•Flow development on pipe bottom is qualitatively the same as in vertical pipes.•Disturbance waves are created at pipe bottom and spread circumferentially.•Edges of disturbance waves may oscillate, ...spreading upward and retracting back.•The strongest circumferential asymmetry is observed for wave height.•Liquid lifting due to droplets and wave splashing cannot sustain annular film.
Transformation of gas-liquid flow in a horizontal pipe is investigated during the transition from stratified to annular flow pattern. Using Brightness-Based Laser-Induced Fluorescence technique, spatiotemporal evolution of liquid film thickness is analyzed over the downstream distance range of about 900 mm (45 pipe diameters), starting from the inlet. The measurements are carried out for three values of azimuthal angle θ: 0 (pipe bottom), 90°, and 180°, to track the circumferential spreading of liquid film and disturbance waves. At large gas velocities, thin liquid film is dragged upwards before the formation of large waves. The disturbance waves are created at the pipe bottom and spread circumferentially as they propagate downstream, over the already-wetted pipe walls. At large enough liquid flow rates, the spreading disturbance waves reach the pipe ceiling and form full rings around the circumference. The frequency and velocity of the disturbance waves eventually become the same around the pipe circumference; the disturbance wave amplitude and the base film thickness decrease with θ. The base film is more uniform around the circumference compared to the wave amplitude. At lower liquid flow rates, the disturbance waves cover only a part of pipe circumference. Their edges demonstrate oscillatory circumferential spreading, which ends by deceleration and decay of the edges. The upper part of the pipe may be wetted by a thin base film layer, covered only with ripples, or remain dry. At low gas speeds and large liquid flow rates, occasional splashing of large waves over the pipe ceiling without pre-wetting by the thin film is possible; however, the remaining film drains downward and no stable annular film is maintained. Liquid droplets, entrained from the pipe bottom and depositing in the upper parts of the pipe, are gathered in trains of creeping pendant droplets at the very top part of the pipe; no continuous wetting is achieved due to droplet deposition.
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The study of the ship airwake is critical as it explores the effect of unsteady wake flow on helicopter operations. This paper studied the near-wake flow topology of a generic ship at Re = 8 × 104 to ...assess the capability of a hybrid RANS/LES (Reynolds-averaged Navier–Stokes/large-eddy simulation) approach, known as IDDES (improved delayed detached-eddy simulation). The impact of computational parameters, including the mesh grid, residual level, time-step size, momentum discretization scheme and transient formulation, on the wake flow was investigated. The numerical results were validated by using the previous experimental data and LES results. The results show that except of the mesh resolution all other computational parameters varied in the current study do not have significant effect on the global drag forces, but showing large differences on the prediction of the local wake flow structures. This point has not been evidently reported in the previous work. The finer grid resolution is sufficient to produce an accurate qualitative and quantitative prediction of the flow structures, while using a poor grid resolution (coarse mesh) leads to inaccurate prediction of the flow topology. The recommended parameters for the time-step size (7.5 × 10−5s) and residual level (1 × 10−4) provide sufficient accuracy of wake predictions, showing good agreement with reference studies. For the convective term of the momentum equation in IDDES, the bounded central differencing scheme is proposed for its discretization, while the bounded second-order implicit is proposed to be adopted as the transient formulation.
•The ability of IDDES to predict the bi-stable phenomenon of a generic ship has been evaluated.•The effect of computational parameters on the ship flow topology has been studied.•IDDES results show good agreement with available numerical and experimental data.•Consistency of global quantities can not guarantee the same trend for the local flow topology.•Optimum computational parameters are proposed for the study of the bi-stable ship wake flow.
