1,721,074 research outputs found
Direct numerical simulations of drag reduction in turbulent channel flow over bio-inspired herringbone riblet-texture
No full textThe use of drag reducing surface textures is a promising passive method to reduce fuel consumption. Probably most wellknown is the utilisation of shark-skin inspired ridges or riblets parallel to the mean flow. They can reduce drag up to 10%. Recently another bio-inspired texture based on bird flight feather riblets has been proposed. It differs from the standard riblets in two ways. First, the riblets are arranged in a converging/diverging or herringbone pattern. Second, the riblet height or groove depth changes gradually. Drag reductions as high as 20% have been claimed [2]. The objective of the present work is to study the drag reducing properties and mechanisms of this texture. To that purpose Direct Numerical Simulations (DNSs) of turbulent plane channel flow have been performed. Structured roughness has been applied to both walls and several geometric parameters have been varied. Marginal drag reductions on the order of 2.5% and significant drag increases well beyond 100% were found. The latter is attributed to a strong secondary flow that mixes momentum through the whole channel. In future optimization studies we might look for conditions at which secondary motions affect the near-wall cycle of turbulence only
Predicting growth rates of interfaces and internal layers in a turbulent boundary layer using a first order jump model
No full textExperimental research is presented on the characteristics of interfaces and internal layers that are present in a turbulent boundary layer (TBL). Both the turbulent non-turbulent interface (T/NT) and internal shear layers are detected in snapshots of the stereo-PIV data. It turns out that the internal layers exhibit similar characteristics compared to the T/NT interface. A theoretical approximation of the large scale boundary layer growth indicates that the correct boundary layer growth can be obtained by employing a modified first order jump model on the conditional statistics. Employing the same framework to the internal shear layers indicates that shear layers tend to move slower in close proximity to the wall, whereas they accelerate when moving away from the wall. Based on previous research it is believed that these internal layers separate large regions of approximately uniform momentum. Hence, boundary entrainment velocities may be interpreted as growth rates of large scale motions in a TBL
Optimization of multiplane μpIV for wall shear stress and wall topography characterization
No full textMultiplane μPIV can be utilized to determine the wall shear stress and wall topology from the measured flow over a structured surface. A theoretical model was developed to predict the measurement error for the surface topography and shear stress, based on a theoretical analysis of the precision in PIV measurements. The main parameters that affect the accuracy of the measurement are identified. The effect of different parameter settings is studied by means of Monte Carlo simulations, and the results are compared with an experimental test case. The results are used to determine the recommended parameter settings for this measurement approach
Particle Image Velocimetry; Progress towards Industrial Application
No full textThis book is a detailed presentation of the cooperative research activity undertaken by 20 European teams to contribute to the development of Particle Image Velocimetry. This research was performed over a period of two years, in the frame of a European Brite-Euram contract and was greatly enhanced by its cooperative aspect. The book presents extensive comparisons of both recording and analysis of PIV images. A database has been built, which is presented in detail and which is accessible via Internet. Industrial application in a large wind tunnel, validation of up-to-date numerical predictions and new developments including extension to 3C are also addressed in the different chapters of this book. In addition, 25 individual contributions are provided in the form of scientific papers which cover various aspects of the development of the method (PTV, video recording, stereo-PIV, holo-PIV). This book will be of interest to scientists and engineers active in the field
Turbulent Particle Transport and its Effect on Flotation
No full textThis thesis studies the hydrodynamics of froth flotation, a process that is used in industry as a means of removing small inclusions from the treated liquid by gas bubble injection. This work is particularly motivated by the removal of aluminium and silicon oxides from liquid steel. Hydrodynamic aspects have a strong influence on the efficiency of froth flotation. The flow around a rising bubble, especially the turbulent flow in the near-wake, is of particular importance as it may cause preferential concentration of particles as well as significantly change the collision and attachment rates of particles with the surface of the bubble. Both effects are studied in this work. Preferential concentration of particles in the near-wake is a result of the time-averaged balance of inertial and pressure forces on a dispersed particle, resulting in a drift towards the wake. It can therefore be observed by an effective time-averaged particle slip velocity over the wake boundary. Likewise, a direct confirmation of preferential concentration is possible by measurements of the average concentration of particles in the wake. These measurements were done in the wake of a solid mock-up of a spherical-cap bubble in the VerMeer vertical water tunnel. The particle slip velocity was obtained from simultaneous two-phase Particle Image Velocimetry measurements and numerically integrated in post-processing. The particle concentration was obtained directly from the average scattered light intensity. Both experimental results are compared to the predictions of a model for the preferential concentration in the wake, derived from a local balance of inertial, gravitational and diffusive fluxes. The influence of turbulence on particle attachment rates was measured in the newly constructed DABuT (Dynamic Air Bubble Trap) facility, using bubbles of different shapes and volumes. The research was again focused on the semi-spherical bubbles, these were also found to have the highest attachment rates. The results of the measurements are compared to a model based on a turbulent attachment flux and the effective shielding of particle attachments by a mono layer of particles building up and finally covering the rear side of the bubble in proximity to the wake. In conclusion, both effects contribute to the overall efficiency of flotation. the results of this work suggest that spherical-cap bubbles — although potentially difficult to generate — have the highest potential for the optimisation of flotation processes in industry.Process and EnergyMechanical, Maritime and Materials Engineerin
Dynamics of Precursor Films: Experiment and Theory
