Precessing cylinders at the second and third resonance: Turbulence controlled by geostrophic flow

Copyright © 2015 American Physical SocietyWe investigate, via both asymptotic analysis and direct numerical simulation, precessionally driven flow of a homogeneous fluid confined in fluid-filled circular cylinders that rotate rapidly about their symmetry axis and precess about a different axis and that are marked by radius-height aspect ratios Γ=1.045945 and Γ=1.611089 . At these radius-height aspect ratios, the Poincaré force resonates directly with the two special inertial modes that have the simplest vertical structure. An asymptotic analytical solution in closed form describing weakly precessing flow is derived in the mantle frame of reference for asymptotically small Ekman numbers, showing quantitative agreement with the result of direct nonlinear numerical simulation. Our numerical simulation makes use of a finite-element method with the three-dimensional tetrahedralization of a cylindrical cavity that allows the construction of dense nodes in the vicinity of the bounding surface of the cavity for resolving the thin viscous boundary layer. It is found that axisymmetric geostrophic flow in the alternating eastward and westward direction can be generated and maintained by nonlinear and viscous effects in the viscous boundary layer. It is also found that, when the precessing rate is moderate and, consequently, the geostrophic flow is weak, nonlinear interaction between the resonant inertial mode and the nonesonant inertial modes driven by the Poincaré force and the boundary-layer influx leads to strongly turbulent flow with irregular temporal-spatial fluctuation. When the cylinders are strongly precessing such that the geostrophic flow becomes predominant, however, the effect of the geostrophic flow controls/stabilizes its nonlinear dynamics, leading to weakly turbulent flow that can be largely described by a dominant quasisteady geostrophic component and a weak nonaxisymmetric component localized in the region where the geostrophic flow is weak.Leverhulme TrustMacau FDCTNSF

Elucidation of the Biosynthetic Gene Cluster and the Post-PKS Modification Mechanism for Fostriecin in Streptomyces pulveraceus

SummaryFostriecin is a unique phosphate monoester antibiotic that was isolated from Streptomyces pulveraceus as a protein phosphatase 2A (PP2A) and PP4A selective inhibitor. However, its biosynthetic mechanism remains to be elucidated. In this study, a 73 kb gene cluster encoding a six modular Type I polyketide synthases (PKS) and seven tailoring enzymes was identified by cosmid sequencing from the producer. The functions of two tailoring enzymes were characterized by gene disruption and an in vitro enzyme activity assay. Remarkably, the isolation of three malonylated fostriecin analogs from post-PKS gene knockout mutants indicated malonylated-polyketide formation could be a normal biosynthetic process in the formation of the unsaturated six-membered lactone in fostriecin. Based on this study, a comprehensive post-PKS modification mechanism for fostriecin biosynthesis was proposed