Generation of Large-Scale Vorticity in a Homogeneous Turbulence with a Mean Velocity Shear

An effect of a mean velocity shear on a turbulence and on the effective force which is determined by the gradient of Reynolds stresses is studied. Generation of a mean vorticity in a homogeneous incompressible turbulent flow with an imposed mean velocity shear due to an excitation of a large-scale instability is found. The instability is caused by a combined effect of the large-scale shear motions (''skew-induced" deflection of equilibrium mean vorticity) and ''Reynolds stress-induced" generation of perturbations of mean vorticity. Spatial characteristics, such as the minimum size of the growing perturbations and the size of perturbations with the maximum growth rate, are determined. This instability and the dynamics of the mean vorticity are associated with the Prandtl's turbulent secondary flows. This instability is similar to the mean-field magnetic dynamo instability. Astrophysical applications of the obtained results are discussed.Comment: 8 pages, 3 figures, REVTEX4, submitted to Phys. Rev.

Supercritical Assisted Electrospray for the Production of Controlled Size Loaded PVP Microparticles

The production of polymeric micro- and nanoparticles with regular morphology and controlled size and distribution is a relevant target for many fields, from pharmaceutical to nutraceutical, from energetics to fine chemical. In this work, an improved micronization process is proposed, called supercritical assisted electrospray, for the production of biocompatible polyvynilpyrrolidone (PVP) particles loaded with quercetin. This supercritical process can largely improve the traditional electrospray performance, thanks to the addition of supercritical CO2 in the liquid polymeric solution at the beginning of the process. Operating in this manner, an expanded liquid is formed, characterized by a reduced viscosity and surface tension. Repeatable PVP microparticle diameters and distributions were obtained, ranging between 0.47 ± 0.24 µm and 6.01 ± 3.60 µm at PVP concentrations from 1% to 7% w/w, and between 1.71 μm ± 1.07 μm and 2.08 μm ± 1.18 μm, for 1% and 3% w/w PVP particles loaded at 7% w/w quercetin, respectively, working at 120 bar, 35 ℃ and 30 kV