1,720,953 research outputs found
Towards predicting cavitation noise using scale-resolving simulations: The importance of inflow turbulence
No full textThere is increasing attention for the effects of anthropogenic underwater radiated noise (URN) on marine fauna. This is expected to lead to regulations with respect to the maximum permitted sound emissions of ships. It is known that cavitating tip vortices, generated by ship propellers, are some of the key contributors to URN. Consequently, there is a need to evaluate propeller designs with respect to noise generation in a design stage. Computational fluid dynamics (CFD) has the potential to offer detailed insights into cavitating vortex dynamics and noise sources, at a reasonable cost. URN can be efficiently estimated using CFD in combination with an acoustic analogy. In order to use such predictions in a design process, it is essential to understand and quantify the errors associated with the numerical predictions of noise sources. This thesis investigates the reliability of such evaluations and aims to reduce occurring modelling errors.To compute noise sources, it is necessary to simulate cavitation dynamics using scale-resolving simulations (SRS). Here, part of the turbulence kinetic energy spectrum is resolved in space and time, as opposed to being modelled using Reynolds averaged Navier-Stokes (RANS). The SRS method of choice in this work is the partially averaged Navier-Stokes (PANS) method. Bridging models, such as PANS, exhibit a smooth transition and absence of commutation errors between RANS and large eddy simulation (LES) zones, in contrast to hybrid models such as detached eddy simulation (DES). The formulation allows for a theoretical decoupling of the discretisation and modelling errors, thereby enabling verification and validation processes.PANS allows the user to select the ratios of resolved-to-total turbulence kinetic energy and dissipation (rate). Appropriate settings and methods to estimate these settings a priori are investigated. Furthermore, a new PANS closure is developed, which offers improved convergence behaviour compared to more commonly used models, and is better suited to application for multiphase flows. It has been shown repeatedly in literature that SRS should be accompanied by physical inflow boundary conditions, where time-varying fluctuations, resembling turbulence, should be inserted upstream of the object of interest, to prevent laminar solutions. However, from literature it is clear that for maritime applications this is often neglected. To the knowledge of the author, there is no previous application of such an inflow in combination with cavitation. In this PhD thesis, a synthetic inflow turbulence generator (ITG) is implemented, and tested for several test cases in wetted and cavitating conditions. For these cases, the numerical errors, consisting of discretisation, iterative and statistical errors are evaluated.Firstly, the results when using the ITG are compared against recycling flow results for a turbulent channel flow, using different SRS methods. It was shown that the ITG can deliver a resolved turbulent inflow at lower computational cost. Secondly, the effect of neglecting such an inflow was tested for the Delft Twist 11 hydrofoil, where it was shown that simulating such a flow with a low ratio of resolved-to-total turbulence kinetic energy can lead to flow separation at the wing leading edge. This is in contrast to experimentally observed behaviour. The inclusion of the ITG can reduce this modelling error, although the sheet cavity dynamics remain largely unaffected. Finally, an elliptical wing with a cavitating tip vortex is simulated. The observed vortex dynamics are compared against a semi-analytical model from literature. To obtain vortex dynamics, the ITG was shown to be necessary. The far-field noise generated by the vortex is quantified and related to the cavity dynamics.Some of the main contributions of this research are improved insight in the use of SRS in cavitating conditions, in simulating cavity dynamics and in using an ITG to obtain flow fields representative of experimental conditions. In this way it has enhanced our understanding of the ability and limitations in the prediction of acoustic sources due to cavitation. To improve predictions of cavitation dynamics it is recommended to address the cavitation model and the method which describes the cavity interface, to reduce the discrepancy in average cavity size between simulations and experimental observations.Ship Hydromechanics and Structure
The Development of a Partially Averaged Navier-Stokes KSKL Model
No full textA new partially averaged Navier-Stokes (PANS) closure is derived based on the k-kL (KSKL) model. The aim of this new model is to incorporate the desirable features of the KSKL model, compared to the k-ω shear stress transport model, into the PANS framework. These features include reduced eddy-viscosity levels, a lower dependency on the cell height at the wall, well-defined boundary conditions, and improved iterative convergence. As well as the new model derivation, the paper demonstrates that these desirable features are indeed maintained, for a range of modeled-to-total turbulence kinetic energy ratios (fk), and even for multiphase flow.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Ship Hydromechanics and Structure
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
On the accuracy of partially averaged Navier–Stokes resolution estimates
No full textPartially Averaged Navier–Stokes computations can employ three different approaches for specifying the ratio of modelled-to-total turbulence kinetic energy fk. Use can be made of either a constant, a spatially- or a spatially- and temporally-varying value. This work compares different estimates for fk found in literature and evaluates them for two test-cases: a circular cylinder at Re=3900 and a turbulent channel flow at Reτ=395. Additionally, the estimates are compared to the a posteriori computed ratio of modelled-to-total turbulence kinetic energy()hfk˜ obtained from the PANS flow solution. The trends observed for the estimates are similar, although the magnitude varies significantly. All spatially varying fk approaches reduce the PANS model to a DES-like model, thereby entangling modelling and discretisation errors. At the same time, fk˜ shows that the behaviour of these estimates is incorrect: fk becomes too large near the wall of the object and in the far field. It is observed that fk˜ is always lower than the set value, when using fk fixed in space and time. Finally, it is clear that the estimates, applied to internal, boundary layer, flows yield too high values for fk. In order to minimise errors and increase the reliability of industrial CFD results, the approach with a constant fk is still preferable, assuming suitably fine grids are used.Ship Hydromechanics and Structure
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
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