Flow networks with dendritic tube organization are common in natural systems. Although the Hess-Murray law has been extensively reported in the literature, there have also been reports of systems ...that deviate from this law. Using 3D dendritic flow networks of tubes, this study compares designs with various homothety reduction factor for diameters and lengths of tubes. The geometric constraint that is applied to these networks is equal tube volume at each branching level. The assessment is based on the flow resistance of networks calculated based on the Computational Fluid Dynamics (CFD) results. This study shows, among other things, that the performance of dendritic designs is highly dependent on the geometric features such as the svelteness of the network, and on tube alignment at different levels of bifurcation. It is also worth mentioning that flow asymmetry can develop in the dendritic networks that are symmetrical in terms of design. These findings should be considered while designing networks for engineering systems.
The majority of tidal energy convertors (TECs) currently under development are of a non-yawing horizontal axis design. However, most energetic regions that have been identified as candidate sites for ...installation of TEC arrays exhibit some degree of directional and magnitude asymmetry between incident flood and ebb flow angles and velocities, particularly in nearshore environments where topographic, bathymetric and seabed frictional effects and interactions are significant. Understanding the contribution of directional and magnitude asymmetry to resource power density along with off axis rotor alignment to flow could influence site selection and help elucidate optimal turbine orientation. Here, 2D oceanographic model simulations and field data were analysed to investigate these effects at potential deployment locations in the Irish Sea; an energetic semi-enclosed shelf sea region. We find that observed sites exhibiting a high degree of asymmetry may be associated with a reduction of over 2% in annual energy yield when deployment design optimisation is ignored. However, at the majority of sites, even in the presence of significant asymmetry, the difference is <0.3%. Although the effects are shown to have less significance than other uncertainties in resource assessment, these impacts could be further investigated and quantified using CFD and 3D modelling.
•Flow direction and magnitude asymmetry on tidal stream resource is quantified.•Turbine yaw misalignment combined with tidal asymmetry is further assessed.•2D oceanographic models and field data are used to investigate these effects.•Formulation of a simple spatial assessment methodology is presented.•Reduction of over 2% in turbine annual energy yield when optimisation ignored.
A method of tidal stream energy resource assessment around the Ushant Island in the Iroise Sea, using surface velocity time series from High Frequency radars (HFR) and ADCP measurements, is ...presented. Remotely sensed velocities provided by the radars allow to augment the industry standard approach of 3D numerical modeling and in-situ ADCP surveying to make a large-scale quantification of tidal stream resource. They capture the real ocean dynamics and thus provide context on the complex spatial variability of tidal currents that are so often feature at potential tidal energy sites. The observations show current velocities of 4 m/s northwest of the Island and in the Fromveur Strait, with 1 m/s value exceeded 60% and 70% of time respectively. Emphasis is given to the peculiarities in tidal flow asymmetry and to the study of the variation of vertical velocity profiles during different tidal stages. Radar derived velocities reveal a pronounced asymmetry between the flood and ebb flow around the Ushant Island, quantified by dimensionless number a - velocity asymmetry. The largest range of asymmetry variation, from 0.5 to 2.5, is observed in the Fromveur Strait. Harmonic analysis demonstrated that a joint variation of phase of the principal semi-diurnal (M2) and quarter-diurnal (M4) tidal velocity component accounts for flow asymmetry variation in the strait. Asymmetry in current direction is also quantified. ADCP measurements show that the vertical velocity profiles follow a 1/α power law with a power law exponent mostly depending on geographic location than on tidal stage. It was demonstrated that, in the Fromveur Strait, the 1/7 power law is appropriate to characterize the velocity profile. The combination of two sources of data enables characterization of the velocity variations in three spatial dimensions and in time thus increasing accuracy of the hydrokinetic resource assessment from HF radar observations. The estimation shows that the mean technical resource is 50% smaller in the lower half than in the upper half of the water column. The theoretical resource on average is three times higher than the technical resource and appears to be more sensitive to variations in the shape of the velocity profile in the lower layer.
•A novel method of assessing the hydrokinetic resource at promising tidal stream energy sites in the Iroise Sea is presented.•By merging surface velocity time series from HF radars with ADCP data, the major metrics of the tidal flow are quantified.•Current velocities show a pronounced asymmetry between the flood and ebb flow varying in a range from 0.5 to 2.5.•The 1/7 power law is appropriate to characterize the velocity profile, during at least 3-h period of the strongest flow.•Time series of the theoretical and technical power were reconstructed in the lower and upper half of the water column.
