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The clogging behavior of the micro-orifice under a flow accelerated condition was investigated after 500 h of immersion in high-temperature water. The results indicated the residual area of the micro-orifice was reduced to one-third of its original size after 500 h of immersion due to the deposition of corrosion products. In this process, the clogging behavior of micro-orifice can be divided into three stages: the stable deposition stage, the quick recovery stage, and the dynamic equilibrium stage. The corrosion products were porous and consisted of many deposited particles. The process of particle deposition and removal was carried out simultaneously.

The Antarctic ice sheet has been losing mass over past decades through the accelerated flow of its glaciers, conditioned by ocean temperature and bed topography. Glaciers retreating along retrograde slopes (that is, the bed elevation drops in the inland direction) are potentially unstable, while subglacial ridges slow down the glacial retreat. Despite major advances in the mapping of subglacial bed topography, significant sectors of Antarctica remain poorly resolved and critical spatial details are missing. Here we present a novel, high-resolution and physically based description of Antarctic bed topography using mass conservation. Our results reveal previously unknown basal features with major implications for glacier response to climate change. For example, glaciers flowing across the Transantarctic Mountains are protected by broad, stabilizing ridges. Conversely, in the marine basin of Wilkes Land, East Antarctica, we find retrograde slopes along Ninnis and Denman glaciers, with stabilizing slopes beneath Moscow University, Totten and Lambert glacier system, despite corrections in bed elevation of up to 1 km for the latter. This transformative description of bed topography redefines the high- and lower-risk sectors for rapid sea level rise from Antarctica; it will also significantly impact model projections of sea level rise from Antarctica in the coming centuries.A high-resolution update of Antarctic bed topography using mass conservation reveals broad stabilizing ridges for glaciers flowing across the Transantarctic Mountains, and stabilizing slopes beneath Moscow University, Totten and Lambert glacier system.

The efficiency of tidal stream turbines in a large array depends on the balance between negative effects of turbine-wake interactions and positive effects of bypass-flow acceleration due to local blockage, both of which are functions of the layout of turbines. In this study we investigate the hydrodynamics of turbines in an infinitely large array with aligned or staggered layouts for a range of streamwise and lateral turbine spacing. First, we present a theoretical analysis based on an extension of the linear momentum actuator disc theory for perfectly aligned and staggered layouts, employing a hybrid inviscid-viscous approach to account for the local blockage effect within each row of turbines and the viscous (turbulent) wake mixing behind each row in a coupled manner. We then perform large-eddy simulation (LES) of open-channel flow for 28 layouts of tidal turbines using an actuator line method with doubly periodic boundary conditions. Both theoretical and LES results show that the efficiency of turbines (or the power of turbines for a given bulk velocity) in an aligned array decreases as we reduce the streamwise turbine spacing, whereas that in a staggered array remains high and may even increase due to the positive local blockage effect (causing the local flow velocity upstream of each turbine to exceed the bulk velocity) if the lateral turbine spacing is sufficiently small. The LES results further reveal that the amplitude of wake meandering tends to decrease as we reduce the lateral turbine spacing, which leads to a lower wake recovery rate in the near-wake region. These results will help to understand and improve the efficiency of tidal turbines in future large arrays, even though the performance of real tidal arrays may depend not only on turbine-to-turbine interactions within the array but also on macro-scale interactions between the array and natural tidal currents, the latter of which are outside the scope of this study.

