4 results
Wake structure of an array of cylinders in shallow flow
- Fei He, Marco Ghisalberti, Hongwei An, Scott Draper, Paul Branson, Chengjiao Ren, Liang Cheng
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- Journal:
- Journal of Fluid Mechanics / Volume 986 / 10 May 2024
- Published online by Cambridge University Press:
- 10 May 2024, A30
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Although there is a range of approaches for classifying the wake structure behind an array of obstacles, these approaches provide inconsistent results across different array systems. This motivates the present study to integrate and reconcile these approaches into one that is consistent across different systems. This new, transferable classification approach is based on the dimensionless flow blockage of the array and the wake stability parameter. To demonstrate this approach, a series of laboratory experiments was conducted to characterise the wake structure behind an array of emergent cylinders across a practically relevant parameter space that has not previously been explored. Two arrays with the same values of flow blockage and wake stability parameters but different sizes display the same wake structure, demonstrating the controlling influence of these two parameters on the wake structure. This approach classifies four different wake structures, which are distinct in that they display differences in instantaneous and time-averaged flow fields, temporal velocity oscillations, shear layer growth and the length of the steady wake region. The dependence of the wake structure on the two parameters is a consequence of (i) the controlling influence of blockage on the fraction of incident flow passing through the array and (ii) the ability of shallowness to suppress wake instabilities and, to a lesser extent, also influence the velocity through the array. This paper provides a predictive framework for the wake structure based on knowledge of the array geometry, and the depth and velocity of incident flow across the entire relevant practical parameter space.
Cylinder wakes in shallow oscillatory flow: the coastal island wake problem
- Paul M. Branson, Marco Ghisalberti, Gregory N. Ivey, Emil J. Hopfinger
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- Journal:
- Journal of Fluid Mechanics / Volume 874 / 10 September 2019
- Published online by Cambridge University Press:
- 04 July 2019, pp. 158-184
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Topographic complexity on continental shelves is the catalyst that transforms the barotropic tide into the secondary and residual circulations that dominate vertical and cross-shelf mixing processes. Island wakes are one such example that are observed to significantly influence the transport and distribution of biological and physical scalars. Despite the importance of island wakes, to date, no sufficient, mechanistic description of the physical processes governing their development exists for the general case of unsteady tidal forcing. Controlled laboratory experiments are necessary for the understanding of this complex flow phenomenon. Here, three-dimensional velocity field measurements of cylinder wakes in shallow-water oscillatory flow are conducted across a parameter space that is typical of tidal flow around shallow islands. The wake form in steady flows is typically described in terms of the stability parameter $S=c_{f}D/h$ (where $D$ is the island diameter, $h$ is the water depth and $c_{f}$ is the bottom boundary friction coefficient); in tidal flows, there is an additional dependence on the Keulegan–Carpenter number $KC=U_{0}T/D$ (where $U_{0}$ is the tidal velocity amplitude and $T$ is the tidal period). In this study we demonstrate that when the influence of bottom friction is confined to a Stokes boundary layer the stability parameter is given by $S=\unicode[STIX]{x1D6FF}^{+}/KC$ where $\unicode[STIX]{x1D6FF}^{+}$ is the ratio of the wavelength of the Stokes bottom boundary layer to the depth. Three classes of wake form are observed with decreasing wake stability: (i) steady bubble for $S\gtrsim 0.1$; (ii) unsteady bubble for $0.06\lesssim S\lesssim 0.1$; and (iii) vortex shedding for $S\lesssim 0.06$. Transitions in wake form and wake stability are shown to depend on the magnitude and temporal evolution of the wake return flow. Scaling laws are developed to allow upscaling of the laboratory results to island wakes. Vertical and lateral transport depend on three parameters: (i) the flow aspect ratio $h/D$; (ii) the amplitude of tidal motion relative to the island size, given by $KC$; and (iii) the relative influence of bottom friction to the flow depth, given by $\unicode[STIX]{x1D6FF}^{+}$. A model of wake upwelling based on Ekman pumping from the bottom boundary layer demonstrates that upwelling in the near-wake region of an island scales with $U_{0}(h/D)KC^{1/6}$ and is independent of the wake form. Finally, we demonstrate an intrinsic link between the dynamical eddy scales, predicted by the Ekman pumping model, and the island wake form and stability.
The Cognitive Behaviour Therapy Scale for Children and Young People (CBTS-CYP): Development and Psychometric Properties
- Paul Stallard, Pam Myles, Amanda Branson
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- Journal:
- Behavioural and Cognitive Psychotherapy / Volume 42 / Issue 3 / May 2014
- Published online by Cambridge University Press:
- 04 February 2014, pp. 269-282
- Print publication:
- May 2014
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Background: There is increased interest in developing training in cognitive behaviour therapy (CBT) with children and young people. However, the assessment of clinical competence has relied upon the use of measures such as the Cognitive Therapy Scale-Revised (CTS-R: Blackburn et al., 2001) which has been validated to assess competence with adults. The appropriateness of this measure to assess competence when working with children and young people has been questioned. Aim: This paper describes the development and initial evaluation of the Cognitive Behaviour Therapy Scale for Children and Young People (CBTS-CYP) developed specifically to assess competence in CBT with children and young people. Method: A cross section of child CBT practitioners (n = 61) were consulted to establish face validity. Internal reliability, convergent validity and discriminative ability were assessed in two studies. In the first, 12 assessors independently rated a single video using both the Cognitive Behaviour Therapy Scale for Children and Young People (CBTS-CYP) and Cognitive Therapy Scale-Revised (CTS-Revised: Blackburn et al., 2001). In the second, 48 different recordings of CBT undertaken with children and young people were rated on both the CBTS-CYP and CTS-R. Results: Face validity and internal reliability of the CBTS-CYP were high, and convergent validity with the CTS-R was good. The CBTS-CYP compared well with the CTS-R in discriminative ability. Conclusion: The CBTS-CYP provides an appropriate way of assessing competence in using CBT with children and young people. Further work is required to assess robustness with younger children and the impact of group training in reducing inter-rater variations.
2 - International finance and economic development
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- By Paul Krugman, MIT and CEPR, Richard E. Baldwin, Institut Universitaire des Hautes Etudes Internationales, Genève and CEPR, William H. Branson, Princeton University and CEPR
- Edited by Alberto Giovannini, Columbia University, New York
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- Book:
- Finance and Development
- Published online:
- 05 November 2011
- Print publication:
- 25 March 1993, pp 11-28
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Summary
Although the 1980s was a decade of enormous activity in international financial markets, little of this activity translated into net resource transfers from capital-rich to capital-scarce countries. Indeed, as a result of the debt crisis resource transfers generally went from South to North. As the 1990s begin, however, the prospects for substantial external finance for development seems brighter, at least in some areas. In Europe, Portugal and Spain have recently attracted substantial inflows of external capital; it is widely believed, although some are sceptical, that reforming Eastern European nations may also be able to attract considerable external finance. In North America, Mexican economic reforms and the prospect of a free trade agreement have enabled that nation to resume voluntary access to the world capital market, with large inflows of direct investment in particular. In both the European and North American context, capital inflows have been widely seen both as a vote of confidence in the future economic growth of the recipients and as a key force propelling those growth prospects.
Some observers (e.g. Hale, 1991) have gone further and suggested that with the victory of the West in the Cold War, the stage is now set for a second golden age of capitalism. In this new golden age, the optimists suggest, international net capital flows may again rise to levels as a share of world product comparable to those in the pre- World War I era, and a worldwide convergence of per capita income will be the result.