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The determination of the leakage and consequently the friction factor is an important part of analyzing the flow through a seal. This is done experimentally by means of a flat plate tester, which allows for the simplified representation of the seal pattern on a flat plate surface tested under a range of clearances and pressure drops. The setup mounts a smooth plate opposite a second plate which may be smooth or have a roughened surface while the separation between plates is held constant. The present study analyzes the phenomenon of friction factor 'upset' ? wherein it was seen that as the pressure drop across the parallel plates is increased, there is a sudden increase in the friction factor (i.e. a decrease in flow rate) at a certain Reynolds number and for any further increase in the pressure differential, the friction factor shows the expected trend and decreases slowly. This phenomenon was initially believed to be an anomaly in the rig and was attributed to choking at an upstream flow control valve. The present author differs from that view and hypothesized that the reason for the abrupt change is linked to the flow mechanics of the system and the current study analyzes the same. Preliminary analysis of available data has established that the cause for the 'upset' was not related to the switch from a normal mode resonance driven by the Helmholtz frequency of the cavities on the stator to a shear layer instability, as was seen earlier by Ha. The friction factor jump for this case is therefore proposed to be due to a change of the instability modes as the fluid passes over the cavities in the plate. A detailed analysis of the physics of the flow will be carried out via a numerical simulation using a Large Eddy Simulation (LES) model from ANSYS Fluent. Results will be validated through comparisons with experimental data from the flat plate test rig.

Yes / This experimental study of forward gravure coating considers the effects of operating variables on air entrainment, ribbing instabilities and the thickness of the film formed. The data show that this coating method can yield very thin films of thickness of order of 15 - 20% at most of the equivalent cell depth of a gravure roller. Air free and non ribbed stable uniform films can however only be obtained in a narrow window of operating conditions at very low substrate capillary number (CaS ~ 0.02) equivalent to substrate speeds typically less than 20m/min. The paper draws a similarity with flow features observed with smooth forward roll coating and slide coating. It is shown that the onset of ribbing and the flux distribution between the gravure roller and the substrate at the exit of the nip obey approximately the same rules as in smooth forward roll coating, whereas the onset of air entrainment actually corresponds to a low-flow limit of coatability similar to that observed in slide coating.

Crashback operation of a propeller is a common emergency slowing maneuver for ships and submarines. The reversing of the propeller while the vessel is moving forward results in large loads on the propeller blades and highly detached flow, which presents both practical concerns and fundamental fluid physics inquiries. This thesis contains a comprehensive numerical analysis of two propellers in crashback operation. Available numerical and experimental data for David Taylor Model Basin (DTMB) 4381 propeller are used for validation of the computational fluid dynamics solver used, REX. A second propeller, Maritime Research Institute Netherlands (MARIN) 7371R is used to classify the common crashback flow behavior into regimes. Four regimes were identified, each existing for a range of operating conditions. The most prominent and deciding feature of the flow regimes is the presence of a ring vortex, resulting from the opposing action of the free-stream flow and the propeller induced flow. The position, shape and strength changes between regimes, dominating the dynamics of the flow by altering the induced flow into the propeller disk. Flow conditions resulting from regime transitions are described. Changes in the ring vortex structure lead to two stable flow conditions of interest. One condition produces a reduction of thrust despite the increase in flow speed into the propeller and negligible side-forces. The other condition creates large side-forces capable of rotating a vessel, resulting from an asymmetry forming in the ring vortex. Additionally, massive flow separation occurs at high free-stream speeds that cause extreme blade loading. An extensive description of each flow regime is provided, with further investigation and discussion of the flow regimes that present more practical concerns and novel characteristics of the crashback flow.

