6 results
Breaking wave field statistics with a multi-layer model
- Jiarong Wu, Stéphane Popinet, Luc Deike
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- Journal:
- Journal of Fluid Mechanics / Volume 968 / 10 August 2023
- Published online by Cambridge University Press:
- 31 July 2023, A12
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The statistics of breaking wave fields are characterised within a novel multi-layer framework, which generalises the single-layer Saint-Venant system into a multi-layer and non-hydrostatic formulation of the Navier–Stokes equations. We simulate an ensemble of phase-resolved surface wave fields in physical space, where strong nonlinearities, including directional wave breaking and the subsequent highly rotational flow motion, are modelled, without surface overturning. We extract the kinematics of wave breaking by identifying breaking fronts and their speed, for freely evolving wave fields initialised with typical wind wave spectra. The $\varLambda (c)$ distribution, defined as the length of breaking fronts (per unit area) moving with speed $c$ to $c+{\rm d}c$ following Phillips (J. Fluid Mech., vol. 156, 1985, pp. 505–531), is reported for a broad range of conditions. We recover the $\varLambda (c) \propto c^{-6}$ scaling without wind forcing for sufficiently steep wave fields. A scaling of $\varLambda (c)$ based solely on the root-mean-square slope and peak wave phase speed is shown to describe the modelled breaking distributions well. The modelled breaking distributions are in good agreement with field measurements and the proposed scaling can be applied successfully to the observational data sets. The present work paves the way for simulations of the turbulent upper ocean directly coupled to a realistic breaking wave dynamics, including Langmuir turbulence, and other sub-mesoscale processes.
Revisiting wind wave growth with fully coupled direct numerical simulations
- Jiarong Wu, Stéphane Popinet, Luc Deike
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- Journal:
- Journal of Fluid Mechanics / Volume 951 / 25 November 2022
- Published online by Cambridge University Press:
- 04 November 2022, A18
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We investigate wind wave growth by direct numerical simulations solving for the two-phase Navier–Stokes equations. We consider the ratio of the wave speed $c$ to the wind friction velocity $u_*$ from $c/u_*= 2$ to 8, i.e. in the slow to intermediate wave regime; and initial wave steepness $ak$ from 0.1 to 0.3; the two being varied independently. The turbulent wind and the travelling, nearly monochromatic waves are fully coupled without any subgrid-scale models. The wall friction Reynolds number is 720. The novel fully coupled approach captures the simultaneous evolution of the wave amplitude and shape, together with the underwater boundary layer (drift current), up to wave breaking. The wave energy growth computed from the time-dependent surface elevation is in quantitative agreement with that computed from the surface pressure distribution, which confirms the leading role of the pressure forcing for finite amplitude gravity waves. The phase shift and the amplitude of the principal mode of surface pressure distribution are systematically reported, to provide direct evidence for possible wind wave growth theories. Intermittent and localised airflow separation is observed for steep waves with small wave age, but its effect on setting the phase-averaged pressure distribution is not drastically different from that of non-separated sheltering. We find that the wave form drag force is not a strong function of wave age but closely related to wave steepness. In addition, the history of wind wave coupling can affect the wave form drag, due to the wave crest shape and other complex coupling effects. The normalised wave growth rate we obtain agrees with previous studies. We make an effort to clarify various commonly adopted underlying assumptions, and to reconcile the scattering of the data between different previous theoretical, numerical and experimental results, as we revisit this longstanding problem with new numerical evidence.
Reducing radiation dose in paediatric interventional cardiac catheterisation
- Jiarong Bai, Feng Wang, Haosheng Yang, Ying Lu, Lin Wu
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- Journal:
- Cardiology in the Young / Volume 29 / Issue 7 / July 2019
- Published online by Cambridge University Press:
- 09 July 2019, pp. 967-971
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Objective:
Radiation exposure during paediatric cardiac catheterisation procedures should be minimised to “as low as reasonably achievable”. The aim of this study was to evaluate the effectiveness of a modified radiation safety protocol in reducing patient dose during paediatric interventional cardiac catheterisation.
Methods:Radiation dose data were retrospectively extracted from January 2014 to December 2015 (Standard group) and prospectively collected from January 2016 to December 2017 (Low-dose group) after implementation of a modified radiation safety protocol. Both groups included five most common procedures: atrial septal defect closure, patent ductus arteriosus closure, perimembranous ventricular septal defect closure, pulmonary valvuloplasty, and supraventricular tachycardia ablation.
