This document addresses problems encountered when establishing the very low air-flow speeds required for experimental investigations of the mechanisms of low-Reynolds-number boundary-layer turbulence. Small-scale motions in the near-wall region are important features of turbulent boundary-layer dynamics, and, if these features are to be resolved by measurements in air with conventionally-sized hot-wire probes, a well-behaved canonical turbulent boundary layer must be developed at free stream flow speeds no higher than 4 m/s. However, at such low speeds, the turbulent boundary layers developed on the walls of a wind tunnel are very susceptible to perturbation by non-turbulent time-dependent flow structures which originate upstream from the test section in the laminar flow at the inlet and in the contraction. Four different non-turbulent flow structures have been identified. The first is a result of quasi-two-dimensional separation of the laminar boundary-layer from the surfaces of the wind-tunnel contraction. Potential flow simulations show that susceptibility to this form of separation is reduced by increasing the degree of axisymmetry in the cross-section geometry and by decreasing the streamwise curvature of the concave surfaces. The second source of time-dependence in the laminar boundary-layer flow is an array of weak streamwise vortices produced by Goertler instability. The Goertler vortices can be removed by boundary-layer suction at the contraction exit. The third form of flow perturbation, revealed by visualisation experiments with streamers, is a weak large-scale forced-vortex swirl produced by random spatial fluctuations of temperature at the wind-tunnel inlet. This can be prevented by thorough mixing of the inlet flow; for example, a centrifugal blower installed at the inlet reduces the amplitude of temperature nonuniformity by a factor of about forty and so prevents buoyancy-driven swirl. When subjected to weak pressure gradients near the start of a wind-tunnel contraction, Goertler vortices in laminar wall layers can develop into three-dimensional separations with strong counter-rotating trailing vortices. These trailing vortices are the fourth source of unsteady flow in the test-section. They can be suppressed by a series of appropriately located screens which remove the low-speed-streak precursors of the three-dimensional separations. Elimination of the above four contaminating secondary flows permits the development of a steady uniform downstream flow and well-behaved turbulent wall layers. Measurements of velocity in the turbulent boundary layer of the test-section have been obtained by hot-wire anemometry. When a hot-wire probe is located within the viscous sublayer, heat transfer from the hot-wire filament to the wall produces significant errors in the measurements of both the mean and the fluctuating velocity components. This error is known as wall-proximity effect and two successful methods are developed for removing it from the hot-wire signal. The first method is based on the observation that, if all experimental parameters except flow speed and distance from the wall are fixed, the velocity error may be expressed nondimensionally as a function of only one parameter, in the form DeltaU^+=f(y^+). The second method, which also accommodates the effect of changing the hot-wire overheat ratio, is based on a dimensional analyis of heat transfer to the wall. Velocity measurements in the turbulent boundary layer at the mid-plane of a nearly square test-section duct have established that, when the boundary-layer thickness is less than one quarter of the duct height, mean-velocity characteristics are indistinguishable from those of a two-dimensional flat-plate boundary layer. In thicker mid-plane boundary layers, the mean-velocity characteristics are affected by stress-induced secondary flow and by lateral constriction of the boundary-layer wake region. A significant difference between flat-plate and duct boundary layers is also observed in momentum-balance calculations. The momentum-integral equation for a duct requires definitions of momentumd and displacement thickness which are different from those given for flat-plate boundary layers. Momentum-thickness growth rates predicted by the momentum-integral equation for a duct agree closely with measurements of the newly defined duct momentum thickness. Such agreement cannot be obtained in terms of standard flat-plate momentum thickness. In duct boundary layers with Reynolds numbers Re_theta between 400 and 2600, similarity in the wake-region distributions of streamwise turbulence statistics has been obtained by normalising distance from the wall with the flat-plate momentum thickness, theta_2. This result indicates that, in contrast with the mean velocity characteristics, the structure of mid-plane turbulence does not depend on the proportion of duct cross-section occupied by boundary layers and is essentially the same as in a flat-plate boundary layer. For Reynolds numbers less than 400, both wall-region and wake-region similarity fail because near-wall turbulence events interact strongly with the free stream flow and because large scale turbulence motions are directly influenced by the wall. In these conditions, which exist in both duct and flat-plate turbulent boundary layers, there is no distinct near-wall or wake region, and the behaviour of turbulence throughout the boundary layer depends on both wall variables and on outer region variables simultaneously. / Thesis (Ph.D.)--School of Mechanical Engineering, 1998.

