutta de quarta ordem. Os resultados de ambos os métodos mostraram uma boa concordância. / The efficiency of combustion engines are strict related to the mixing between reagents. That is very desire in every aerospace propulsion system, when the combustion is continuous such as rocket engines, ramjets and scramjets. The combustion in scramjet occurs in supersonic speed ith a very small resilience time, know how two fluids are mixed in this case is very important because are direct related to the efficiency of the combustion. In such cases small disturbances in a jet flow can be amplified until occur the transition from a laminar flow to a turbulent flow, that is desired because the great capacity of mixing of turbulent flows. In this work are studied the jet flow stability when the jet are composed by two fluids with small disturbances in the base for supersonic flows in bidimensional and tridimensional cases. The numerical results are obtained by numerical simulation and linear stability theory (LST). High order finite difference schemes are adopted for spatial derivatives. The integration in time are caried out by a fouth order Runge-Kutta scheme. The results obtained by numerical simulation and linear stability theory show good agreement.

No description available.

Several applications, including those in the energy sector that require high thermal efficiency, such as those in the solar energy industry, require a careful thermal analysis of heat exchange components. In this regard, thermal resistance is a major cause of exergy destruction and must be minimised as much as possible, but also adequately designed. In the past, a number of correlations have been developed to predict heat transfer coefficients in compact heat exchangers. The designers of such heat exchangers often exploit the development of thermal boundary layers to achieve higher overall efficiency due to increases in local heat transfer coefficients. However, most of the correlations that have been developed for heat exchangers neglect the specific effect of the thermal boundary layer development in the inlet region, and instead only offer effective average heat transfer coefficients, which most users assume to be constant throughout the heat exchanger. This is often an over-simplification and leads to over-designed heat exchangers. In this study, focus is placed on annular flow passages with uniform heating on the inner wall. This geometry has many applications. This study aims to collect experimental heat transfer data for water at various flow rates and inlet geometries, to process the data and determine local and overall heat transfer coefficients, and to develop an improved local heat transfer coefficient correlation. Experimental tests were performed on a horizontal concentric tube-in-tube heat exchanger with a length of 1.05 m and a diameter ratio of 0.648. The surface of the inner tube was treated with thermochromic liquid crystals (TLCs), which allowed for high-resolution temperature mapping of the heated surface when combined with an automated camera position system in order to determine local heat transfer coefficients. Conventional in-line and out-of-line annular inlet configurations were evaluated for Reynolds numbers from 2 000 to 7 500, as well as the transition from laminar to turbulent flow for a single in-line inlet configuration. It was found that the local heat transfer coefficients were significantly higher at the inlets, and decreased as the boundary layers developed. With the high resolution of the results, the local heat transfer coefficients were investigated in detail. Local maximum and minimum heat transfer coefficients were identified where the thermal boundary layers merged for high turbulent flow cases. The annular inlet geometries only influenced the heat transfer for Reynolds numbers larger than 4 000, for which larger inlets are favoured. Out-of-line inlet geometries are not favoured for heat transfer. A new heat transfer correlation was developed from the experimental data, based on an existing heat transfer correlation for turbulent flow in an annular flow passage, considering the boundary layer development. The new correlation estimated the area-weighted heat transfer coefficients within 10% of the experimental data and closely followed trends for local heat transfer coefficients. / Dissertation (MEng)--University of Pretoria, 2017. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

No description available.

No description available.

The present work seeks to fully investigate, describe and characterize the distinct flow regimes existing within a mixing vessel at various rotational speeds. This investigation is computational in nature and simulates the flow within a baffled tank containing a Rushton turbine of the standard configuration. For a Re based on impeller diameter and blade rotational speed (Re â ¡ Ï ND2/μ) the following flow regimes were identified and investigated in detail: Reverse/reciprocating flows at very low Re (<10); stalled flows at low Re (â 10); laminar pumping flow for higher Re and transitional pumping flow (10 squared < Re <10 to the 4th). For the three Re numbers 1, 10 and 28, it was found that for the higher Re number (28), the flow exhibited the familiar outward pumping action associated with radial impellers under turbulent flow conditions. However, as the Re number decreases, the net radial flow during one impeller revolution was reduced and for the lowest Re number a reciprocating motion with negligible net pumping was observed. In order to elucidate the physical mechanism responsible for the observed flow pattern at low Re, the forces acting on a fluid element in the radial direction were analyzed. Based on this analysis, a simplified quasi-analytic model of the flow was developed that gives a satisfactory qualitative, as well as quantitative representation of the flow at very low Re. Investigation of the transitional flow regime (Re â 3000) includes a compilation and characterization of ensemble and turbulent quantities such as the Reynolds stress components, dissipation length η and time scales Ï , as well a detailed investigation of the near-impeller flow and trailing vortex. Calculation and compilation of all terms in the turbulent kinetic energy transport equation was performed (including generation and the illusive turbulent pressure work). Specifically, the most important transport mechanism was turbulent convection/diffusion from the impeller disk-plane/trailing vortex region. Mean flow transport of turbulent kinetic energy was primarily towards the impeller disk-plane and radially outward from the trailing vortex region. The turbulent pressure work was found to partially counteract turbulent convection. Turbulent dissipation followed by turbulent viscous work were found to be the least important mechanism responsible for turbulent transport with both terms being maximized within the vortex region and at the disk-plane down-stream from the vortices. / Ph. D.