1,720,996 research outputs found
Proceedings of the EUROPIV 2 Workshop held in Zaragoza, Spain, March 31 - April 1, 2003
No full textThe Particle Image Velocimetry (PIV) measurement technique has undergone a strong development in the last 10 years. This book presents the proceedings of an international workshop held in Zaragoza, Spain, on March 31st and April 1st, 2003, containing contributions from worldwide leading teams in the development of the PIV method. Most of these papers have been funded by the EC via the European EUROPIV 2 consortium to improve the performances of this measurement technique toward applications in the European Aeronautical industry, including results which are of strong interest for the worldwide community in Fluid Dynamics
Repeller or Attractor? Selecting the Dynamical Model for the Onset of Turbulence in Pipe Flow
No full textThe collapse of turbulence, observable in shear flows at low Reynolds numbers, raises the question if turbulence is generically of a transient nature or becomes sustained at some critical point. Recent data have led to conflicting views with the majority of studies supporting the model of turbulence turning into an attracting state. Here we present lifetime measurements of turbulence in pipe flow spanning 8 orders of magnitude in time, drastically extending all previous investigations. We show that no critical point exists in this regime and that in contrast to the prevailing view the turbulent state remains transient. To our knowledge this is the first observation of superexponential transients in turbulence, confirming a conjecture derived from low-dimensional systems.Process and EnergyMechanical, Maritime and Materials Engineerin
Particle Manipulation using Microfluidics
No full textThe progress made by microfabrication technologies has escalated the interest in particle manipulation on micron scale. Particles are not only limited to solid particles but also include cells, droplets and bubbles. Activities involved in manipulation are trapping, sorting, separation and so on. The devices designed to execute such activities are aimed for a particular application. However, there lies a gap in literature to design an all-in-one device. Microfluidics research carried out at the Laboratory for Aero and Hydrodynamics is currently working on designing such devices which can carry out different manipulation tasks.This thesis is aimed at designing and fabricating such a device where two particles can be manipulated simultaneously. The application of designing such a device would be to study cell-cell interaction, droplet coalescence and so on. To achieve this, a microfluidic device is designed as a Hele-Shaw flow cell. Since the height-averaged velocity in a Hele-Shaw flow cell is irrotational, analytical solution is possible for such a design. Although a Hele-Shaw flow is viscous flow between two closely spaced plates, it produces same streamline patterns as a 2D ideal flow. This enables the use of basic building blocks of an ideal flow field such as sources, sinks, uniform flow and so on. In this case, sources and sinks are superimposed to get the flow field inside the microfluidic device. The microfluidic device is fabricated using Polydimethylsiloxane (PDMS), a commonly used polymer to fabricate microfluidic devices. The design of the device is validated by comparing experimental flow fields, streamlines and stagnation points to the corresponding analytical solution. Single particle manipulation activities are carried out in the device. This is executed by manually controlling the flow rates or the strength of sources and sinks. Firstly, the particle is trapped at the stagnation point for a certain timespan. Finally, some manipulation activities are carried out using a single particle with the aid of streamlines produced by the sources and sinks.Mechanical Engineerin
Towards high resolution Particle Image Velocimetry: Estimating turbulence statistics from the ensemble correlation
No full textThe research in resolving the smaller scales of turbulence has gained significant attention in recent years, owing to the advancements in the measurement techniques. High resolution experimental studies are required to investigate high Reynolds number flows where the scales of turbulence are very small. This is of particular importance in addressing the theoretical issues in studying high Reynolds number wall turbulence which finds applications in transport, energy, and aerospace industries. Hot-Wire Anemometry (HWA) is often employed to reach very high spatial resolution. Early research in Particle Image Velocimetry (PIV), a qualitative flow visualization technique, was not able to achieve a higher resolution as comparable with that of HWA. However, recent developments in PIV allow high resolution measurements, using information on the time averaged ensemble correlation. The ensemble correlation can be used to reduce the size of the interrogation windows employed in a PIV analysis, without compromising for the particle image density in an interrogation window. This is achieved by using a large number of image pairs. In this thesis, ensemble correlation is used to study the turbulence in two specific cases: a turbulent jet and high Reynolds number pipe flow. The ensemble correlation, though being a time averaged quantity, contains information on the velocity Joint Probability Distribution Functions (JPDFs) which can be used to estimate the turbulence statistics. This information can be retrieved from the shape of the ensemble correlation. It is assumed that the ensemble correlation is the convolution of the auto-correlation and the velocity JPDFs. The velocity JPDFs are retrieved by estimating the second moments of the ensemble correlation by fitting a Gaussian profile, and further using a correction for auto-correlation. The second moments can be used to estimate the Reynolds stresses in the flow. The proposed method is validated by implementing it to study the turbulent jet. For this purpose, 9000 image pairs are acquired. The results show that the retrieved moments predict the Reynolds stresses accurately. Further, high Reynolds number pipe flow experiments are performed in the Alpha Loop facility at Deltares. The bulk Reynolds number is varied from 0.3 to 0.6 million, acquiring 20,000 image pairs per measurement series. Also, the pressure drop is measured across the pipe along with the PIV measurements to estimate the roughness of the pipe. The results from the PIV measurements show that the mean velocity profile follows a similar trend as observed in the literature. A small bias is observed between the mean velocity obtained from the ensemble correlation and that obtained by averaging the instantaneous velocity vectors, in regions of high velocity gradients. The Reynolds shear stress estimated from the shape of the ensemble correlation is underestimated. However, the streamwise turbulent fluctuations estimated follow a trend, similar to that observed in the literature.TKI project ‘Turbulent pipe flow at high Reynolds number’, DeltaresMechanical Engineerin
