1,720,986 research outputs found
Hydrodynamic and morphodynamic response to pipe soil interaction
No full textRecent developments of the petrochemical industry in the last two decades, helped the rapid growth of pipeline installations and the related research. The costs associated with a pipeline failure or damage, in terms of reparation at deep water depth and loss of
production, are so high that it is fundamental to evaluate all the physical conditions and the extreme events that a pipeline have to resist during its life.
In the past, the main idea was to totally prevent the pipeline instability, blocking the buildup of axial stresses (with the introduction of a number of expanding axial joints) or to restrain the vertical or lateral displacements of the pipeline (with an appropriate burying).
This type of approaches, being more and more unfeasible and expensive, is becoming
obsolete so that the design of offshore pipelines is to have them placed directly on the seabed.
In detail, when a pipeline is installed on the seabed it disturbs the local environment. As a result, vortices are formed in the neighborhood of the pipeline and a pressure gradient may occur between the upstream and downstream sides of the pipeline. Piping and stagnation
eddy may combine, generating a scour hole under the structures. A two-dimensional scour,induced by the waves and currents, may develop into a three-dimensional scour hole along the axis of the pipeline. If the pipeline is stiff enough and the span length is not very large,the mid-span may be suspended above the scour hole, or, when the span length is longer,the pipeline may start to sag. The pipeline continues its sagging process during the development of the scour hole, until the mid-span comes into contact with the scour hole bottom.
The research concerning the pipe-soil interaction is of fundamental interest for the offshore engineering; the scour hole and the scour development are very important design parameters for the safety of most structures. The scour around an offshore structures is one of the classical topics in the ocean engineering, while one classical problem of the hydraulic engineering is the prediction of the scour hole around bridge piers. The scouring problem is
not theoretically completely resolved in a satisfactory way as yet. One of the main reasons is the highly complex three-dimensional turbulence flow pattern generated around the structures, thus the only studies available in the literature concern the equilibrium condition
of the scouring and the time development of the scour hole. Mao (1986) was the first who
described the role of the small vortices that are formed in the upstream and downstream sides of the pipeline. They were thought of being responsible to carry sand particles away from the footing area of the pipeline until a small opening is formed underneath the pipeline which leads to the tunnel scour. Sumer et al., (2001) found that the pressure difference between the upstream and downstream sides of a pipeline drives a seepage flow underneath the pipe. When this seepage flow becomes excessive, piping occurs; a mixture of water and sand breaks through underneath the pipe, resulting in the onset of scour. The
pressure difference together with other effects (such as vortices forming in front of and at the lee-wake of the pipe) are the agitating forces for the piping process. Once the scour breaks out, it will propagate along the length of the pipeline.
A complementary way to evaluate and understand the process of local scour around
pipelines is offered by numerical models. Over the last two decades, two main types of
numerical models for scouring prediction have been developed. One first type of numerical algorithms was based on potential flow theory (Hansen et al 1986; Li & Cheng 1999). However, such a theory cannot adequately describe the flow below the pipeline subjected to scouring conditions and is unable to predict the downstream side of the bed profile and the vortex shedding generated downstream the body of interest. More recently, turbulenceaveraged
Navies-Stokes solvers (both RANS and LES) coupled with a suitable sediment
transport equation were introduced (Brørs 1999; Li & Cheng, 2001; Liang & Cheng,
2005). The main limits of these algorithms is in the capturing of the inception of the
scouring.
The aim of this research is to investigate the mechanics of scouring both numerically and experimentally and to promote the understanding of the pipe-soil interaction by means of an analysis of the local flow-sediment dynamics. A two-dimensional turbulent flow is investigated through an experimental test using a non-intrusive technique for the particle
motion (Particle Tracking Velocimetry, PTV). The basic image processing for the PTV was
performed by means of the software “YATS” (Miozzi, 2004). Such a software implements
a tracking technique based on the correlation between interrogation windows of consecutive images. Eleven monochromatic waves were tested both in a rigid and an erodible seabed,the former is made of steel plates above wood platforms and it was useful to analyze the vorticity and turbulence generated around the pipe, for the latter it was used a granular
sand, with a median diameter D50= 0.6 mm, in order to investigate the maximum scour
depth and the scour development.
Further, an innovative numerical model, based on the Level Set technique for a two-phase flow, has been implemented to evaluate the transitional and equilibrium stages of the scour development. The scouring of the sand bed is modeled by coupling the solution of the flow in the fluid domain with a simplified model used to describe the erosion of a granular sand
layer. The domain is divided into three parts: i) a pure-water domain, ii) a solid domain with the consolidated sand and iii) a water-sand mixture. The fluid regions are modeled as a single-fluid domain with variable properties of mass and viscosity and are governed by the
mass-conservation and the Navier-Stokes equation. The scour model uses a bulk
approximation of a conservation of mass and advection scheme to predict the transport of
the sediment. The model consists in two basic components one account for the sediment
drifting and another for the sediment lifting (Mattioli et al., 2010). Drifting processes act on the suspended sediments, force the sand to pile up on the compact bottom considering that
sand is composed of single grains that would fall on the bottom in the absence of a velocity field. Lifting processes only take place at the boundary between the solid and the fluid phase.