Results:Median air Kerma was 48.4, 50.5, 29.75, 149, 218, and 12.9 mGy for atrial septal defect closure, pulmonary valvuloplasty, patent ductus arteriosus closure <20 kg, ventricular septal defect closure <20 kg, ventricular septal defect closure ≧20 kg, and supraventricular tachycardia ablation in Standard group, respectively, which significantly decreased to 18.75, 20.7, 11.5, 41.9, 117, and 3.3 mGy in Low-dose group (p < 0.05). This represents a reduction in dose to each patient between 46 and 74%. Among five procedural types in Low-dose group, dose of ventricular septal defect closure was the highest with median air Kerma of 62.5 mGy, dose area product of 364.7 μGy.m2, and dose area product per body weight of 21.5 μGy.m2/kg, respectively, along with the longest fluoroscopy time of 9.9 minutes.
Conclusion:We provided a feasible radiation safety protocol with specific settings on a case-by-case basis. Increasing awareness and adequate training of a practical radiation dose reduction program are essential to improve radiation protection for children.
On the internal flow of a ventilated supercavity
- Yue Wu, Yun Liu, Siyao Shao, Jiarong Hong
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- Journal:
- Journal of Fluid Mechanics / Volume 862 / 10 March 2019
- Published online by Cambridge University Press:
- 16 January 2019, pp. 1135-1165
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This study presents an experimental investigation on the internal flow of a ventilated supercavity using fog flow visualization and particle image velocimetry (PIV) measurements. The ventilated supercavity is generated on a backward-facing cavitator and studied in the high-speed water tunnel at St. Anthony Falls Laboratory. Fog particles are introduced into the supercavity through the ventilation line, and then illuminated by a laser sheet for flow visualizations and PIV measurements. The experiments are performed on the supercavities with two closure types, i.e. the re-entrant jet (RJ) and the twin vortex (TV), under the same water tunnel flow condition but different ventilation rates. The flow visualization revealed three distinct regions within the supercavity, including the ventilation influence region near the cavitator, the extended internal boundary layer along the liquid–gas interface and the reverse flow region occupying a large centre portion of the supercavity. The streamwise and vertical extent of the ventilation influence region, the streamwise growth of the internal boundary layer and the reverse flow within the supercavity are then quantified through PIV flow measurements. Compared to the RJ case, the results indicate that the TV supercavity yields a longer vertical extent of the ventilation influence region, a thinner internal boundary layer and a stronger reverse flow. The internal flow results suggest that at the upstream of the location of the maximum cavity diameter, the gas enters the forward flow (including the internal boundary layer and the forward moving portion of the ventilation influence region) from the reverse flow, while at the downstream of that location, the gas is stripped from the internal boundary layer and enters the reverse flow due to the increasing adverse pressure gradient in the streamwise direction. The above results are combined with visualization results of the supercavity geometry and closure patterns to further explain the influence of gas leakage mechanisms on cavity growth and closure transition. Specifically, visualization of the cavity geometry change during the RJ to TV supercavity transition indicates external flow separation associated with a critical incline angle of the bottom liquid–gas interface at the closure contributes to the onset of RJ closure. The closure visualization shows the coexistence of the toroidal vortex and twin-vortex tubes for the RJ supercavity leads to two gas leakage mechanisms: one associated with the shedding of toroidal vortices ($Q_{RJ}$) and the other due to the gas entrained by the internal boundary layer and leaking from the twin-vortex tubes ($Q_{TV}$). For the RJ supercavity, with increasing ventilation input, due to the reduction of $Q_{RJ}$, the supercavity needs to elongate to increase the gas entrained by the internal boundary layer (i.e. $Q_{TV}$) to balance the ventilation increase. The elongation of the supercavity leads to reduced flow separation, and eventually a transition to the TV supercavity with ventilation above a critical value. For the TV supercavity, $Q_{RJ}$ is absent. An increase of ventilation input can be balanced by the increase of $Q_{TV}$ associated with the widening of the twin-vortex tubes, and therefore, no appreciable elongation of cavity length is observed.
