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Mean dynamics and transition to turbulence in oscillatory channel flow
- Alireza Ebadi, Christopher M. White, Ian Pond, Yves Dubief
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- Journal:
- Journal of Fluid Mechanics / Volume 880 / 10 December 2019
- Published online by Cambridge University Press:
- 18 October 2019, pp. 864-889
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The mean dynamics in oscillatory channel flow is examined to investigate the dynamical mechanisms underlying the transition to turbulence in oscillatory wall-bounded flow. The analyses employ direct numerical simulation data acquired at three Stokes Reynolds numbers: $Re_{s}=648$, 801 and 1009, where the lower $Re_{s}$ flow is transitional over the entire cycle and the two higher $Re_{s}$ flows exhibit flow characteristics similar to steady turbulent wall-bounded flow during part of the cycle. The flow evolution over a half-period of oscillation for all three $Re_{s}$ is as follows: near-wall streamwise velocity streaks develop during the early accelerating portion of the cycle; then at some later point in the cycle that depends on $Re_{s}$, the near-wall streaks breakdown (demarking the onset of the nonlinear development stage), and the near-wall Reynolds stress grows explosively; the Reynolds stress remains elevated for part of the cycle before diminishing (yet remaining finite) during the late decelerating portion of the cycle. This process is then repeated indefinitely. The present findings demonstrate that transition to turbulence occurs when the nonlinear development stage begins during the accelerating portion of the cycle. This crucially leads to the diminishing importance of the centreline momentum source, the emergence of a locally accelerating/decelerating internal layer centred about the edge of the Stokes layer and the wall-normal rearrangement of the mean forces prior to the start of the decelerating portion of the cycle. The rearrangement of mean forces culminates in a four layer structure in the mean balance of forces. This is significant on a number of accounts since empirical and theoretical evidence suggests that the formation of a four layer structure is an important characteristic of a self-similar hierarchal structure that underlies logarithmic dependence of the mean velocity profile in steady turbulent wall-bounded flows (Klewicki et al., J. Fluid Mech., vol. 638, 2009, pp. 73–93). When the nonlinear development stage begins during the decelerating portion of the cycle (i.e. at $Re_{s}=648$), a four layer structure is not observed in the mean balance of forces and the flow remains weakly transitional over the entire cycle.
Quality assessment of systematic reviews of vitamin D, cognition and dementia
- Fariba Aghajafari, Dimity Pond, Nigel Catzikiris, Ian Cameron
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- Journal:
- BJPsych Open / Volume 4 / Issue 4 / July 2018
- Published online by Cambridge University Press:
- 09 July 2018, pp. 238-249
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Background
There is conflicting evidence regarding the association of vitamin D with cognition performance and dementia.
AimsWe aimed to summarise the evidence on the association of vitamin D with cognitive performance, dementia and Alzheimer disease through a qualitative assessment of available systematic reviews and meta-analyses.
MethodWe conducted an overview of the systematic reviews of all study types with or without meta-analyses on vitamin D and either Alzheimer disease, dementia or cognitive performance up to June 2017.
ResultsEleven systematic reviews were identified, nine of which were meta-analyses with substantial heterogeneity, differing statistical methods, variable methodological quality and quality of data abstraction. A Measurement Tool to Assess Systematic Reviews checklist scores ranged from 4 to 10 out of 11, with seven reviews of ‘moderate’ and four of ‘high’ methodological quality. Out of six meta-analyses on the association between low serum concentration of 25-hydroxyvitamin D and risk of dementia, five showed a positive association. Results of meta-analyses on the association between low serum concentration of 25-hydroxyvitamin D and memory function tests showed conflicting results.
ConclusionsThis systematic evaluation of available systematic reviews provided a clearer understanding of the potential link between low serum vitamin D concentrations and dementia. This evaluation also showed that the quality of the available evidence is not optimal because of both the low methodological quality of the reviews and low quality of the original studies. Interpretation of these systematic reviews should therefore be made with care.
Declaration of interestNone.