Shear orientation of nematic phases of clay nanosheets: processing of barrier coatings

When suspensions are exposed to shear forces, the particles may form ordered structures depending on their shapes, concentrations, and the material. For some processes, e.g., for wet-film coating, it is important to know how fast these structures form in shear fields and for how long the structures persist when the shear is relaxed. To obtain information on the particle structure formation and the decay time, the effective viscosity of nematic suspensions of Na-hectorite nanosheets was investigated by rheology employing a cone-plate measurement geometry. The necessary time for the formation textured nematic films could be deduced by carrying out effective viscosity measurements at constant time steps. Information could also be obtained on the lifetime of the platelet textures when shear is relaxed. All this information was employed to identify geometrical requirements for slot dies to produce barrier liners with nanosheet layers oriented parallel to PET substrates. Thereby, we obtained green and simple coatings that are in line with state-of-the-art high-performance materials such as metalized plastic foils in terms of oxygen barrier properties.

The Impact of Midlevel Shear Orientation on the Longevity of and Downdraft Location and Tornado-like Vortex Formation within Simulated Supercells

Despite an increased understanding of environments favorable for tornadic supercells, it is still sometimes unknown why one favorable environment produces many long-tracked tornadic supercells and another seemingly equally-favorable environment produces only short-lived supercells. One relatively unexplored environmental parameter that may differ between such environments is the degree of backing or veering of the midlevel shear vector, especially considering that such variations may not be captured by traditional supercell or tornado forecast parameters. We investigate the impact of the 3-6 km shear vector orientation on simulated supercell evolution by systematically varying it across a suite of idealized simulations. We found that the orientation of the 3-6 km shear vector dictates where precipitation loading is maximized in the storms, and thus alters the storm-relative location of downdrafts and outflow surges. When the shear vector is backed, outflow surges generally occur northwest of an updraft, produce greater convergence beneath the updraft, and do not disrupt inflow, meaning that the storm is more likely to persist and produce more tornado-like vortices (TLVs). When the shear vector is veered, outflow surges generally occur north of an updraft, produce less convergence beneath the updraft, and sometimes undercut it with outflow, causing it to tilt at low levels, sometimes leading to storm dissipation. These storms are shorter lived and thus also produce fewer TLVs. Our simulations indicate that the relative orientation of the 3-6 km shear vector may impact supercell longevity and hence the time period over which tornadoes may form.

Effects of Low-Level Flow Orientation and Vertical Shear on the Structure and Intensity of Tropical Cyclones

This article explores the simultaneous effect of vertical wind shear (VWS) and low-level mean flow (LMF) on tropical cyclone (TC) structure evolution. The structural evolution of 180 western North Pacific TCs from 2002 to 2014 was measured by a new parameter, the RV ratio, which is defined as the ratio of a TC’s radius of 34-kt (17.5 m s−1) wind to its maximum wind speed at the ending point of the intensification period. Whereas TCs with RV ratios in the lowest quartile of all 180 samples favored intensification over expansion, and 82% of these TCs experienced rapid intensification, TCs with RV ratios in the topmost quartile favored size expansion over intensification. A novel result of this study is that TC RV ratios were found to correlate with the LMF orientation relative to the deep-layer VWS vector. Specifically, whereas an LMF directed toward the left-of-shear orientation favors TC intensification, a right-of-shear LMF favors TC size expansion. This study further analyzed the TC rainfall asymmetry and asymmetric surface flow using satellite observations. Results show that for a TC affected by an LMF with right-of-shear orientation, the positive surface flux anomaly in the upshear outer region promotes convection in the downshear rainband region. On the other hand, a left-of-shear LMF induces a positive surface flux anomaly in the downshear outer region, thus promoting convection in the upshear inner core. Enhancement of the symmetric inner-core convection favors intensification, whereas enhancement of the downshear rainband favors expansion.