The effect of mildly rapid strain on turbulent pipe flow
No full textThe way in which mean strain affects the turbulent structures is imperative to understand various natural flows such as flow over a hill, the flow of a river in the delta, jet streams in the upper atmosphere etc. Further, it also has industrial implications viz; flow over bodies such as airfoil, turbomachinery, gas pipelines. The strained pipe flows, in particular, have huge engineering interest due to its prevalence in industrial fittings wherein a larger pipe diameter is connected to a smaller one and vice versa. This subject also has a fundamental interest as strain highlights the interaction of various scales of turbulence. This scale interactionessentially dictates transfer of energy in turbulence and hence is fundamental in understanding turbulence dynamics itself. The present work deals with the experimental study of the response of pipe turbulence to axisymmetric, irrotational strain using high-resolution planar Particle Image Velocimetry (PIV). A mildly rapid strain (s*s ~ 3.2) is imposed on turbulence via a spatial contraction. It is seen that turbulence is suppressed upon straining. As a response to mean strain, transverse Reynold stress increases at the expense of streamwise Reynolds stress and anisotropy is induced in the turbulence. Despite strain being only mildly rapid, Rapid Distortion Theory (RDT) is found to predict the correct trend of normal Reynolds stress although traverse Reynolds stress is over-predicted. The effect of strain on different scales of turbulence is discerned. The large scales of turbulence are seen to get compressed in the radial direction although they do not get affected significantly in the streamwise direction. Near-wall coherent structures which were initially inclined w.r.t. the wall are seen to get aligned with the flow as they also get severely compressed in the radial direction. On the other hand, the small scales of turbulence are found to be spatially organised in the form of sheets or layers. Upon straining, these sheets are found to get aligned with the mean flow. Further, they get elongated in streamwise and compressed in the radial direction. It is observed that the small scales are more severely distorted than the large scales upon staining inside the contraction. At the Reynolds number (Re) range employed in this thesis, there is no substantial difference in which turbulence is strained inside the contraction at disparate Re. Downstream of the contraction, at the axis of the pipe, the anisotropy of Reynolds stress is found to recover slowly. Further, this relaxation is seen to be Re dependent with higher Re turbulence relaxing slightly faster.Mechanical Engineering | Process and Energy Technolog
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
A novel technique for interface reconstruction in transparent media
No full textA novel technique for measuring capillary/gravity waves travelling on air/water interfaces is investigated. The main motivation is to measure the free surface deformations of an air cavity used for air lubrication of ship hulls. Due to limitations in the available techniques, two novel methods are proposed, out of which the Stereo Particle Image Velocimetry synthetic Schlieren (Stereo-PIV-SS) technique is found to be more promising. This technique uses the input of random dot pattern images captured with and without an interface by two cameras mounted in a Stereo configuration. These images are processed to generate "pseudo" particle motion vectors using a Stereo-PIV code. An analytical study is conducted to find a simple linear relation between the pseudo vectors and interface height. Two sets of experiments are conducted to test this technique. One with defined transparent solid Plexiglas and glass profiles and another on small amplitude and slope water waves. For the experiments on water waves, the Free Surface Synthetic Schlieren (FS-SS) technique is used as validation data. In addition to experiments, a synthetic image reconstruction study is conducted to test the quality of the images generated by an in-house code. The investigation reveals that the Stereo-PIV-SS technique is capable of capturing the flat interface height with sub millimeter precision for all interface heights tested in the range of 5 to 60 mm. Nevertheless, the technique can capture a wave profile with sub millimeter precision only for a limited range of wave parameters. The exact limits can only be defined by understanding more about the interplay of various wave parameters, which include the wave orientation, influencing the reconstruction. However, this technique can capture the profiles of waves having wavelength of the order of 1 cm and amplitude of the order of 1 mm, without requiring information about the mean interface height. Moreover, it is low cost, easy to use and apply since it requires only a standard PIV setting. The first attempt to study this new technique has been promising and it can be applied to specified wave regimes. The method can be developed further by understanding how to remove the effect of wave orientation on the reconstruction and including other wave parameters in the interface height correlation.Mechanical Engineerin
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