The description of hydrodynamics associated with the extensive reef system on the shelf break adjacent to the Amazon River is still a challenge for ocean sciences. Despite the discharge of more than ...one billion tons of cohesive sediment per year, the outer continental shelf of the world's largest river presents very low concentrations of suspended sediment near the bottom and an absence of modern fine sediment deposits nearly one hundred kilometers before the shelf break. The offshore limit of the subaqueous delta consists of a sigmoidal clinoform standing between 40 and 70 m in depth, a depositional feature that cannot be explained solely by estuarine-like gravitational circulation. This paper aims to test the hypothesis that internal tides have a major role in the control of offshore fine sediment transport. For that, we implement a set of tridimensional, non-hydrostatic, and high-resolution (up to 2 m, vertical, and 2 km, horizontal) Delft3D models. The experiments showed that even disregarding river plume buoyancy, wind drag, superficial waves, and ocean currents, the exclusive interaction between barotropic tidal currents, bathymetry, and the stratification structure of the ocean is capable of generating asymmetrical current patterns compatible with modern deposition. The maximum shelf slope and the relative depth between the outer shelf and the pycnocline represent the main factors influencing the generation and shoreward propagation of internal tides. Over time, spring-neap cycles are eventually capable of reverting cross-shore subtidal transport tendencies, while seasonal variability in ocean stratification modulates the intensity of baroclinic processes.
•The present study reveals for first time in literature the relevance of internal tides in Amazon continental shelf fine sediment transport•Internal tides occur permanently along the Amazon shelf break with spring tidal ranges up to 80 m•The internal tidal range and the maximum continental slope steepness present a correlation of 80%•Internal tides induce a bottom boundary layer on the outer shelf where shoreward fine sediment transport is dominant•The asymmetry patterns induced by internal tides are compatible with the modern deposition dynamics and with the Amazon reef system distribution
Laminarization of a turbulent flow due to wall heating has been known for more than 50 years, to the point that it is sometimes used as means of reducing friction. However this phenomenon has been ...mainly studied for cylindrical pipes and with imposed heat flux but not for channel flows and with imposed temperature boundary conditions, especially with asymmetric ones (that is to say in presence of a transverse thermal gradient). Based on the recent success of some Reynolds-averaged Navier-Stokes (RANS) models to correctly describe the influence of a strong transverse temperature gradient on turbulent Poiseuille flows, when compared to similar direct numerical simulations (DNS) or large eddy simulations (LES) results, these approaches are used here to investigate reverse transition. Since the choice of the turbulence model has a non-negligible influence on the results, however, it is necessary to use different models to get an indication of the uncertainty associated with them. The proposed methodology is based on the use of RANS closures that do not involve any wall functions due to the strong gradient in the wall layer that has to be modeled. Thus, two first-moment closures and a second-moment closure are considered: the k−ω−SST and the k−ε−v2¯/k, and the EB-RSM. The latter two rely on an elliptic blending. The turbulent heat flux is modeled with a simple gradient diffusion hypothesis (SGDH) and a generalized gradient diffusion hypothesis (GGDH) for the first-moment and second-moment closures respectively. In summary, more than 800 calculations are performed for the above three models in order to analyze the reverse transition, and to open room for debate on the possibility for such approaches to correctly reproduce the experimentally observed behavior.
Earnings asymmetric timeliness captures both accrual and operating cash flow (CFO) asymmetric timeliness. Because recognition of operating cash flows does not reflect differential verification ...thresholds for recognizing unrealized gains versus losses, CFO asymmetry adds noise or bias to tests of conditional conservatism. We show that CFO asymmetry is predictable in the cross-section, and varies systematically with life-cycle characteristics. Removing CFO from earnings and using accruals-based measures of asymmetric timeliness eliminates several biases that prior studies have attributed to other sources. Moreover, accrual asymmetric timeliness varies in the cross-section as theory predicts. Going forward, we recommend researchers use accruals-based asymmetric timeliness measures when testing for conditional conservatism.
