A climatology of the North Atlantic subpolar gyre boundary

<p>Circulation at the boundary of the subpolar North Atlantic influences both the horizontal (gyre) and vertical (overturning) components of the flow structure. While boundary current transport projects directly onto subpolar gyre strength, recent modelling studies have highlighted that buoyancy fluxes between the basin interior and the boundary, followed by rapid buoyancy export by boundary currents, are crucial steps in projecting air-sea interaction onto the strength of the Atlantic Meridional Overturning Circulation (AMOC). This work seeks observational insights into these key boundary processes.<br>To achieve this, we have constructed a robust boundary climatology from quality controlled CTD and Argo hydrography since the turn of the millennium. Following the 1000 m isobath north of 47 °N and aggregating data into 100 km bins, we build a picture of the typical large-scale temperature and salinity structure for each month.<br>This product will allow us to identify where and when important interior-boundary buoyancy fluxes take place over a seasonal cycle. A first step is to evaluate geostrophic flow into the boundary, and hence describe the vertical structure of advective buoyancy exchange. By appealing to satellite altimetry and Argo trajectories, we can also estimate turbulent eddy fluxes both at the surface and 1000 m depth. Models indicate these parameters are key in dictating the pathways for the AMOC lower limb, and we will place our observational findings in the context of these results.<br>Boundary current strength is another key parameter dictating the export of dense water from the subpolar gyre. We will appeal to satellite altimetry to build corresponding climatologies for barotropic boundary flow. Furthermore, along-slope density gradients give rise to a baroclinic boundary current forcing term, which we aim to investigate here. Water density generally increases as we follow the gyre counter-clockwise, with the notable exception of the West Greenland Current section, and our product allows us to partition the spatially-varying contribution of temperature and salinity towards these density gradients. For example, we can evaluate the impact of cooling along the eastern boundary, or surface freshening around southern Greenland, on the dynamics of boundary flow. Ultimately, we would like to understand the evolution of the dynamical balance experienced by a hypothetical fluid parcel traversing the entire subpolar gyre.</p>

Physical Mechanisms Investigation of Sharkskin-Inspired Compressor Cascade Based on Large Eddy Simulations

In this paper, the effectiveness of sharkskin-inspired riblets in reducing the aerodynamic loss of compressor cascade flow was investigated by using high-fidelity numerical simulation method. Two key normalized parameters were selected to parameterize various riblet designs, and the corresponding relative change in cascade performance was first investigated based on the uRANS simulations with/without transition model. Then, the large eddy simulations were conducted to investigate the cascade flow with the selected riblet design cases. By comparing the flow resistance, transition positions, vortex formations and turbulence fluctuations of the boundary flow, the flow control mechanisms of the riblets were finally studied. Simulation results show that compared with the prototype case, the total pressure loss can be reduced by up to 20.5% in the fully turbulent environment. This is because the spanwise fluctuation of the turbulent vortices is impeded, and the turbulent vortices are lifted above the riblet tip. However, when considering transition from laminar to turbulent boundary flow, the aerodynamic performance of compressor cascade strongly depends on the riblet position relative to the transition on cascade SS. The total pressure loss can only be reduced by up to 8.1%, and even most riblet designs will degrade the cascade performance. The major reason is that the riblets are located upstream of the transition zone, especially at the small incidence angles. Due to the installation of riblets, the contact area between the laminar flow and the wall surface is increased, and the downstream laminar-to-turbulent transition is promoted.

Physical Mechanisms Investigation of Sharkskin-Inspired Compressor Cascade Based on Large Eddy Simulations

In order to survive in a complex environment, nature has produced efficient and versatile resource-rich structures. One of the novel drag reduction designs comes from the efficient movement of sharks through microscope riblets aligned along the flow direction. In this paper, the effectiveness of sharkskin-inspired riblets in reducing the aerodynamic loss of compressor cascade flow was investigated by using high-fidelity numerical simulation method. Two key normalized parameters were selected to parameterize various riblet designs, and the corresponding relative change in cascade performance was first investigated based on the uRANS simulations with/without transition model. Then, the large eddy simulations in conjunction with the wall-adapted local eddy viscosity model were conducted to investigate the cascade flow with the selected riblet design cases. By comparing the flow resistance, transition positions, vortex formations and turbulence fluctuations of the boundary flow, the flow control mechanisms of the riblets were finally studied. Simulation results show that compared with the prototype case, the total pressure loss can be reduced by up to 20.5% in the fully turbulent environment. This is because the spanwise fluctuation of the turbulent vortices is impeded, and the turbulent vortices are lifted above the riblet tip. Low-speed streaks inside the riblet valleys generate relatively low shear stresses, while the high-shear stresses occur only at the riblet tips. However, when considering transition from laminar to turbulent boundary flow, the aerodynamic performance of compressor cascade strongly depends on the riblet position relative to the transition on cascade SS. The total pressure loss can only be reduced by up to 8.1%, and even most riblet designs will degrade the cascade performance. The major reason is that the riblets are located upstream of the transition zone, especially at the small incidence angles. Due to the installation of riblets, the contact area between the laminar flow and the wall surface is increased, and the downstream laminar-to-turbulent transition is promoted.

Element-Free Galerkin method for dynamic boundary flow problems

This paper focuses on the study of dynamic boundary flow problems based on the Element-Free Galerkin method. First, Navier–Stokes equation is discretized with the Galerkin method. The inertial term in the equation is discretized with the method of the speed term and direct deduction, respectively. The penalty function method is used to deal with the pressure and the essential boundary condition in the equation, and the discretization of two-dimensional N–S equation based on the EFG method is established. However, irregular changes in boundary conditions are often encountered in practical fluid problems. For example, the motion of flapping-foil is not uniform relative to the flow. In this paper, numerical experiments are carried out for the flow problems with non-uniform boundary motions. The problem which a plate with non-uniform drag movement above a rectangular tank filled with water is studied with the EFG method. The feasibility of the proposed algorithm is verified by comparison with the FEM method. Then, the procession of the water in the tank is stimulated. In the end, the influence of different calculation time steps on the accuracy of the solution is discussed.

Correction Method for Hydraulic Conductivity Measurements Made Using a Fixed Wall Permeameter

Hydraulic conductivity measurement through a fixed wall permeameter is a common practice to obtain the fluid transmissibility characteristics of soil matrix; however, sidewall leakage due to rigid wall effect may significantly influence the observed values for coarse-grained soils. In this study, the boundary flow error was identified through characterizing the geometrical properties of voids adjacent to the sidewall, and a parameter known as the boundary void ratio (eb) was proposed to account for this effect. The findings suggest that a fixed wall cell containing coarse soils would unavoidably generate extra voids at the interface between soil grains and inner rigid wall, contributing to a larger eb at the wall than void ratio within the soil bed; the measured hydraulic conductivity is increased primarily due to the apparatus-induced error. A two-dimensional geometric model was then established to estimate the eb value for uniformly sized coarse soils confined by a rigid permeameter wall, based on which a method was obtained for eliminating the boundary flow error from a fixed wall cell. The mathematical method was finally validated against experimental data from existing literature. It can be concluded that the boundary condition at sidewall featuring unwanted gaps lead to overestimation of the coefficient of permeability; however, the proposed correction method could adequately eliminate the boundary flow error for uniformly sized coarse-grained soils tested within a rigid wall cell.