Many potential small wind turbine locations are near obstacles such as buildings and shelterbelts, which can have a significant, detrimental effect on the local wind climate. This thesis describes the creation of a new model which can predict the wind speed, turbulence intensity, and wind power density at any point in an obstacle’s region of influence, relative to unsheltered conditions. Artificial neural networks were used to learn the relationship between an obstacle’s characteristics and its effects on the local wind. The neural network was trained using measurements collected in the wakes of scale models exposed to a simulated atmospheric boundary layer in a wind tunnel. A field experiment was conducted to validate the wind tunnel measurements. Model predictions are most accurate in the far wake region. The estimated mean uncertainties associated with model predictions of velocity deficit, power density deficit, and turbulence intensity excess are 5.0%, 15%, and 12.8%, respectively. / Industrial collaborators: Weather INnovations Inc., Wenvor Technologies Inc. / Ontario Centre of Excellence for Energy

The longitudinal fluctuations at a point in the core of air flowing through a 15 cm. diameter pipe at a mean centerline velocity of 13.4 and 29.5 m/sec. were measured with a hot-wire anemometer. This signal, after analog to digital conversion, was stored in the form of digital samples on an ICT computer drum storage device. This method of data recording includes the effect of all eddy frequencies from DC upwards and the presence of large, slow eddies in the longitudinal direction became apparent in the subsequent autocorrelations. The longitudinal dispersion of a tracer material injected on the axis of the pipe was measured over short distances with pulses of approx. 20 msecs. duration of radioactive Krypton-85, detected at two downstream stations by small surface-barrier radiation detectors. By varying the separation of these two stations, an asymptotic mixing coefficient was established which was very much greater than the corresponding transverse mixing coefficient measured by other workers. The method proposed by Philip (4) for the prediction of the Lagrangian time autocorrelation from the Eulerian velocity measurements was examined with the correlation data of Baldwin and the data obtained in this investigation. The method applied to the unfiltered correlation data in the present measurements in a non-isotropic field to predict a longitudinal turbulent Peclet no. was found to predict a value in the region measured experimentally. When the present velocity data was filtered to remove the low-frequency components and give a turbulence intensity equal to that measured in a radial direction in previous dispersion measurements, the mixing coefficient predicted with Philip's method was found to agree very well with the transverse mixing coefficient reported in these investigations. A value is also suggested for the longitudinal Peclet number in the absence of the low frequency fluctuations. / Thesis (Ph.D.)-University of Natal, Durban, 1970.