<p>Fluids conveyed in deformable conduits are often encountered in  microfluidic applications, which makes fluid--structure interactions (FSIs) an unavoidable phenomenon. In particular, experiments reported the existence of FSI instabilities in compliant microchannels at low Reynolds numbers, Re, well below the established values for rigid conduits. This observation has significant implications for new strategies for mixing at the microscale, which might harness FSI instabilities in the absence of  turbulence. In this thesis, we conduct research on the modeling and stability of microscale FSIs. Understanding the steady response, the dynamics and the stability of these FSIs are the three major objectives. This thesis begins with the analysis of the steady-state scalings and the linear stability of a previously derived mathematical model, through which we emphasize the power of reduced modeling in making the FSI problems tractable. Next, we turn to a more realistic problem regarding FSIs in a common configuration of low-Re flows through long, shallow rectangular three-dimensional microchannels. Through a scaling analysis, which takes advantage of the geometric separation of scales, we find that the flow can be simplified under the lubrication approximation, while the wall deforms like a variable-stiffness Winkler foundation at the leading order. Coupling these dominant effects, we obtain a new fitting-parameter-free flow rate--pressure drop relation for a thick-walled microchannel, which rationalizes previous experiments. Then, we derive a one-dimensional (1D) steady model, at both vanishing and finite Re, by coupling the reduced flow and deformation models. To satisfy the displacement constraints along the channel edges, weak tension is introduced to regularize the underlying Winkler-foundation-like mechanism. This model is then made dynamic by introducing flow unsteadiness and the elastic wall's inertia. We conduct a global stability analysis of this system by perturbing the non-flat steady state with infinitesimal perturbations. We identify the existence of globally unstable modes, typically in the weakly inertial flow regime, whose features are consistent with experimental observations. The unstable eigenmodes oscillate at frequencies close to the natural frequency of the wall, suggesting that the instabilities are resonance phenomena. We also capture the transient energy amplification of perturbations through a linear non-normality analysis of the proposed reduced 1D FSI model.</p>

Current and future legislation demands ever decreasing levels of pollution from gas turbine engines, and with combustor performance playing a critical role in resultant emissions, a need exists to develop a greater appreciation of the fundamental causes of unsteadiness. Particle Image Velocimetry (PIV) provides a platform to enable such investigations. This thesis presents the development of PIV measurement methodologies for highly turbulent flows. An appraisal of these techniques applied to gas turbine combustors is then given, finally allowing a description of the increased understanding of the underlying fluid dynamic processes within combustors to be provided. Through the development of best practice optimisation procedures and correction techniques for the effects of sub-grid filtering, high quality PN data has been obtained. Time average statistical data at high spatial resolution has been collected and presented for generic and actual combustor geometry providing detailed validation of the turbulence correction methods developed, validation data for computational studies, and increased understanding of flow mechanisms. These data include information not previously available such as turbulent length scales. Methodologies developed for the analysis of instantaneous PIV data have also allowed the identification of transient flow structures not seen previously because they are invisible in the time average. Application of a new `PDF conditioning' technique has aided the explanation of calculated correlation functions: for example, bimodal primary zone recirculation behaviour and jet misalignments were explained using these techniques. Decomposition of the velocity fields has also identified structures present such as jet shear layer vortices, and through-port swirling motion. All of these phenomena are potentially degrading to combustor performance and may result in flame instability, incomplete combustion, increased noise and increased emissions.