Thwaites Glacier represents 15% of the ice discharge from the West Antarctic Ice Sheet and influences a wider catchment 1 – 3 . Because it is grounded below sea level 4 , 5 , Thwaites Glacier is thought to be susceptible to runaway retreat triggered at the grounding line (GL) at which the glacier reaches the ocean 6 , 7 . Recent ice-flow acceleration 2 , 8 and retreat of the ice front 8 – 10 and GL 11 , 12 indicate that ice loss will continue. The relative impacts of mechanisms underlying recent retreat are however uncertain. Here we show sustained GL retreat from at least 2011 to 2020 and resolve mechanisms of ice-shelf melt at the submetre scale. Our conclusions are based on observations of the Thwaites Eastern Ice Shelf (TEIS) from an underwater vehicle, extending from the GL to 3 km oceanward and from the ice–ocean interface to the sea floor. These observations show a rough ice base above a sea floor sloping upward towards the GL and an ocean cavity in which the warmest water exceeds 2 °C above freezing. Data closest to the ice base show that enhanced melting occurs along sloped surfaces that initiate near the GL and evolve into steep-sided terraces. This pronounced melting along steep ice faces, including in crevasses, produces stratification that suppresses melt along flat interfaces. These data imply that slope-dependent melting sculpts the ice base and acts as an important response to ocean warming. Thwaites Eastern Ice Shelf observations from a new underwater vehicle show that high melt rates occur where ice is sharply sloped at the ocean interface, with lower melt where the ice is comparatively flat.

Ice discharge from large ice sheets plays a direct role in determining rates of sea-level rise. We map present-day Antarctic-wide surface velocities using Landsat 7 and 8 imagery spanning 2013–2015 and compare to earlier estimates derived from synthetic aperture radar, revealing heterogeneous changes in ice flow since ∼ 2008. The new mapping provides complete coastal and inland coverage of ice velocity north of 82.4° S with a mean error of < 10 m yr−1, resulting from multiple overlapping image pairs acquired during the daylight period. Using an optimized flux gate, ice discharge from Antarctica is 1929 ± 40 Gigatons per year (Gt yr−1) in 2015, an increase of 36 ± 15 Gt yr−1 from the time of the radar mapping. Flow accelerations across the grounding lines of West Antarctica's Amundsen Sea Embayment, Getz Ice Shelf and Marguerite Bay on the western Antarctic Peninsula, account for 88 % of this increase. In contrast, glaciers draining the East Antarctic Ice Sheet have been remarkably constant over the period of observation. Including modeled rates of snow accumulation and basal melt, the Antarctic ice sheet lost ice at an average rate of 183 ± 94 Gt yr−1 between 2008 and 2015. The modest increase in ice discharge over the past 7 years is contrasted by high rates of ice sheet mass loss and distinct spatial patters of elevation lowering. The West Antarctic Ice Sheet is experiencing high rates of mass loss and displays distinct patterns of elevation lowering that point to a dynamic imbalance. We find modest increase in ice discharge over the past 7 years, which suggests that the recent pattern of mass loss in Antarctica is part of a longer-term phase of enhanced glacier flow initiated in the decades leading up to the first continent-wide radar mapping of ice flow.

Supercritical pressure fluids are widely used in heat transfer and energy systems. The benefit of high heat transfer performance and the successful avoidance of phase change from the use of supercritical pressure fluids are well-known, but the complex behaviours of such fluids owing to dramatic thermal property variations pose strong challenges to the design of heat transfer applications. In this paper, the turbulent flow and heat transfer of supercritical pressure$\\textrm {CO}_2$in a small vertical tube influenced by coupled effects of buoyancy and thermal acceleration are numerically investigated using direct numerical simulation. Both upward and downward flows with an inlet Reynolds number of 3540 and pressure of 7.75 MPa have been simulated and the results are compared with corresponding experimental data. The flow and heat transfer results reveal that under buoyancy and thermal acceleration, the turbulent flow and heat transfer exhibit four developing periods in which buoyancy and thermal acceleration alternately dominate. The results suggest a way to distinguish the dominant factor of heat transfer in different periods and a criterion for heat transfer degradation under the complex coupling of buoyancy and thermal acceleration. An analysis of the orthogonal decomposition and the generative mechanism of turbulent structures indicates that the flow acceleration induces a stretch-to-disrupt mechanism of coherent turbulent structures. The significant flow acceleration can destroy the three-dimensional flow structure and stretch the vortices resulting in dissipation.