This document addresses problems encountered when establishing the very low air-flow speeds required for experimental investigations of the mechanisms of low-Reynolds-number boundary-layer turbulence. Small-scale motions in the near-wall region are important features of turbulent boundary-layer dynamics, and, if these features are to be resolved by measurements in air with conventionally-sized hot-wire probes, a well-behaved canonical turbulent boundary layer must be developed at free stream flow speeds no higher than 4 m/s. However, at such low speeds, the turbulent boundary layers developed on the walls of a wind tunnel are very susceptible to perturbation by non-turbulent time-dependent flow structures which originate upstream from the test section in the laminar flow at the inlet and in the contraction. Four different non-turbulent flow structures have been identified. The first is a result of quasi-two-dimensional separation of the laminar boundary-layer from the surfaces of the wind-tunnel contraction. Potential flow simulations show that susceptibility to this form of separation is reduced by increasing the degree of axisymmetry in the cross-section geometry and by decreasing the streamwise curvature of the concave surfaces. The second source of time-dependence in the laminar boundary-layer flow is an array of weak streamwise vortices produced by Goertler instability. The Goertler vortices can be removed by boundary-layer suction at the contraction exit. The third form of flow perturbation, revealed by visualisation experiments with streamers, is a weak large-scale forced-vortex swirl produced by random spatial fluctuations of temperature at the wind-tunnel inlet. This can be prevented by thorough mixing of the inlet flow; for example, a centrifugal blower installed at the inlet reduces the amplitude of temperature nonuniformity by a factor of about forty and so prevents buoyancy-driven swirl. When subjected to weak pressure gradients near the start of a wind-tunnel contraction, Goertler vortices in laminar wall layers can develop into three-dimensional separations with strong counter-rotating trailing vortices. These trailing vortices are the fourth source of unsteady flow in the test-section. They can be suppressed by a series of appropriately located screens which remove the low-speed-streak precursors of the three-dimensional separations. Elimination of the above four contaminating secondary flows permits the development of a steady uniform downstream flow and well-behaved turbulent wall layers. Measurements of velocity in the turbulent boundary layer of the test-section have been obtained by hot-wire anemometry. When a hot-wire probe is located within the viscous sublayer, heat transfer from the hot-wire filament to the wall produces significant errors in the measurements of both the mean and the fluctuating velocity components. This error is known as wall-proximity effect and two successful methods are developed for removing it from the hot-wire signal. The first method is based on the observation that, if all experimental parameters except flow speed and distance from the wall are fixed, the velocity error may be expressed nondimensionally as a function of only one parameter, in the form DeltaU^+=f(y^+). The second method, which also accommodates the effect of changing the hot-wire overheat ratio, is based on a dimensional analyis of heat transfer to the wall. Velocity measurements in the turbulent boundary layer at the mid-plane of a nearly square test-section duct have established that, when the boundary-layer thickness is less than one quarter of the duct height, mean-velocity characteristics are indistinguishable from those of a two-dimensional flat-plate boundary layer. In thicker mid-plane boundary layers, the mean-velocity characteristics are affected by stress-induced secondary flow and by lateral constriction of the boundary-layer wake region. A significant difference between flat-plate and duct boundary layers is also observed in momentum-balance calculations. The momentum-integral equation for a duct requires definitions of momentumd and displacement thickness which are different from those given for flat-plate boundary layers. Momentum-thickness growth rates predicted by the momentum-integral equation for a duct agree closely with measurements of the newly defined duct momentum thickness. Such agreement cannot be obtained in terms of standard flat-plate momentum thickness. In duct boundary layers with Reynolds numbers Re_theta between 400 and 2600, similarity in the wake-region distributions of streamwise turbulence statistics has been obtained by normalising distance from the wall with the flat-plate momentum thickness, theta_2. This result indicates that, in contrast with the mean velocity characteristics, the structure of mid-plane turbulence does not depend on the proportion of duct cross-section occupied by boundary layers and is essentially the same as in a flat-plate boundary layer. For Reynolds numbers less than 400, both wall-region and wake-region similarity fail because near-wall turbulence events interact strongly with the free stream flow and because large scale turbulence motions are directly influenced by the wall. In these conditions, which exist in both duct and flat-plate turbulent boundary layers, there is no distinct near-wall or wake region, and the behaviour of turbulence throughout the boundary layer depends on both wall variables and on outer region variables simultaneously. / Thesis (Ph.D.)--School of Mechanical Engineering, 1998.