Low-emissions combustor design is crucially important to gas turbine engine manufacturers. Unfortunately, many designs are susceptible to unsteady oscillations that can result in structural fatigue and increased noise. Computational approaches that resolve flow unsteadiness, for example Large Eddy Simulation (LES), are being explored as one avenue to help understand such phenomena. However, in order to quantifY the accuracy of LES predictions, benchmark validation data in suitably chosen test cases are required. Comprehensive experimental data covering both time-averaged and timeresolved features are currently scarce. It was the aim of this thesis, therefore, to provide such data .in a configuration representing the near-field of a typical gas turbine fuel injector. It was decided to focus on the fuel injector since many unsteady events are believed to originate because of the transient interactions between the fuel injector flow and the main combustor flow. A radial fed two-stream fuel injector, based on a preexisting industrial gas-turbine Turbomeca design was used, since this geometry was known to be susceptible to unsteadiness. The fuel injector was investigated under isothermal conditions to place emphasis on the fluid mechanical behaviour of the fuel injector, including detailed capture of any unsteady phenomena present. Light Sheet Imaging (LSI) systems were used as the primary experimental technique to provide high quality spatially and temporally resolved instantaneous velocity and scalar field information in 2D planes (using ParticieImage Velocimetry (PIV) and Planar LaserInduced Fluorescence (PUF) techniques). Several methods were employed to extract information quantifYing the flow unsteadiness and improve visualisation of timedependent large-scale turbulent structures. Proper Orthogonal Decomposition (POD) analysis enabled clear identification of the dominant modes of energy containing structures. The results indicated that periodic high-energy containing vortex structures occurred in the swirl stream shear layer, emerging from the fuel injector. These formed a two-strong two-weak rotating vortex pattern which propagated down the main duct flow path. The formation of these vortices was found to be a function of the swirl number and originated due to an interaction between the forward moving swirl flow and the furthest upstream penetration point ofthe recirculation zone present in the main duct flow. Dependent on the magnitude of the swirl number (influencing the swirl stream cone angle) and the geometry of the fuel injector, the vortex formation point was sometimes found inside the fuel injector itself. If the vortices originated inside the fuel injector they appeared much more coherent in space and time and of higher energy. A second unsteady high energy containing phenomenon was also identified, namely a Precessing Vortex Core (PVC), which was damped out if the fuel injector contained a central jet. The dynamics of the PVC interacted with the dynamics of the swirl stream shear layer vortices to reduce there strength. Transient scalar measurements indicated that there was a clear connection between the unsteady vortex pattern and the rate of mixing, resulting in bursts of high heat release and is therefore identified as one source of combustor oscillations. Future fuel injector designs need to pay close attention to these unsteady features in selecting swirl number and internal geometry parameters.

A centrifugal compressor is a mechanical machine with purpose to convert kineticenergy from a rotating impeller wheel into the fluid medium by compressingit. One application involves supplying boost air pressure to downsized internalcombustion engines (ICE). This allows, for a given combustion chamber volume,more oxygen to the combustion process, which is key for an elevated energeticefficiency and reducing emissions. However, the centrifugal compressor is limitedat off-design operating conditions by the inception of flow instabilities causingrotating stall and/or surge. These instabilities appear at low flow rates andtypically leads to large vibrations and stress levels. Such instabilities affectthe operating life-time of the machine and are associated with significant noiselevels.The flow in centrifugal compressors is complex due to the presence of a widerange of temporal- and spatial-scales and flow instabilities. The success fromconverting basic technology into a working design depends on understandingthe flow instabilities at off-design operating conditions, which limit significantlythe performance of the compressor. Therefore, the thesis aims to elucidate theunderlying flow mechanisms leading to rotating stall and/or surge by means ofnumerical analysis. Such knowledge may allow improved centrifugal compressordesigns enabling them to operate more silent over a broader operating range.Centrifugal compressors may have complex shapes with a rotating partthat generate turbulent flow separation, shear-layers and wakes. These flowfeatures must be assessed if one wants to understand the interactions among theflow structures at different locations within the compressor. For high fidelityprediction of the complex flow field, the Large Eddy Simulation (LES) approachis employed, which enables capturing relevant flow-driven instabilities underoff-design conditions. The LES solution sensitivity to the grid resolution usedand to the time-step employed has been assessed. Available experimentaldata in terms of compressor performance parameters, time-averaged velocity,pressure data (time-averaged and spectra) were used for validation purposes.LES produces a substantial amount of temporal and spatial flow data. Thisnecessitates efficient post-processing and introduction of statistical averagingin order to extract useful information from the instantaneous chaotic data. Inthe thesis, flow mode decomposition techniques and statistical methods, suchas Fourier spectra analysis, Dynamic Mode Decomposition (DMD), ProperOrthogonal Decomposition (POD) and two-point correlations, respectively, areemployed. These methods allow quantifying large coherent flow structures atvfrequencies of interest. Among the main findings a dominant mode was foundassociated with surge, which is categorized as a filling and emptying processof the system as a whole. The computed LES data suggest that it is causedby substantial periodic oscillation of the impeller blade incidence flow angleleading to complete system flow reversal. The rotating stall flow mode occurringprior to surge and co-existing with it, was also captured. It shows rotating flowfeatures upstream of the impeller as well as in the diffuser. / <p>QC 20171117</p>