Actually, the model is unable to reproduce the equilibrium stationary condition. Clearly the main problem with this sort of computations is related with the large difference in the time scales for the flow evolution and for the evolution of the seabed configuration: the morphological time scale is much larger than the hydrodynamic one. The vortex analysis performed for the rigid-bottom configuration, shows a good agreement with the experimental available results. The vortex motion is well reproduced for both KC=6 and
KC=26. The simulations performed with an erodible sandy seabed made use of only 40
wave cycles, the model seems to not reproduce correctly the scouring under a pipeline. The erosion below the cylinder, is reached too far downstream of the cylinder, while it is expected to occur about 1D downstream of the pipe, the numerical result shows a scour hole about 3D far from the pipe. Regarding the maximum scour depth, the model seems to reproduce fairly well the experimental available results, this is mainly due to the value of β
(lifting contribution) here used as a calibration parameter.
The experimental study performed with the rigid-bottom configuration analyzes the
boundary layer thickness, the streamwise vorticity dimension and the TKE equation. The
boundary layer thickness δ increases with KC, following a logarithmic law (figure C.1 -
Appendix C), while, the streamwise vortex dimension measured at the downstream side of
the pipe, increases with KC following a linear law (figure C.2 - Appendix C). The TKE
analysis, defines the contribute of each terms inside the equation (evolution + advection = production + transport + diffusion + dissipation). The results for the most energetic wave
condition reproduced show that the evolution term, upstream of the pipe (x/D=-1) shows a
peak for z/D=0.25 and z/D=1 at x/D=-0.5, it is almost negative over the pipe (x/D=0) and it decreases at the downstream side of the pipe for x/D=1 and x/D=1.5. The advection term is almost positive both upstream and downstream of the pipe. The production term is almost positive near the pipeline at the upstream end (x/D=1 and x/D=0.5), the profile exhibits a double peaks in correspondence of x/D=0. Far from the pipeline (x/D= -1.5 and x/D= 1.5),
the production term decreases. The transport term increases rapidly reaching a maximum
for x/D=0.5 showing a peak for z/D=0.6 on the top of the pipe. The dissipation term
exhibits a peak near the bottom for all the sections studied, while the diffusion term is one order of magnitude smaller than all the other terms and seems not to give a fundamental contribution on the turbulence dynamics. An approximate balance between the evolution term and the production term upstream of the cylinder, a balance between evolution plus advection and the transport term above the cylinder and a balance between the advection and transport terms downstream of the pipeline were found.
For the experimental analysis, to classify the scouring process, three distinctive regimes were recognized: i) no scour regime for KC15 acquired a shape similar to the surface of a non-linear wave. A hill was created at a distance of x/D=0.5 between two throughs with a gentle slope, due to the “jet” contribution of the
tunnel erosion and the vortex developed downstream the pipe. The asymmetry of the scour hole is one aspect of the scouring mechanism not completely well explained as yet, further analysis will be performed in order to better understand the sediment dynamics and explain the physics of the phenomenon. An intuitive aspect found observing the experimental
results is that, even if the piping and seepage flow do not occur, the sediment motion may create a scour hole around the cylinder. If the slope of the scour hole is sufficiently large,some sediment under the pipe may slide down, leading to the failure of the pipe foundation.
This aspect is extremely relevant because it can give a new definition for the onset of scour.
The empirical equation of Sumer and Fredsoe (1990) and Cevik and Yuksel (1999) on the
measured S/D vs. KC, shows a good agreement with the experimental results. The
measured scour widths in term of W/D, plotted against KC compared with the empirical equation of Catano-Lopera and Garcia (2007) and Sumer and Fredsoe (2002), shows a worse agreement. The Catano-Lopera formula overestimates the scour width for KC 15, while, the Sumer and Fredsoe fails for KC>15.
In Chapter 1 a literature review concerning the mechanics of scour is presented.
In Chapter 2 a numerical DNS solver of the Navier-Stokes equation is proposed. The scour
conditions were simulated as forced by both currents and waves and some comparisons
with the available experimental literature test were performed (Chapter 3).
Chapter 4 illustrates a physical model on the pipe-soil interaction between a pipe and a rigid-erodible bed. Two different configurations were tested, in the former a PTV was applied to reconstruct the hydrodynamics around the pipeline, while the latter focuses on the influence of short and long waves on a pipe over a coarse sand bed. The results of the experimental tests are shown in Chapter 5.
Chapter 6 closes the present research
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
- …
