Showing papers by "John W. M. Bush published in 2005"

Dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau

Abstract: SUMMARY Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses depend on the mean velocity ( ¯ U ), viscosity (µ) and channel thickness (h), uniquely through the term µ ¯ U/h 2 . These stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 10 18 P asfl owing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr −1 .

Meniscus-climbing insects

Abstract: Water-walking insects and spiders rely on surface tension for static weight support and use a variety of means to propel themselves along the surface. To pass from the water surface to land, they must contend with the slippery slopes of the menisci that border the water's edge. The ability to climb menisci is a skill exploited by water-walking insects as they seek land in order to lay eggs or avoid predators; moreover, it was a necessary adaptation for their ancestors as they evolved from terrestrials to live exclusively on the water surface. Many millimetre-scale water-walking insects are unable to climb menisci using their traditional means of propulsion. Through a combined experimental and theoretical study, here we investigate the meniscus-climbing technique that such insects use. By assuming a fixed body posture, they deform the water surface in order to generate capillary forces: they thus propel themselves laterally without moving their appendages. We develop a theoretical model for this novel mode of propulsion and use it to rationalize the climbers' characteristic body postures and predict climbing trajectories consistent with those reported here and elsewhere.

Particle concentration evolution and sedimentation-induced instabilities in a stably stratified environment

Abstract: We present the results of a combined experimental and theoretical investigation of sedimentation in a stratified fluid. The theory of sedimentation in a homogeneous ambient is extended to include the influence of a spatially varying particle settling speed. The results of an experimental investigation of latex particles settling in a stably stratified salt water solution are reported. Density variations in the suspending fluid reduce the particle settling speed which increases particle concentrations, thus enhancing the effects of hindered settling. A criterion is developed for the convective instability of an initially uniform suspension settling in a stably stratified ambient. If, as depth increases, the magnitude of the ambient density gradient decreases sufficiently rapidly, an initially uniform particle concentration will give rise to a statically unstable density profile. Experimental observations provide qualitative verification of this new stability criterion.

The stratified Boycott effect

Abstract: We present the results of an experimental investigation of the flows generated by monodisperse particles settling at low Reynolds number in a stably stratified ambient with an inclined sidewall. In this configuration, upwelling beneath the inclined wall associated with the Boycott effect is opposed by the ambient density stratification. The evolution of the system is determined by the relative magnitudes of the container depth, ; subsequently, smaller overlying layers develop. Within each layer, convection erodes the initially linear density gradient, generating a step-like density profile throughout the system that persists after all the particles have settled. Particles are transported across the discrete density jumps between layers by plumes of particle-laden fluid.

Visualization of a fish wake using tobacco mosaic virus

Abstract: TMV and viewed through a crossedpolarizer. Flow disturbances cause the rod-shaped TMV toorient along flow lines and so differentially refract incominglight to produce bright spots in regions of strong shear. Theviral solution is invisible to the naked eye and only becomesapparent under examination through a polarizer. An attrac-tive feature of this technique is that it is harmless to fish,providing a safe alternative for visualizing their wakes.The black neon tetra Fig. 1 of body length 3.6 cmmaintains a steady position by flicking its fins and creatingvortices. The fish’s breathing is marked by a glowing whiteregion created by the suction into its mouth. In Fig. 2, thefish swims upward generating a conical wake whose breadthbrakes by extending its pectoral fins and blowing a vortexring from its mouth.Polarized light can also be used to illuminate a suffi-ciently low dilution of Kalliroscope Fig. 3 that it is invis-ible to the naked eye and benign to fish.