1,720,975 research outputs found
Experimental and numerical study of hydrodynamic responses of a twin-tube submerged floating tunnel considering vortex-induced vibration
Get PDFA pontoon-supported submerged floating tunnel (SFT) is a reasonable means crossing a fjord with large depth and width. A relevant design of such a tunnel consists of two identical tubes in a tandem configuration and suspended approximately 20-30 m below the mean sea level. The two tubes are rigidly connected by large diameter diagonal truss elements. The natural frequencies of the low-order modes of the full-scale SFT (the first vertical motion-dominant mode and the first two lateral motion-dominant modes) are well within the energy content of the spectrum of vortex shedding induced forces and the difference-frequency wave excitation force. The vortexinduced vibration (VIV) and wave-induced hydroelastic response represent the main challenges in the design of the SFT. Regarding the above two considerations, this work aims at investigating the following aspects: (1) VIV features of a tandem twin-cylinder model and the lift and drag coefficients used in the simplified time domain VIV prediction method. (2) Hydrodynamic features of the SFT considering the 2nd order wave force acting on pontoons, short-crested waves and VIV.
A series of self-oscillation tests were first conducted with an elastically supported rigidly connected tandem twin rigid-cylinder model, to gain a better understanding of the VIV features of an SFT with respect to the ow velocity, spacing distance and submergence. Nondimensional parameters including the reduced velocity, spacing and submergence ratios build relations between the test model and prototype. The crossow (CF) response amplitude and lift force coe_cients were observed to be strongly related to the spacing distance and ow velocity, as expected. The interaction between the twin cylinders leads to a smaller critical reduced velocity (corresponding to the initiation of the VIV) and response frequency than those of a single cylinder when the spacing ratio is less than 4. The lift forces on the two cylinders strongly depend on the reduced velocity, but their rates of change are different. This difference induces an extra torsional moment, which is also sinusoidal in time but quadratic in the ow velocity. An abrupt in crease in the torsional moment is observed at a small reduced velocity when the spacing ratio is 2, and a large final constant value is found in the case of a spacing ratio equal to 4. In addition to the CF lift forces, VIV also strongly amplifies the inline (IL) drag forces, and they achieve their maxima coincidently with the response amplitude. Two drag forces are in direction with each other in the VIV condition rather than opposite directions in stationary result for spacing ratios of 2 and 3. The influence of the free surface on the response amplitude and drag forces can be neglected when the newly defined nondimensional depth h' (h0 = fnhU , where U, fn and h denote the flow velocity, natural frequency and submergence, respectively) is larger than 0.55.
Regarding the wave-induced hydroelastic features, the 2nd order wave forces dominate the lateral motion by exciting the low frequency eigenmodes but have small effects on the vertical motion. The long-crest first order waves induce nearly twice as large lateral bending moments (Mz) as shortcrest waves, by including the 2nd order forces, the short-crest waves lead to a large Mz due to that they can excite both the 1st and 2nd natural modes in the lateral direction. Based on the simplified VIV prediction method (in which, the cross flow hydrodynamic force is modelled as a summation of the vortex-induced force relying on self-oscillation tests and the damping force based on the Morison equation), the maximum standard deviation of the vertical motion (STD/D 0:31) and the corresponding bending moment at the middle part is much larger than that induced by the long-crest waves. Except for the VIV in the central long span between pontoons, the VIV excitation is limited
Ramp Tests: A Novel Approach to VIV Model Testing of Flexible Cylinders Using Continuously Varying Towing Speeds
Get PDFIn this paper we present a small portion of the results from the 2011 SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 150,000 in order to study the Vortex-Induced Vibration (VIV) response in uniform current profiles. This paper presents the experimental results collected from a series of tests where the towing speed was continuously varied while the cylinder and carriage traversed the basin. As the cylinder is accelerated (or decelerated) the incident current speed is continuously changing which means that multiple modes can be excited consecutively in a single tow through the basin. These varying towing speed tests are collectively referred to as ‘ramp tests’. The response data collected in these ‘ramp tests’ are presented in terms of CF response amplitudes and strains and are carefully compared with the response data collected during conventional steady towing speed tests. The data shows that when the acceleration of the carriage is kept below a critical value the ‘ramp tests’ are then able to provide VIV response information which is equivalent to that obtained from many constant speed tests. One ramp test provides the equivalent response data of (up to) 10 constant speed tests in a single run. This paper also introduces a dimensionless parameter γ, which determines if the proposed acceleration for a ramp test is within acceptable limits and may be used as a substitute for many constant speed tests. The parameter can be used to determine the appropriate acceleration or deceleration rate in order to ensure that a ramp test will yield suitable VIV response data. The parameter also allows one to know quickly whether or not fully developed VIV is possible in a given set of unsteady flow conditions.SHEAR7 JIPSHELL International Exploration and Production Co
Dynamic Analysis of Semi-submersible Offshore Fish Farm Operated in China East Sea
Get PDFIn recent years, the aquaculture industry in China has achieved rapid development, which covers more than half of the world aquaculture production. For further development, Chinese aquaculture companies are looking for advanced farm technology available for exposed ocean condition. The semi-submersible offshore fish farm is one of the most foreseeable equipment, which is being tested in the Norwegian Sea. This thesis aims to simulate the dynamic performance of a semi-submersible offshore fish farm operated in Chinese ocean.
