The Evolution of Asymmetries in the Tropical Cyclone Boundary Layer Wind Field during Landfall

The evolution of the tropical cyclone boundary layer (TCBL) wind field before landfall is examined in this study. As noted in previous studies, a typical TCBL wind structure over the ocean features a supergradient boundary layer jet to the left of motion and Earth-relative maximum winds to the right. However, the detailed response of the wind field to frictional convergence at the coastline is less well known. Here, idealized numerical simulations reveal an increase in the offshore radial and vertical velocities beginning once the TC is roughly 200 km offshore. This increase in the radial velocity is attributed to the sudden decrease in frictional stress once the highly agradient flow crosses the offshore coastline. Enhanced advection of angular momentum by the secondary circulation forces a strengthening of the supergradient jet near the top of the TCBL. Sensitivity experiments reveal that the coastal roughness discontinuity dominates the friction asymmetry due to motion. Additionally, increasing the inland roughness through increasing the aerodynamic roughness length enhances the observed asymmetries. Lastly, a brief analysis of in-situ surface wind data collected during the landfall of three Gulf of Mexico hurricanes is provided and compared to the idealized simulations. Despite the limited in-situ data, the observations generally support the simulations. The results here imply that assumptions about the TCBL wind field based on observations from over horizontally-homogeneous surface types - which have been well-documented by previous studies - are inappropriate for use near strong frictional heterogeneity.

Micromechanical Tests on Natural Fibre Composites with Enzymatically Enhanced Fibre–Matrix Adhesion

Natural fibre–reinforced composites are more sustainable than other composites with respect to the raw materials. Their properties are attractive due to high specific properties, and especially so wherever high damping is valued. As the interphase between fibre and matrix is the region of highest stresses, a strong bond between fibre and matrix is essential for any composites’ properties. The present study compares two methods of determining the interfacial shear stress in natural fibre–reinforced composites: the single fibre fragmentation test and the single fibre pullout test. The studied composites are flax fibre reinforced epoxy. For a variety of fibre–matrix interaction, the fibres are treated with a laccase enzyme and dopamine, which is known to improve the fibre–matrix shear strength. In the observed samples, single fibre fragmentation test data, i.e. of fracture mode and fragment length, scatter when compared to pullout data. In single fibre pullout tests, the local interfacial shear strength showed a 30% increase in the laccase-treated samples, compared to the control samples. The method also permitted an evaluation of the frictional stress occurring after surface failure.

Memory effects in friction: the role of sliding heterogeneities

We report on memory effects involved in the unsteady-state frictional response of a contact interface between a silicone rubber and a spherical glass probe when it is perturbed by changes in the orientation of the driving motion or by velocity steps. From measurements of the displacement fields at the interface, we show that observed memory effects can be accounted for by the non-uniform distribution of the sliding velocity within the contact interface. As a consequence of these memory effects, the friction force may no longer be aligned with respect to the sliding trajectory. In addition, stick–slip motions with a purely geometrical origin are also evidenced. These observations are adequately accounted for by a friction model that takes into account heterogeneous displacements within the contact area. When a velocity dependence of the frictional stress is incorporated in this model, unsteady-state regimes induced by velocity steps are also adequately described. The good agreement between the model and experiments outlines the role of space heterogeneities in memory effects involved in soft matter friction.

Radial-axial transport coordination enhances sugar translocation in the phloem vasculature of plants

Understanding the controls of mass transport of photosynthates in the phloem of plants is necessary for describing plant carbon allocation, productivity, and responses to water and thermal stress. While several hypotheses about optimization of phloem structure and function, and limitations of phloem transport under drought have been tested both with models and anatomical data, the true impact of radial water exchange of phloem conduits with their surroundings on mass transport of photosynthates has not been addressed. Here the physics of the Munch mechanism of sugar transport is re-evaluated to include local variations in viscosity resulting from the radial water exchange in two dimensions (axial and radial). Model results show that radial water exchange pushes sucrose away from conduit walls thereby reducing wall frictional stress due to a decrease in sap viscosity and increasing sugar concentration in the central region of the conduit. This leads to increased sugar front speed and axial mass transport for a wide range of phloem conduit lengths and allows sugar transport to operate more efficiently than predicted by previous models. A faster front speed leads to higher phloem resiliency under drought because more sugar can be transported with a smaller pressure gradient.

