Turbulence transition and the edge of chaos in pipe flow

The linear stability of pipe flow implies that only perturbations of sufficient strength will trigger the transition to turbulence. In order to determine this threshold in perturbation amplitude we study the \emph{edge of chaos} which separates perturbations that decay towards the laminar profile and perturbations that trigger turbulence. Using the lifetime as an indicator and methods developed in (Skufca et al, Phys. Rev. Lett. {\bf 96}, 174101 (2006)) we show that superimposed on an overall $1/\Re$-scaling predicted and studied previously there are small, non-monotonic variations reflecting folds in the edge of chaos. By tracing the motion in the edge we find that it is formed by the stable manifold of a unique flow field that is dominated by a pair of downstream vortices, asymmetrically placed towards the wall. The flow field that generates the edge of chaos shows intrinsic chaotic dynamics.Comment: 4 pages, 5 figure

High-temperature LM cathode ion thrusters Quarterly progress report, 5 May - 4 Aug. 1968

Design and operation of high temperature liquid mercury cathode ion thruster

Asymmetry of temporal cross-correlations in turbulent shear flows

We investigate spatial and temporal cross-correlations between streamwise and normal velocity components in three shear flows: a low-dimensional model for vortex-streak interactions, direct numerical simulations for a nearly homogeneous shear flow and experimental data for a turbulent boundary layer. A driving of streamwise streaks by streamwise vortices gives rise to a temporal asymmetry in the short time correlation. Close to the wall or the bounding surface in the free-slip situations, this asymmetry is identified. Further away from the boundaries the asymmetry becomes weaker and changes character, indicating the prevalence of other processes. The systematic variation of the asymmetry measure may be used as a complementary indicator to separate different layers in turbulent shear flows. The location of the extrema at different streamwise displacements can be used to read off the mean advection speed; it differs from the mean streamwise velocity because of asymmetries in the normal extension of the structures.Comment: 10 pages, 7 Postscript figures (low quality due to downsizing

Echoes in classical dynamical systems

Echoes arise when external manipulations to a system induce a reversal of its time evolution that leads to a more or less perfect recovery of the initial state. We discuss the accuracy with which a cloud of trajectories returns to the initial state in classical dynamical systems that are exposed to additive noise and small differences in the equations of motion for forward and backward evolution. The cases of integrable and chaotic motion and small or large noise are studied in some detail and many different dynamical laws are identified. Experimental tests in 2-d flows that show chaotic advection are proposed.Comment: to be published in J. Phys.

On statistically stationary homogeneous shear turbulence

A statistically stationary turbulence with a mean shear gradient is realized in a flow driven by suitable body forces. The flow domain is periodic in downstream and spanwise directions and bounded by stress free surfaces in the normal direction. Except for small layers near the surfaces the flow is homogeneous. The fluctuations in turbulent energy are less violent than in the simulations using remeshing, but the anisotropy on small scales as measured by the skewness of derivatives is similar and decays weakly with increasing Reynolds number.Comment: 4 pages, 5 figures (Figs. 3 and 4 as external JPG-Files

High-temperature LM cathode ion thrusters Quarterly progress report, 5 Feb. - 4 May 1968

Poiseuille flow measurements for high temperature liquid metal cathode ion thruster

Pesticide Residues in Hemlock and Canadice Lakes and their Tributaries in Western New York, 1997-98

In 1997, the U.S.Geological Survey (USGS) and the City of Rochester began a cooperative program to study the presence of pesticides (herbicides and insecticides) that occur at trace levels in Hemlock and Canadice Lakes and their tributaries. The most frequently detected pesticides in streamflow and lake-water samples were herbicides commonly used in agriculture — atrazine, metolachlor, and simazine. None of the concentrations of these compounds in the samples exceeded Federal or State water-quality standards. Differences in the concentrations among stream samples can be attributed to land use and streamflow, and the timing of rainfall in relation to herbicide application. The north (lower) end of Hemlock Lake can receive pesticides in agricultural runoff from northern parts of its watershed and Canadice Creek. These pesticide inputs bypass most of the lake and could periodically affect the water quality periodically affect the water quality at the City of Rochester intake. Pesticide concentrations in samples from the intake during this study, however, were about 100 times less than current Federal and State standards for drinking water. Residues of DDT, dieldrin, and mirex are present in low concentrations in the bottom sediments of both lakes, but none were detected in water samples. The use of these insecticides was banned in 1972,and their persistence in the lakebed sediments is probably due to erosion of contaminated soils from agricultural lands

Brief report: how implicit attitudes toward emotion regulation influence partner-directed aggression

Intimate Partner Violence (IPV) perpetration involves many risk factors related to cognitions and emotions, such as insufficient emotion regulation. Those who inadequately regulate negative emotions have been shown to be more likely to have a history of partner violence. However, during routine activities, such as dealing with an intimate partner, emotions are regulated without effortful processing or monitoring. Because this unmonitored emotion regulation process takes place, implicit, or indirect measures of emotion regulation may be better suited to assess unconscious attitudes toward regulating negative emotions. We examined if implicit attitudes toward emotion regulation are related to the frequency of aggressive reactions and if the frequency differs based on a previous history of IPV. A history of IPV predicted some aggressive responding, but the implicit measure unexpected predicted aggression negatively. Possible explanations, clinical implications of inadequate emotion regulation, and intervention suggestions are discussed