18,567 research outputs found

    Relative periodic orbits in point vortex systems

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    We give a method to determine relative periodic orbits in point vortex systems: it consists mainly into perform a symplectic reduction on a fixed point submanifold in order to obtain a two-dimensional reduced phase space. The method is applied to point vortices systems on a sphere and on the plane, but works for other surfaces with isotropy (cylinder, ellipsoid, ...). The method permits also to determine some relative equilibria and heteroclinic cycles connecting these relative equilibria.Comment: 27 pages, 17 figure

    Plastic deformation of rough rolling contact: An experimental and numerical investigation

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    Quantifying the surface roughness evolution in contacts is a crucial step in the fatigue prediction process. Surfaces are initially conditioned by the running-in process and later altered by surface fatigue. The aim of this study is to understand and predict the evolution of the micro-geometry in the first few over-rolling cycles. Numerical predictions are validated by experiments. A major difficulty in understanding surface degradation is the measurement of the surface roughness evolution at the relevant scales. A twin disc micro-test rig, called μMag, was specially designed for this kind of analysis. The μMag allows the “in situ” observation of changes in the disc surface during interrupted tests, thus avoiding dismounting the specimens, which is a major cause of inaccuracy. The new method also maintains the relative position of the two discs. The precision of the measurements allows one to use the initial surface micro-geometry as input for the numerical contact calculation. Thus, the plastic deformation of the surfaces can be measured during the first cycles and compared to the numerical prediction. Results show a very good agreement between numerical predictions and experimental measurements

    The Physiological Basis for Altered Na\u3csup\u3e+\u3c/sup\u3e and Cl\u3csup\u3e-\u3c/sup\u3e Movement Across the Gills of Rainbow Trout (\u3cem\u3eOncorhynchus mykiss\u3c/em\u3e) in Alkaline (pH=9.5) Water

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    To test the hypothesis that internal ion imbalances at high pH are caused by altered branchial ion transporting capacity and permeability, radiotracers (24Na+ and 36Cl-) were used to measure ion movements across the gills of intact rainbow trout (Oncorhynchus mykiss) during 3 d exposure to pH 9.5. At control pH (pH 8.0), the trout were in net ion balance, but by 8 h at high pH, 60%–70% reductions in Cl- influx (Cl) and Na+ influx (JNa/in) led to net Cl- and Na+ losses of -200 µmol kg-1 h-1. Outflux (diffusive efflux plus renal ion losses) was not initially altered. By 72 h, net Cl- balance was reestablished because of a restoration of JCl/in. Although JNa/in remained 50% lower at this time, counterbalancing reductions in Na+ outflux restored net Na+ balance. One-substrate ion-uptake kinetics analyses indicated that reduced ion influx after 8 h at pH 9.5 was caused by 50% decreases in Cl- and Na+ maximal transport rates (JCl/max, JNa/max), likely reflecting decreased numbers of functional transport sites. Two-substrate kinetic analyses indicated that reduced internal HCO3- and H+ supply for respective branchial Cl-/base and Na+/acid transport systems also contributed to lower JCl/in and, to a lesser extent, lower JNa/in at pH 9.5. Recovery in in of JCl/in after 3 d accounted for restoration of Cl- balance and max likely reflected increased numbers of transport sites. In contrast, JNa/in remained 33% lower after 3 d, but a lower affinity of the max gills for Na+ (fourfold greater KNa/m) accounted for the chronic m reduction in Na+ influx at pH 9.5. Thus, reestablishment of Cl- uptake capacity and counterbalancing reductions in Na+ outflux allows rainbow trout to reestablish net ion balance in alkaline waters
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