Contact Friction Regulation of the UV Laser Textured PVA Hydrogel and Titanium Alloy Interface

The soft, hydrophilic and smooth surface of hydrogel displays extremely complex friction behavior. In this work, the ultraviolet laser post-processing created PVA hydrogel surface textures is found to be a one-step effective way for the contact friction regulation at the hydrogel-titanium alloy interface. Micro-grooves with various spacings and depths are fabricated by adjusting the laser scanline interval, laser energy density and the scanning times. Friction torques are measured by a strain-controlled parallel-plate rheometer to characterize frictional behaviors of micro-grooves. At the sliding velocity range of 0.085mm/s < v < 2.3mm/s, the effect of interlocking and ploughing force are the dominant origins of friction. Frictional stress increases with the decrease of the micro-groove spacing, but not with the increase of the micro-groove depth, which indicates that frictional stress doesn’t simply increase with an increase of hydrogel’s roughness. As the velocity increases from 2.3mm/s to 100mm/s, the surface wettability of textured hydrogel plays an important role in regulating friction. Both smooth hydrogel and laser textured hydrogels show stick-slip phenomenon which occurs in the same velocity range. These results take us a step closer to deriving a more effective, accurate, and dependable guideline for designing laser-textured surface grooves for sliding friction control of hydrogel applications.

Effect of cartilage thickness mismatch in osteochondral grafting from knee to talus on articular contact pressures: A finite element analysis

Objectives: The aim of this study was to investigate the effect of cartilage thickness mismatch on tibiotalar articular contact pressure in osteochondral grafting from femoral condyles to medial talar dome using a finite element analysis (FEA). Materials and methods: Flush-implanted osteochondral grafting was performed on the talar centromedial aspect of the dome using osteochondral plugs with two different cartilage thicknesses. One of the plugs had an equal cartilage thickness with the recipient talar cartilage and the second plug had a thicker cartilage representing a plug harvested from the knee. The ankle joint was loaded during a single-leg stance phase of gait. Tibiotalar contact pressure, frictional stress, equivalent stress (von Mises values), and deformation were analyzed. Results: In both osteochondral grafting simulations, tibiotalar contact pressure, frictional stress, equivalent stress (von Mises values) on both tibial and talar cartilage surfaces were restored to near-normal values. Conclusion: Cartilage thickness mismatch does not significantly change the tibiotalar contact biomechanics, when the graft is inserted flush with the talar cartilage surface.

Seasonal Variation of the Surface Kuroshio Intrusion into the South China Sea Evidenced by Satellite Geostrophic Streamlines

The long-term satellite altimeter and reanalysis data show that large seasonal variations are associated with geostrophic Kuroshio intrusion, but not with the current intensity, width and axis position east of Philippine. To address this issue, we examine the seasonal variability of surface intrusion patterns by a new streamline-based method. The along-streamline analysis reveals that the seasonality of geostrophic intrusion is only attributed to the cyclonic shear part of the flow, while the anticyclonic shear part always leaps across the Luzon Strait. A possible physical mechanism is proposed to accommodate these seasonal characteristics based on globally the vorticity (torque work) balance between the basin-wide negative wind stress curl and the positive vorticity fluxes induced by the lateral wall, as well as locally loss of balance between the torques of frictional stresses and normal stresses owing to the boundary gap. Through modifying the nearshore sea surface level, the northeasterly/southeasterly monsoon increases/decreases the positive vorticity fluxes in response to global vorticity balance, and simultaneously amplifies/alleviates the local imbalance by enhancing/reducing the positive frictional stress torque within the cyclonic shear layer. Therefore, in winter when the positive torque is large enough, the Kuroshio splits and the intrusion occurs, while in summer the stress torque is so weak that the entire current keeps flowing north.