Submitted by RODRIGO EDUARDO PREDOLIN null (rbi.predolin@gmail.com) on 2017-09-14T14:18:56Z No. of bitstreams: 1 defesa mestrado trabalho completo v3r.pdf: 25746886 bytes, checksum: 35a0b098c5eddb9a63a0d1f9107fe4e2 (MD5) / Approved for entry into archive by Luiz Galeffi (luizgaleffi@gmail.com) on 2017-09-15T13:51:52Z (GMT) No. of bitstreams: 1 predolin_re_me_bauru.pdf: 25746886 bytes, checksum: 35a0b098c5eddb9a63a0d1f9107fe4e2 (MD5) / Made available in DSpace on 2017-09-15T13:51:52Z (GMT). No. of bitstreams: 1 predolin_re_me_bauru.pdf: 25746886 bytes, checksum: 35a0b098c5eddb9a63a0d1f9107fe4e2 (MD5) Previous issue date: 2017-08-14 / Sistemas para monitoramento de parâmetros são amplamente utilizados no setor industrial para controle de processos e também na área de pesquisa e desenvolvimento. Estes sistemas permitem uma análise detalhada do comportamento de equipamentos e dispositivos e fornecem informações que auxiliam na melhora do seu desempenho. No estudo da utilização de energias renováveis o uso de sensores é aplicado para mapear o ambiente onde o equipamento está inserido e analisar o seu comportamento e desempenho. No caso de coletores solares, é realizado o monitoramento da temperatura do ambiente, da água no coletor em locais diferentes, da velocidade do vento e da radiação solar, permitindo o seu controle e melhorando o seu desempenho. A instrumentação adequada pode trazer melhorias aos coletores solares, porém dependem de estudos detalhados do seu comportamento através da aquisição de dados do equipamento e do ambiente onde ele está inserido. Para viabilizar esta otimização são necessários equipamentos de coleta de dados específicos que, normalmente, tem alto custo de aquisição. Este é um dos principais empecilhos para uma maior evolução destes equipamentos, principalmente os destinados ao uso residencial. Sendo assim, este trabalho objetiva desenvolver um módulo de coleta de dados de baixo custo para auxíliar no estudo de equipamentos em diversas áreas, incluindo a área de fontes de energia renovável. Um dispositivo deste tipo possibilita a coleta de diversos dados físicos como, por exemplo, a temperatura, a velocidade do vento e vazão d’água. Com o dispositivo há a possibilidade do armazenamento desses dados em um cartão SD, facilitando a sua transferência para o computador. Ao final do trabalho é apresentado o projeto de um módulo e as bibliotecas de maneira a permitir a sua fácil utilização. Alguns valores de parâmetros também foram captados, verificando-se o comportamento adequado do módulo e biblioteca nas condições propostas. / Parameter monitoring systems are widely used in the industrial sector for process control and also in the area of research and development. These systems afford device's behavior analysis with detailed and provide information that helps improve its performance. In the study of renewable energies use, the sensors is applied to map the environment where the equipment is inserted and to analyze its behavior and performance. In the case of solar collectors, the monitoring occurs in the environment temperature, collector water temperature in different places, wind speed and the solar radiation, allowing its control and improving its performance. Proper instrumentation can bring improvements to solar collectors, but depend on detail studies of its behavior based on the acquisition of data from the equipment and the environment where it is inserted. To make this optimization feasible, specific data collection equipment is required, which normally has a high acquisition cost. This is one of the main impediments to keep the evolution of this equipment, especially those destined for residential use. Therefore, this work aims to develop a low cost data collection module to aid in the initial study of equipments used in several sector, include the renewable energy sources sector. This kind of device turn possible the collection of various physical data, such as temperature, wind speed and water flow. With this device there is the possibility of storing this data on an SD card, making it easy to transfer to the computer. At the end of the work, the design of the module and its libraries are presented in a way that allows easy use. Some parameter values were also captured, verifying the proper behavior of the module and library in the proposed conditions.

