Algal nutrient competition in continuous culture

Natural phytoplankton from Lake Constance was exposed to competition experiments in continuous culture at 6 different molar rations of PratioSi as potentially limiting nutrients. As predicted by theory there was competitive exclusion of all species but one at single nutrient limitation, and coexistence of two species in steady state when P was limiting for one and Si for the other species. In one case coexistence of three species occurred. Equilibrium species composition, which stabilized after 3 to 6 weeks of cultivation, was independent of the species composition of the inoculum and only controlled by the nutrient regime. Comparability of experimental results and field observations is discussed

A new look at energy release rates for quasistatically propagating cracks in inelastic materials

A mapping technique is used to derive an integral expression for the energy release rate for a quasistatically propagating crack. The derivation does not depend on any assumptions in regard to the contitutive behavior of the material. It leads to a contour integral around the crack tip, plus an area integral over the region enclosed by this contour. Only the stress and displacement fields appear in the integrands. Although for stationary crack solutions known to the authors the area integral is not convergent, for propagating crack solutions in elastoplastic material, the integrals are convergent, and lead to zero energy release rate. This confirms conclusions by Rice from an independent point of view.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42773/1/10704_2004_Article_BF00012388.pd

The analytic solution of near-tip stress fields for perfectly plastic pressure-sensitive material under plane stress condition

Different from dense metals, many engineering materials exhibit pressure-sensitive yielding and plastic volumetric deformation. Adopting a yield criterion that contains a linear combination of the Mises stress and the hydrostatic stress, the analytic solutions of plane-stress mode I perfectly-plastic near-tip stress fields for pressuresensitive materials are derived. Also, the relevant characteristic fields are presented. This perfectly plastic solution, containing a pressure sensitivity parameter μ, is shown to correspond to the limit of low-hardening solutions, and when μ=0 it reduces to the perfectly plastic solution of near-tip fields for the Mises material given by Hutchinson [1]. The effects of material pressure sensitivity on the near-tip fields are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42771/1/10704_2004_Article_BF00034180.pd

Collective excitations of trapped Bose condensates in the energy and time domains

A time-dependent method for calculating the collective excitation frequencies and densities of a trapped, inhomogeneous Bose-Einstein condensate with circulation is presented. The results are compared with time-independent solutions of the Bogoliubov-deGennes equations. The method is based on time-dependent linear-response theory combined with spectral analysis of moments of the excitation modes of interest. The technique is straightforward to apply, is extremely efficient in our implementation with parallel FFT methods, and produces highly accurate results. The method is suitable for general trap geometries, condensate flows and condensates permeated with vortex structures.Comment: 6 pages, 3 figures small typos fixe

Optomechanical characterization of acoustic modes in a mirror

We present an experimental study of the internal mechanical vibration modes of a mirror. We determine the frequency repartition of acoustic resonances via a spectral analysis of the Brownian motion of the mirror, and the spatial profile of the acoustic modes by monitoring their mechanical response to a resonant radiation pressure force swept across the mirror surface. We have applied this technique to mirrors with cylindrical and plano-convex geometries, and compared the experimental results to theoretical predictions. We have in particular observed the gaussian modes predicted for plano-convex mirrors.Comment: 8 pages, 8 figures, RevTe