On the account of significance of bioconvection in biotechnology and several biological systems, valuable contributions have been performed by scientists in current decade. In current framework, a theoretical bioconvection model is constituted to examine the analyzed the thermally developed magnetized couple stress nanoparticles flow by involving narrative flow characteristics namely activation energy, chemical reaction and radiation features. The accelerated flow is organized on the periodically porous stretched configuration. The heat performances are evaluated via famous Buongiorno’s model which successfully reflects the important features of thermophoretic and Brownian motion. The composed fluid model is based on the governing equations of momentum, energy, nanoparticles concentration and motile microorganisms. The dimensionless problem has been solved analytically via homotopic procedure where the convergence of results is carefully examined. The interesting graphical description for the distribution of velocity, heat transfer of nanoparticles, concentration pattern and gyrotactic microorganism significance are presented with relevant physical significance. The variation in wall shear stress is also graphically underlined which shows an interesting periodic oscillation near the flow domain. The numerical interpretation for examining the heat mass and motile density transfer rate is presented in tubular form.

Confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large, low‐gradient main channel commonly occur in nature, but they have not yet been investigated. Measurements of the three‐dimensional velocity field, turbulence, sediment transport, bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the Upper Rhone River, Switzerland. The difference between the low‐flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone. Due to this bed discordance, the tributary flow penetrates into the main channel mainly in the upper part of the water column, whereas the main‐channel flow is hardly hindered by the tributary in the lower part of the water column, giving rise to a two‐layer flow structure in the confluence zone. In confluences with high supply of coarse sediment from the tributary, the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity. The sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near‐bed flow originating from the main channel. The sediment transport capacity is further increased by the three‐dimensionality of the flow, which is characterized by maximum velocities occurring near the bed, and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary. A conceptual model is proposed for the hydro‐morpho‐sedimentary processes, and compared to existing conceptual models for confluences with different characteristics. Key Points Experimental investigation in a channel confluence under live bed conditions Configuration not considered in previous studies Three‐dimensional flow velocity and turbulence measurements

This paper reviews analytical solutions for the accelerated flow of an incompressible Newtonian fluid in a pipeline. This problem can be solved in one of two ways according to the (1) imposed pressure gradient or (2) flow rate. Laminar accelerated flow solutions presented in a number of publications concern cases where the two driving mechanisms are described by simple mathematical functions: (a) impulsive change; (b) constant change; (c) ramp change, etc. The adoption of a more complex and realistic description of the pressure gradient or flow rate will be associated with a profound mathematical complexity of the final solution. This is particularly visible with the help of the universal formula derived by several researchers over the years and discussed in this paper. In addition to the solutions strictly defined for laminar flow, an interesting extension of this theory is the theory of underlying laminar flow for the analysis of turbulent accelerated pipe flows (TULF model developed by García García and Alvariño). The TULF model extends the Pai model developed more than 60 years ago, which has been previously used for steady flows only. The discussed solutions extend the theory of analytical solutions of simplified two-dimensional Navier–Stokes equations and can be used not only to study the behavior of liquids during accelerating pipe flow but they can also be used to test the accuracy of commercial CFD codes.

The Greenland Ice Sheet is losing mass at an accelerating rate due to increased surface melt and flow acceleration in outlet glaciers. Quantifying future dynamic contributions to sea level requires accurate portrayal of outlet glaciers in ice sheet simulations, but to date poor knowledge of subglacial topography and limited model resolution have prevented reproduction of complex spatial patterns of outlet flow. Here we combine a high-resolution ice-sheet model coupled to uniformly applied models of subglacial hydrology and basal sliding, and a new subglacial topography data set to simulate the flow of the Greenland Ice Sheet. Flow patterns of many outlet glaciers are well captured, illustrating fundamental commonalities in outlet glacier flow and highlighting the importance of efforts to map subglacial topography. Success in reproducing present day flow patterns shows the potential for prognostic modelling of ice sheets without the need for spatially varying parameters with uncertain time evolution. Quantifying Greenland's future contribution to sea level requires accurate portrayal of its outlet glaciers in ice sheet simulations. Here, the authors show that outlet glacier flow can be captured if ice thickness is well constrained and vertical shearing as well as membrane stresses are included in the model.