No full textDespite the fact that moving droplets are very common, a moving contact line is a longstanding fundamental challenge in the field of fluid mechanics. This fundamental challenge is the main topic of this thesis in the context of ASML immersion lithography equipment. In Chapter 2 an overview of existing theoretical and experimental work is given, that focuses on the mechanism of singularity removal in case of a moving contact line. In order to get more insight in this mechanism, it was proposed to study the nanoscale region of a contact line while it is moving. In this region van der Waals forces come into play and the contact line is referred to as a precursor film. Two types of precursor films exist, namely adiabatic and diffusive, which are present in case of a moving or non-moving contact line respectively. The current experimental results demonstrate the existence of a precursor film, but remain qualitative only in case of a moving precursor film. Hence, there is no adequate comparison of experimental results with the complete theory. Furthermore, conflicting results exist regarding the existence of a precursor film in a partial wetting situation. Nevertheless, a good understanding of the behavior of precursor films in the case of complete wetting is present, and the experimental results for the diffusive precursor film are in good agreement with theory. However, there is no experimental data regarding (the film profile of) adiabatic precursor films. This is related to the inherent difficulty of probing the region of a precursor film due to the great disparity of length and time scales involved. Moreover, understanding the behavior of adiabatic films is the key to understanding moving contact lines. Therefore, a measurement technique with sufficient spatial as well as temporal resolution is developed to probe this region. Chapter 3 describes Total Internal Reflection Fluorescence Microscopy (TIRFM), which formed the basis for the developed measurement technique. TIRFM is an advanced microscopy technique that limits the illumination to a very thin layer at the substrate by using an evanescent wave. This offers an unprecedented signal-to-noise ratio since only the fluorescent particles or fluorophores within the penetration depth of the evanescent wave are producing signal towards the detector. Furthermore, the experimental setup is designed and built to accurately measure the precursor film profile while it is moving.Process and EnergyMechanical, Maritime and Materials Engineerin
Quantification of hemodynamics during vascular development
No full textThe work described in this PhD thesis has been carried out to improve insight into the relation between changing vascular network structures and their corresponding hemodynamics. Accurate quantitative information about the changing hemodynamics has been obtained experimentally from developing extraembryonic vascular networks of chicken embryos, which are a commonly-used model for cardiovascular research. First, a two-dimensional velocity field is measured by an optical measurement method called Particle Image Velocimetry. From this velocity field, both characteristic hemodynamic parameters and the corresponding vascular structure are extracted. These parameters are used to model the vascular network to enable further analysis. The relation between the quantitative results and qualitative observations has been studied, as well as the agreement with theoretical design rules for optimal networks. The non-intuitive and complex hemodynamical and structural changes presents in all networks indicate that measuring these changes is crucial to gain more insight in this process. Further, the changes in hemodynamics parameters can be related to the observed structural development. The quality of the data sets is comparable, which means that the data set is suitable for further analysis with known accuracy, and it is particularly suitable for validation of vascular development models. The followed method for characterizing vascular networks can easily be applied to other two-dimensional networks having optical access. Besides, this method can also contribute to quantitatively describing the effects of, for example, mechanical and chemical interventions when applied to developing networks both before and after the interventions. This work is preceded by a study on the accuracy of the flow measurement method, microscopic Particle Image Velocimetry. The influence of experimental parameters has been investigated for a correct interpretation of the measured flow velocities.Process and EnergyMechanical, Maritime and Materials Engineerin
Particle image velocimetry for microscale blood flow measurement
No full textThe development, the regulation, and the pathology of the circulatory system (e.g. cardiogenesis, thermoregulation, atherosclerosis) are determined by blood flow induced mechanical forces. While this proposition has been confirmed during recent years, the exact mechanisms still remain unclear. This is mainly due to the fact that those forces could hardly be measured. The goal of the research described in this thesis is the development of a measurement technique that can provide such data for living organisms. Fluid mechanical forces can be deduced from spatial velocity information, as provided by particle image velocimetry. Small, artificial, fluorescent particles with a hydrophilic coating are used to determine the fluid velocity. Fluorescence enables the interference-free recording of the particle motion, and the coating makes the particles "invisible" to the biological system. Three dimensional flow is accessed by a combined measurement of multiple planes and a numerical reconstruction method. For the first time the three dimensional flow distribution in a beating embryonic chicken heart with an inner diameter of 200 micrometers could accurately be determined. The measurements explain high gene expression levels found at the inner curvature of bends.Mechanical Maritime and Materials Engineerin
Digital holographic particle image velocimetry
No full textThis thesis contributes to the development of an instrument that can measure fluid flow velocity in a three-dimensional (3D) measurement domain by holographic imaging.Mechanical Maritime and Materials Engineerin
Moving liquid droplets with inertia: Experiment, simulation, and theory
No full textThis thesis is a work on a contact line instability at a finite Reynolds number, 0 < Re < O(100). This problem corresponds to an immersion droplet applied in a liquid- immersion lithography machine. We perform extensive works to understand this instability problem by means of experimental, numerical, and theoretical ways. First, in order to measure the 3D internal flow pattern, we perform 3D-3C velocimetry techniques, i.e. tomographic particle image velocimetry and 3D particle tracking velocimetry. Furthermore, we observe droplet shape changes by shadowgraphy. Second, based on these experimental results, we develop a modified three-dimensional lubrication model including inertial effects. In this model, the pressure is described as a combination of dynamic pressure effects and capillary pressure effects. By this extended model, we obtain an analytical solution describing the relationship between opening angles and receding angles. Additionally, we show a self-similar flow pattern near the dewetting contact lines. Last, a simplified numerical model is introduced for a liquid immersion droplet. For the numerical computation, we adopt a flat cylinder and apply a zero flux boundary condition at the gas-liquid interface. The numerical results are compared with measurement results and give a good agreement.Process and EnergyMechanical, Maritime and Materials Engineerin
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