<p>Heat dissipation requirements of next-generation power electronics in electric vehicles, high-performance computing, and radar systems will far exceed the capabilities of conventional heat sink technologies such as single-phase liquid cold plates and air-cooled heat sinks. The leading candidate technology that promises to meet these needs is microchannel flow boiling. Compared to conventional heat sink technologies, flow boiling provides some of the highest heat transfer coefficients available and can dissipate heat at a lower pumping power and with more uniform surface temperatures. However, there are unique challenges associated with flow boiling that currently prevent practical implementation of the technology, including limited modeling capabilities, inherent critical heat flux (CHF) limitations, and the presence of two-phase flow instabilities. This thesis is targeted primarily at addressing the impact of dynamic two-phase flow instabilities on heat transfer and CHF in microchannel heat sinks, in contrast with earlier literature that has focused on prediction and characterization of the flow dynamics.</p> <p><br></p> <p>Two dynamic instabilities of importance in microchannel heat sinks are pressure drop oscillations (PDO) and parallel channel instabilities, both resulting from an interaction between the inertia of a two-phase mixture within a heated channel and a source of compressibility outside of the channel. However, the individual impact of these instabilities on heat transfer performance has not been quantified. In this thesis, an experimental facility is developed to isolate the individual and combined impact of PDOs and parallel channel instabilities on surface temperature and CHF in single- and parallel-microchannels. This is achieved by introducing a measurable compressible volume directly upstream of the test section and isolating the test section from any unwanted compressibility within components throughout the rest of the system. Experiments are first performed targeting the investigation of PDOs in single channels and then targeting PDOs and parallel channel instabilities in multi-channel heat sinks. In the case of parallel channels, inlet restrictors are introduced to suppress channel-to-channel interactions and provide a baseline case of stable boiling. Throughout these experiments, only moderate increases in time-average surface temperature are observed (6 °C) and reduction of CHF is negligible, despite drastically different flow pattern observations when instabilities are present. These observations are in stark contrast with other cases in the literature, for which significant deterioration of surface temperatures and CHF have been attributed to the presence of PDOs. For example, significant temperature oscillations have been observed in the literature studying silicon-etched microchannel heat sinks experiencing PDOs. A predictive model is clearly required to understand and detect the conditions when dynamic instabilities should be considered in heat sink design.</p> <p><br></p> <p>To better understand the conditions when PDOs might have significant impact on heat transfer performance, an investigation of thermal capacitance is performed using a dynamic two-phase model and a targeted experimental approach in heat sinks having different thermal masses. The model reveals that, if thermal capacitance is low, PDOs become more severe, and the amplitude of temperature oscillations increase. These predictions are confirmed by experimental observations, and, in addition, premature CHF is observed in the heat sink with lower thermal mass. With sufficient thermal capacitance, the system recovers before triggering CHF, preventing deterioration of performance due to PDOs. Among the wide range of flow conditions considered in this thesis, the reduction of thermal mass resulted in the greatest impact on transient response of a heat sink during flow boiling instabilities. This reveals thermal capacitance as a critical parameter when determining if dynamic instabilities will deteriorate performance in a microchannel heat sink application. This allows engineers to make an informed judgement on whether adding features to suppress instabilities, at the cost of increased pumping power, is warranted. In order for the practical implementation of two-phase heat sinks to be realized, further development of dynamic modeling capabilities is required, and these models should be backed by systematic experimental investigations into conditions where instabilities should be considered.</p>