Made available in DSpace on 2016-08-17T14:53:11Z (GMT). No. of bitstreams: 1 Shirlen Viana Leal.pdf: 556717 bytes, checksum: 0c051baae450dc752c45ceb9e82ddd19 (MD5) Previous issue date: 2010-06-04 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The thermoresistive - based hot-wire anemometer operating at a controlled temperature is a classical architecture that is vastly found in the literature. Nevertheless, this architecture may present a problem due to dynamic range variation of its output voltage for significant variation of the fluid temperature. In this work, the classical architecture is revisited and the influence of the fluid temperature is verified. An alternative architecture of a controlled temperature anemometer implemented with a controlled current source with automatic compensation of the fluid temperature influence on the output voltage dynamic range is proposed. Simulations of the classical architecture and of the proposed system using the parameters from a commercial sensor are presented and compared. Results show that the output voltage dynamic range for the proposed architecture employing the automatic compensation is practically constant. / O anemômetro de fio quente, baseado em sensor termo-resistivo operando à temperatura constante, é uma arquitetura clássica que é vastamente encontrada na bibliografia. Todavia, esta arquitetura pode apresentar um problema devido à variação da faixa dinâmica de sua tensão de saída para variações significativas da temperatura do fluido. Neste trabalho, a arquitetura clássica foi revisada e a influência da temperatura do fluido foi observada. Uma arquitetura alternativa de um anemômetro à temperatura controlada, implementada com uma fonte de corrente com compensação automática da influência da temperatura do fluido na faixa dinâmica da tensão de saída é proposta. Simulações da arquitetura clássica e do sistema proposto usando parâmetros de um sensor comercial são apresentadas e comparadas. Concluise, a partir dos resultados obtidos, que a faixa dinâmica da tensão de saída para a arquitetura proposta, empregando a compensação automática, é praticamente constante.

An investigation of the flow characteristics and sediment transport processes has been carried out in a two-stage meandering channel. Three phases of experiments have been conducted with various floodplain roughnesses. The dimensions of the flume are 13m long and 2.4m wide with a fixed valley slope of 11500. The meandering main channel has a sinuosity of 1.384 with top width of 0.4m. In each phase of the experiment, hydraulic data pertaining to stage-discharge, bed topography and sediment transport rate were measured at various overbank flow depths. Several flow depths were chosen to measure the three-dimensional velocities by means of Laser Doppler Anemometer and the morphological bedforms were recorded using the Photogrammetric technique. The boundary shear stresses were also measured by means of a Preston Tube and Vane Indicator. The experimental results showed that the presence of the energy losses due to momentum exchange and turbulence, bedforms roughness and floodplain roughness induced additional flow resistance to the main channel flow, particularly for shallow overbank flows. The combination of these losses affected a significant reduction in velocity and boundary shear stress in the main channel which, subsequently led to the reduction of sediment discharge at low relative depth for most tested cases. The reduction was more pronounced when the floodplain roughness increased. The examination of the three-dimensional velocity indicated that the formation of bedforms in the main channel is significantly affected by the flow structures, especially the secondary flow. A new method for predicting velocity and sediment transport rate has been introduced based on the two-dimensional equation (Spooner's) coupled with the self-calibrated empirical transport formula. The proposed method gave accurate prediction for depthaveraged velocity and sediment transport rate for two-stage meandering channel.

Flush-mounted cavity hot-wire probes have been around since two decades, but have typically not been applied as often compared to the traditional wall hot-wires mounted several wire diameters above the surface. While the latter suffer from heat conduction from the hot wire to the substrate in particular when used in air flows, the former is belived to significantly enhance the frequency response of the sensor. The recent work using a cavity hotwire by Gubian et al. (2019) came to the surprising conclusion that the magnitute of the fluctuating wall-shear stress τ+w,rms reaches an asymptotic value of 0.44 beyond the friction Reynolds number Re τ ∼ 600. In an effort to explain this result, which is at odds with the majority of the literature, the present work combines direct numerical simulations (DNS) of a turbulent channel flow with a cavity modelled using the immersed boundary method, as well as an experimental replication of the study of Gubian et al. in a turbulent boundary layer to explain how the contradicting results could have been obtained. It is shown that the measurements of the mentioned study can be replicated qualitatively as a result of measurement problems. We will present why cavity hot-wire probes should neither be used for quantitative nor qualitative measurements of wall-bounded flows, and that several experimental short-comings can interact to sometimes falsely yield seemingly correct results.

