548 research outputs found
Mechanics of inhomogeneous turbulence and interfacial layers
No full textThe mechanics of inhomogeneous turbulence in and adjacent to interfacial layers bounding turbulent and non-turbulent regions are analysed. Different mechanisms are identified according to the straining by the turbulent eddies in relation to the strength of the mean shear adjacent to, or across, the interfacial layer. How the turbulence is initiated and the topology of the region of turbulence are also significant factors. Specifically the cases of a layer of turbulence bounded on one, or two, sides by a uniform and/or shearing flow, and a circular region of a rotating turbulent vortex are considered and discussed.
The entrainment processes at fluctuating interfaces occur both at the outer edges of turbulent shear layers, with and without free-stream turbulence (e.g. jets, wakes and boundary layers), at internal boundaries such as those at the outside of the non-turbulent core of swirling flows (e.g. the ‘eye-wall’ of a hurricane) or at the top of the viscous sublayer and roughness elements in turbulent boundary layers. Conditionally sampled data enables these concepts to be tested. These concepts lead to physically based estimates for critical modelling parameters such as eddy viscosity near interfaces, entrainment rates, maximum velocity and displacement heights
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
Single-pixel PIV measurements of high-re turbulent pipe flow
No full textIn this paper we present PIV measurements of turbulent pipe flow at Reynolds numbers between 3.4×105 and 6.9×105. We apply a so-called 'single-pixel correlation' that yields a superior spatial resolution (Westerweel et al., 2004). We use the location and shape of the averaged correlation peak to obtain the mean velocity and normal and Reynolds stresses (Scharnowski et al., 2012). A novel aspect of the single-pixel correlation approach is the extension to determine the spatial correlation of the velocity fluctuations. In this paper we present the results for Re = 4.98×105, corresponding to a shear Reynolds number Reτ = 10.3×103, with a spatial resolution of ∆y+ = 18.Fluid MechanicsMulti Phase System
Flow induced by beating artificial cilia: An experimental and numerical study
No full textArtificial cilia-driven fluid transport in microfluidic devices is a newly emerging research topic that came into focus during the last five years. Artificial cilia can be described as wall-attached elongated structures that perform an externally actuated periodic beat cycle to manipulate fluids on sub-millimeter scale. The periodic motion of the cilia can be used to transport, mix or filter fluids. It is the variety of potential microfluidic manipulation processes that makes artificial cilia-induced microfluidic manipulation so attractive, and it probably is one of the main reasons for the rapid growth in the number of recent scientific papers in this field. The main research objective of this thesis is to study the influence of beating artificial cilia on the fluid transport. To this purpose, the research was done in three main steps. Firstly, fluid and particle velocities above a surface of natural cilia were measured to determine typical transport rates. This served as guideline for the second step in which a novel artificial cilia device was utilized and critically evaluated experimentally. While a nearly synchronized cilia beat was investigated in the experiments, in a third step the influence of a coordinated but phase-shifted beating of cilia on the fluid transport was studied numerically.Laboratory for Aero and HydrodynamicsMechanical, Maritime and Materials Engineerin
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
Droplet collisions in turbulence
No full textLiquid droplets occur in many natural phenomena and play an important role in a large number of industrial applications. One of the distinct properties of droplets as opposed to solid particles is their ability to merge, or coalesce upon collision. Coalescence of liquid drops is of importance in for example the food industry, where the shelf life of emulsion type of products is often elongated with absence of coalescence, rocket propulsion, where fuel and oxidizer are separately injected and ignite upon coalescence, the semiconductor industry, where droplets of molten tin are targeted by a laser to produce extreme ultraviolet light, and during the formation of rain. One might not realize it immediately, but the outcome of a droplet collision is not always coalescence, but also bouncing can occur. In most of the mentioned cases the droplet collisions do not take place in a stationary environment but in an environment that flows with respect to the droplets or is even turbulent. The aim of the current study is to gain insight into the effect of external turbulence on the outcome of droplet collisions. When two droplets collide it might appear that the drops are in contact, but in reality there will be a thin film of surrounding fluid in between the droplets. When this film becomes so thin that Van der Waals forces between the molecules of the two drops come into effect a hole will be initiated in the film and the droplets will coalesce. The associated thickness is in the order 60 nm. If the film thickness does not decrease to this value, the droplets will bounce. The time it takes from the start of drop interaction to film rupture is called the drainage time, denoted by t_drainage. The time during which the drops are in apparent contact is called the interaction time t_interaction. A droplet collision will thus lead to coalescence when t_{drainage} 30). The external flow is thus of significant influence on the outcome of droplet collisions in terms of bouncing or coalescence. A possible explanation for this hindered coalescence is the fact that a compressional flow in the direction of the collision axis induces an internal drop flow which counteracts film drainage. If this explanation is valid the measured effect is not due to turbulence. References (1) Qian and Law, Regimes of coalescence and separation in droplet collision. J. Fluid Mech 331, 1997. (2) Miller and Scriven, The oscillations of a fluid droplet immersed in another fluid. J. Fluid Mech 32, 1968.Process and EnergyMechanical, Maritime and Materials Engineerin
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
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
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
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