Investigations of Gas–Particle Two-Phase Flow in Swirling Combustor by the Particle Stokes Numbers

Gas turbulence modulations and particle dispersions of swirling gas–particle two-phase flow in the combustor is investigated under the large spans of the particle Stokes numbers. To fully consider the preferential concentrations and anisotropic dispersions of a particle, a kinetic frictional stress model coupled with a second-order moment two-phase turbulent model and granular temperature equation is improved. The proposed modeling and simulations are in good agreement with the experimental validations. Results show turbulent modulations and particle dispersions exhibit strongly anisotropic characteristics, keeping a close relationship with flow structure. The axial gas velocity and RMS fluctuation velocity of 45.0-μm EGP was approximately 5.0 times and 3.0 times greater than 1000.0 μm Copper particles, and their axial particle velocity was 0.25 times and twice greater than those of 45.0 μm EGP. The degree of modulation in the axial–radial direction is larger than those of radial–tangential and axial–tangential direction. Particle dispersions are sensitive to particle diameter parameters and intensified by higher Stokes number.

Numerical Simulation and Experimental Study on Axial Stiffness and Stress Deformation of the Braided Corrugated Hose

To explore the mechanical properties of the braided corrugated hose, the space curve parametric equation of the braided tube is deduced, specific to the structural features of the braided tube. On this basis, the equivalent braided tube model is proposed based on the same axial stiffness in order to improve the calculational efficiency. The geometric model and the Finite Element Model of the DN25 braided corrugated hose is established. The numerical simulation results are analyzed, and the distribution of the equivalent stress and frictional stress is discussed. The maximum equivalent stress of the braided corrugated hose occurs at the braided tube, with the value of 903MPa. The maximum equivalent stress of the bellows occurs at the area in contact with the braided tube, with the value of 314MPa. The maximum frictional stress between the bellows and the braided tube is 88.46MPa. The tensile experiment of the DN25 braided corrugated hose is performed. The simulation results are in good agreement with test data, with a maximum error of 9.4%, verifying the rationality of the model. The study is helpful to the research of the axial stiffness of the braided corrugated hose and provides the base for wear and life studies on the braided corrugated hose.

Wear of AlCrN and CrAlSiN Coatings Applied to Nonstandard Involute Gears

Wear of nonstandard involute gears with two types of coatings, AlCrN and CrAlSiN, was studied. The coatings were applied by cathodic arc deposition. The gears were tested using a Niemann tester at a graduated load up to the 12th load stage and were compared to noncoated gears. Both Biogear S150 gear oil and PP90 universal hydraulic oil were applied during these tests. The thickness of deposited coatings and wear of gear teeth were studied by SEM and their chemical compositions were determined by EDS analysis. Maximal contact pressure of 1350 MPa was calculated in the region of the tooth flank at the 12th load stage. Maximal frictional stress was also calculated on the tooth flank. The resistance against wear of gears was evaluated based on the critical weight loss and mainly based on the critical surface roughness of gears. The critical roughness was exceeded at the 10th load stage for noncoated gears. For the gears with AlCrN and CrAlSiN coatings, the critical roughness was exceeded at the 11th load stage. Wear of AlCrN and CrAlSiN coatings was nonuniform along the height of tooth. Wear on the tooth flank was characterized by fragmentation of thin coatings and subsequent detaching of fragments from the steel substrate. The steel substrate was worn by microcutting, which caused the highest roughness on the tooth surface. On the tooth pitch, surface protrusions of coatings were smoothed, and coatings cracked and locally detached subsequently. On the tooth face, surface protrusions were also smoothed but coatings remained compact without crack initiations. Both experimental oils, Biogear S150 and PP90, proved to be suitable during Niemann tests as their temperatures did not exceed the limit value of 80 °C.