The development of the constant voltage anemometer (CVA) for the boundary layer data system (BLDS) has been motivated by a need for the explicit autonomous measurement of velocity fluctuations in the boundary layer. The frequency response of a sensor operated by CVA has been studied analytically and experimentally. The thermal lag of the sensor is quantified by a time constant, MCVA. When the time constant is decreased, the half-amplitude cut-off frequency, fCVA, is increased, thereby decreasing the amount of attenuation during measurements. In this thesis, three main approaches have been outlined in theory and tested experimentally to determine the feasibility and effectiveness of implementing them with CVA to limit attenuation: operation at higher Vw, implementation of software compensation, and utilization of smaller diameter sensors. Operation of CVA at higher voltage results in little improvement in frequency response but is accompanied by increased danger of wire burnout. However, sensors do need to be operated at high wire voltages to be more sensitive to velocity fluctuations and less sensitive to temperature fluctuations, without reaching a temperature high enough for wire burnout. Software compensation of the CVA output has been shown not to be useful for measurements with BLDS. The electrical noise present in the CVA measurement system is amplified by the correction algorithm and creates measurements that are not representative of the fluctuations being measured. Decreasing sensor diameter leads to a significant decrease of MCVA and therefore increase of fCVA. Under similar operating conditions, a 2.5 micron diameter sensor showed less roll off in the frequency spectra (measured higher turbulence intensities) than a 3.8 micron diameter sensor for tests in both a turbulent jet and in a turbulent boundary layer. Smaller sensors are more fragile and have been shown to have a decrease in sensitivity as compared to larger sensors; however, for some applications, the increase in frequency response may be worth the trade-off with fragility and sensitivity.

Traditionally, the measurement of turbulence has been conducted using hot-wire anemometry. This thesis presents the implementation of a constant voltage hot-wire anemometer for use with the Boundary Layer Data System (BLDS). A hot-wire calibration apparatus has been developed that is capable of operation inside a vacuum chamber and flow speeds up to 50 m/s. Hot-wires operated with a constant-voltage anemometer (CVA) were calibrated at absolute static pressures down to 26 kPa. A thermal/electrical model for a hot-wire and the CVA circuit successfully predicted the measured CVA output voltage trend at reduced pressure environments; however, better results were obtained when the Nusselt number was increased. A calibration approach that required only one measured flow speed was developed to allow autonomous calibrations of a CVA hot-wire. The single-point calibration approach was evaluated through comparison with the experimental data from the vacuum chamber over a range of 14-50 m/s and at pressures from 26 to 100 kPa. The thermal-electrical model was used to make predictions of CVA output voltage and the corresponding flow speed for conditions that could not be replicated within a laboratory. The first set of predictions were made for conditions from 7.5 to 100 kPa, at a constant temperature of 25⁰C, within a flight speed range of 40 to 150 m/s. Single-point calibrations were developed from these predictions. Additionally, the thermal-electrical model was used to predict hot-wire response for a change in temperature of 25⁰C at 26 kPa and the single-point calibration developed for the pressure range 7.5 to 100 kPa was tested for its ability to adjust. The temperature variation at a single pressure of 26 kPa proved that the single-point function was capable of adapting to off-standard temperatures with the largest deviations of +/- 7% in the mid-range velocities. With a temperature drop, the deviations were below 5%. The second set of thermal-electrical predictions involved conditions for altitude from 0 to 18 km at flow speeds from 40 to 150 m/s. A single-point calibration was developed for altitude conditions. Furthermore, to test the single-point calibration the thermal-electrical model was used to predict hot-re response for a temperature variation of 25⁰C at 18 km. The single-point calibration developed for altitude proved that it was capable of adjusting to a temperature variation of 25⁰C with maximum deviations of about 5% at mid-range velocities. It is proposed that the single-point calibration approach could be employed for CVA measurements with the Boundary Layer Data System (BLDS) to allow hot-wire data to be acquired autonomously during flight tests.