Near-wake behaviour of a utility-scale wind turbine
- Teja Dasari, Yue Wu, Yun Liu, Jiarong Hong
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- Journal:
- Journal of Fluid Mechanics / Volume 859 / 25 January 2019
- Published online by Cambridge University Press:
- 16 November 2018, pp. 204-246
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Super-large-scale particle image velocimetry (SLPIV) and the associated flow visualization technique using natural snowfall have been shown to be effective tools to probe the turbulent velocity field and coherent structures around utility-scale wind turbines (Hong et al.Nat. Commun., vol. 5, 2014, article 4216). Here, we present a follow-up study using the data collected during multiple deployments from 2014 to 2016 around the 2.5 MW turbine at the EOLOS field station. These data include SLPIV measurements in the near wake of the turbine in a field of view of 115 m (vertical) $\times$ 66 m (streamwise), and the visualization of tip vortex behaviour near the elevation corresponding to the bottom blade tip over a broad range of turbine operational conditions. The SLPIV measurements provide velocity deficit and turbulent kinetic energy assessments over the entire rotor span. The instantaneous velocity fields from SLPIV indicate the presence of intermittent wake contraction states which are in clear contrast with the expansion states typically associated with wind turbine wakes. These contraction states feature a pronounced upsurge of velocity in the central portion of the wake. The wake velocity ratio $R_{w}$, defined as the ratio of the spatially averaged velocity of the inner wake to that of the outer wake, is introduced to categorize the instantaneous near wake into expansion ($R_{w}<1$) and contraction states ($R_{w}>1$). Based on the $R_{w}$ criterion, the wake contraction occurs 25 % of the time during a 30 min time duration of SLPIV measurements. The contraction states are found to be correlated with the rate of change of blade pitch by examining the distribution and samples of time sequences of wake states with different turbine operation parameters. Moreover, blade pitch change is shown to be strongly correlated to the tower and blade strains measured on the turbine, and the result suggests that the flexing of the turbine tower and the blades could indeed lead to the interaction of the rotor with the turbine wake, causing wake contraction. The visualization of tip vortex behaviour demonstrates the presence of a state of consistent vortex formation as well as various types of disturbed vortex states. The histograms corresponding to the consistent and disturbed states are examined over a number of turbine operation/response parameters, including turbine power and tower strain as well as the fluctuation of these quantities, with different conditional sampling restrictions. This analysis establishes a clear statistical correspondence between these turbine parameters and tip vortex behaviours under different turbine operation conditions, which is further substantiated by examining samples of time series of these turbine parameters and tip vortex patterns. This study not only offers benchmark datasets for comparison with the-state-of-the-art numerical simulation, laboratory and field measurements, but also sheds light on understanding wake characteristics and the downstream development of the wake, turbine performance and regulation, as well as developing novel turbine or wind farm control strategies.
6 - Classification and correlation of nonmarine Permo-Triassic boundary in China
- Edited by Walter C. Sweet, Ohio State University, Yang Zunyi, China University of Geosciences, Wukan, J. M. Dickins, Bureau of Mineral Resources, Canberra, Yin Hongfu, China University of Geosciences, Wukan
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- Book:
- Permo-Triassic Events in the Eastern Tethys
- Published online:
- 12 October 2009
- Print publication:
- 20 February 1992, pp 56-59
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Summary
Introduction
In China, nonmarine Permian and Triassic strata are distributed mainly in the vast area north of a line formed by the Kunlun, Qinling, and Dabie mountains. They are particularly well developed in the Junggar, Turpan, and Shaanxi-Gansu-Ningxia basins. South of the line, nonmarine Permo-Triassic sediments occur mostly in eastern Yunnan and western Guizhou. In addition, alternating marine and continental Permian and Triassic rocks are also developed in a few areas in both South and North China. In the past decade or so great progress has been made in studies of both the biological and nonbiological features of the nonmarine Permo-Triassic boundary. Existing data reveal distinct changes across the boundary in biologic groups such as vertebrates, bivalves, ostracodes, spores, pollen, and other flora, and in nonbiologic features such as grain size and color of sediments, sedimentary environments and climate. These changes suggest an event between the Permian and Triassic and can be applied to correlation of the nonmarine Permo-Triassic boundary.
Biotic changes
Determination of the Permo-Triassic boundary in nonmarine sequences is made at present mainly on the basis of vertebrates and spore-pollen assemblages, and to a lesser extent, on the basis of ostracodes, bivalves, and other elements of the flora.
Vertebrate fauna
The vertebrate fauna changes markedly from Permian to Triassic. Dicynodonts such as Kunpania scopulusa, Striodon magnus, Jimusaria sinkiangensis, Dicynodon tienshanensis, Jimusaria taoshuyanensis, Turfandon bogdaensis and Kansuodon sp. are represented in Upper Permian strata in the Junggar and Turpan basins of Xinjiang and Gansu provinces, northwest China (Zhao, 1980; Yang et al., 1986). A Pareiasaurus fauna, represented by Tapinocephalidae and Shihtienfenia permica, and Shansisaurus xuecunensis, has been reported from equivalent strata in Shanxi and Shaanxi provinces, North China.