The farm model is constructed based on the offshore farm "Ocean Farm 1", operated by SalMar ASA. The target location of this farm is in China East Sea with a depth of around 100m. Some simplification is thus carried out at the design stage considering the actual situation in China. There are some components of the model: a main body including central pontoon, steel net frame and connectors; fish nets; mooring system.
A dynamic analysis is conducted to study the response of farm structure in both frequency domain and time domain. In order to calculate hydrodynamic properties of the farm structure, a composite model with both Morison beam and panel surface is constructed in GeniE. And simulating the composite model in WADAM, RAOs of response motion in six degrees of freedom are obtained. On the other hand, time domain analysis is executed in WASIM program, and compared with the results of frequency domain analysis. The kinetic results of the farm body acquired in WADAM are used as the input data of SIMO program to implement coupled analysis of mooring system in time domain. A catenary mooring system is modeled in SIMA to check its capability of keeping position in ultimate sea states.
For all of the simulations carried out in SIMO program, the mooring lines presented to be capable of withstanding the hydrodynamic loads from waves and currents. This thesis compares the results of motion under various environment conditions. It indicates that current dominates drift motion of the farm compared with wave drift force. However, when considering the oscillation motion of structure, wave force comes to be the main factor. Due to the symmetry of farm structure, wave from different directions only result in slight difference, and the effect of misalignment of wave and current also shows to be insignificant.
Furthermore, mooring systems with various pre-tension are tested in the same environment. The consequences of simulation verify that the pre-tension of mooring line has a significant effect on the drift of floating farm body. With the increasing of mooring line pre-tension, the performance of mooring system can be enhanced. Finally, the simplified quasi-static mooring system in WASIM is compared with the catenary mooring system in SIMO. And it illustrates the influence of nonlinear effects: the results from SIMO are less in accordance with wave motion
Dynamic Analysis of Semi-submersible Offshore Fish Farm Operated in China East Sea
No full textIn recent years, the aquaculture industry in China has achieved rapid development, which covers more than half of the world aquaculture production. For further development, Chinese aquaculture companies are looking for advanced farm technology available for exposed ocean condition. The semi-submersible offshore fish farm is one of the most foreseeable equipment, which is being tested in the Norwegian Sea. This thesis aims to simulate the dynamic performance of a semi-submersible offshore fish farm operated in Chinese ocean.
The farm model is constructed based on the offshore farm "Ocean Farm 1", operated by SalMar ASA. The target location of this farm is in China East Sea with a depth of around 100m. Some simplification is thus carried out at the design stage considering the actual situation in China. There are some components of the model: a main body including central pontoon, steel net frame and connectors; fish nets; mooring system.
A dynamic analysis is conducted to study the response of farm structure in both frequency domain and time domain. In order to calculate hydrodynamic properties of the farm structure, a composite model with both Morison beam and panel surface is constructed in GeniE. And simulating the composite model in WADAM, RAOs of response motion in six degrees of freedom are obtained. On the other hand, time domain analysis is executed in WASIM program, and compared with the results of frequency domain analysis. The kinetic results of the farm body acquired in WADAM are used as the input data of SIMO program to implement coupled analysis of mooring system in time domain. A catenary mooring system is modeled in SIMA to check its capability of keeping position in ultimate sea states.