•Earnings-based measures of conditional conservatism capture both operating cash flow (CFO) asymmetric timeliness and accrual asymmetric timeliness. We argue that CFO asymmetry does not reflect differential verification thresholds for recognizing unrealized gains versus unrealized losses, but CFO does exhibit asymmetry in good news versus bad news environments.•We show that CFO asymmetry exhibits predictable cross-sectional variation with firms’ life cycle stage, which can bias tests of conditional conservatism.•We provide evidence that sources of bias documented in prior research are related to cash flow asymmetry. CFO asymmetry explains the anomalous lagged earnings asymmetry documented by Patatoukas and Thomas (2011), and explains the bias that Ball et al. (2013b) attribute to expected return/expected earnings covariance.•We show that using accruals as the dependent variable successfully removes many of the biases caused by cash flow asymmetry. We also show that accruals-based asymmetric timeliness estimates vary in cross-section as agency/contracting/litigation theories predict. Going forward, we recommend using accruals as the dependent variable in studies of conditional conservatism.
In this paper we elucidate the asymmetric flow pattern and the haemodynamic quantity distributions and correlations in the pulmonary artery (PA) vasculature in healthy adults having structurally ...normal hearts, to provide reference on the flow characteristics in the PA and the right ventricle.
Velocity data are acquired non-invasively from 18 healthy volunteers by 4D flow magnetic resonance imaging, resolved to 20 phases with spatial resolution 3 × 3 × 3 mm
. Interpolation is applied to improve the accuracy in quantifying haemodynamic quantities including kinetic energy, rotational energy, helicity and energy dissipation rate. These quantities are volumetrically normalised to remove size dependency, representing densities or local intensity.
Flow asymmetry in the PA is quantified in terms of all the flow dynamic quantities and their correlations. The right PA has larger diameter and higher peak stroke velocity than the left PA. It also has the highest rotational energy intensity. Counter-rotating helical streams in the main PA appear to be associated with the unidirectional helical flow noticed in the left and the right PA near the peak systole.
This study provides a fundamental basis of normal flow in the PA. It implies the validity to use these flow pattern-related quantitative measures to aid with the identification of abnormal PA flow non-invasively, specifically for detecting abnormalities in the pulmonary circulation and response to therapy, where haemodynamic flow is commonly characterised by increased vortical and helical formations.
•Testing of RANS models capabilities for turbulent flows with strong transverse temperature gradient.•Comparison of various formulations for the turbulent heat flux.•Comparison of first-moment and ...second-moment closures based on elliptic approach.
The effects of a strong transverse temperature gradient on a turbulent Poiseuille flow are studied numerically using Reynolds-averaged Navier–Stokes (RANS) models. Such a situation is very common for numerous industrial applications. Since a large majority of industrial computations are based on the RANS approach, the aim of the present work is to investigate the ability of different RANS models to reproduce the main physical phenomena at the origin of the asymmetry of the flow and thermal fields. Comparison are performed with available direct numerical simulations (DNS) or large eddy simulation (LES) databases. With the prospect of future application of the models in the industrial context, models based on the widely used eddy-viscosity and simple gradient diffusion (SGDH) hypotheses are compared to more elaborate second-moment closures for the Reynolds stress and turbulent heat flux. The aim is to determine the closure level necessary to reproduce the influence of strong temperature gradients on the turbulent flow, for a wide range of wall-temperature ratios. Eddy-viscosity models prove able to correctly reproduce the asymmetry of the flow and the tendency toward relaminarization close to the hot wall, which are mainly due to the strong variations of the physical properties (namely the molecular viscosity and the density). Discrepancies in the predictions of the different closure levels only appear for the highest temperature ratios. Unfortunately, reliable reference data are lacking for these configurations, which calls for future DNS or refined LES studies.
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