For all of the simulations carried out in SIMO program, the mooring lines presented to be capable of withstanding the hydrodynamic loads from waves and currents. This thesis compares the results of motion under various environment conditions. It indicates that current dominates drift motion of the farm compared with wave drift force. However, when considering the oscillation motion of structure, wave force comes to be the main factor. Due to the symmetry of farm structure, wave from different directions only result in slight difference, and the effect of misalignment of wave and current also shows to be insignificant.
Furthermore, mooring systems with various pre-tension are tested in the same environment. The consequences of simulation verify that the pre-tension of mooring line has a significant effect on the drift of floating farm body. With the increasing of mooring line pre-tension, the performance of mooring system can be enhanced. Finally, the simplified quasi-static mooring system in WASIM is compared with the catenary mooring system in SIMO. And it illustrates the influence of nonlinear effects: the results from SIMO are less in accordance with wave motion
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
Experimental and numerical study of hydrodynamic responses of a twin-tube submerged floating tunnel considering vortex-induced vibration
No full textA pontoon-supported submerged floating tunnel (SFT) is a reasonable means crossing a fjord with large depth and width. A relevant design of such a tunnel consists of two identical tubes in a tandem configuration and suspended approximately 20-30 m below the mean sea level. The two tubes are rigidly connected by large diameter diagonal truss elements. The natural frequencies of the low-order modes of the full-scale SFT (the first vertical motion-dominant mode and the first two lateral motion-dominant modes) are well within the energy content of the spectrum of vortex shedding induced forces and the difference-frequency wave excitation force. The vortexinduced vibration (VIV) and wave-induced hydroelastic response represent the main challenges in the design of the SFT. Regarding the above two considerations, this work aims at investigating the following aspects: (1) VIV features of a tandem twin-cylinder model and the lift and drag coefficients used in the simplified time domain VIV prediction method. (2) Hydrodynamic features of the SFT considering the 2nd order wave force acting on pontoons, short-crested waves and VIV.
A series of self-oscillation tests were first conducted with an elastically supported rigidly connected tandem twin rigid-cylinder model, to gain a better understanding of the VIV features of an SFT with respect to the ow velocity, spacing distance and submergence. Nondimensional parameters including the reduced velocity, spacing and submergence ratios build relations between the test model and prototype. The crossow (CF) response amplitude and lift force coe_cients were observed to be strongly related to the spacing distance and ow velocity, as expected. The interaction between the twin cylinders leads to a smaller critical reduced velocity (corresponding to the initiation of the VIV) and response frequency than those of a single cylinder when the spacing ratio is less than 4. The lift forces on the two cylinders strongly depend on the reduced velocity, but their rates of change are different. This difference induces an extra torsional moment, which is also sinusoidal in time but quadratic in the ow velocity. An abrupt in crease in the torsional moment is observed at a small reduced velocity when the spacing ratio is 2, and a large final constant value is found in the case of a spacing ratio equal to 4. In addition to the CF lift forces, VIV also strongly amplifies the inline (IL) drag forces, and they achieve their maxima coincidently with the response amplitude. Two drag forces are in direction with each other in the VIV condition rather than opposite directions in stationary result for spacing ratios of 2 and 3. The influence of the free surface on the response amplitude and drag forces can be neglected when the newly defined nondimensional depth h' (h0 = fnhU , where U, fn and h denote the flow velocity, natural frequency and submergence, respectively) is larger than 0.55.
Regarding the wave-induced hydroelastic features, the 2nd order wave forces dominate the lateral motion by exciting the low frequency eigenmodes but have small effects on the vertical motion. The long-crest first order waves induce nearly twice as large lateral bending moments (Mz) as shortcrest waves, by including the 2nd order forces, the short-crest waves lead to a large Mz due to that they can excite both the 1st and 2nd natural modes in the lateral direction. Based on the simplified VIV prediction method (in which, the cross flow hydrodynamic force is modelled as a summation of the vortex-induced force relying on self-oscillation tests and the damping force based on the Morison equation), the maximum standard deviation of the vertical motion (STD/D 0:31) and the corresponding bending moment at the middle part is much larger than that induced by the long-crest waves. Except for the VIV in the central long span between pontoons, the VIV excitation is limited
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
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