Stephen Childress

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STATEMENT OF PROBLEM Visual color matching to determine shades in dentistry is inconsistent and unreliable. If accurate, instrumental measurement of tooth color would provide objective, quantified data to match natural teeth to clinical shade guides. PURPOSE This study evaluated and compared the ability of a new computerized colorimeter and a simple(More)
We investigate large Rayleigh number (10– 10) and large Prandtl number (10– 10) thermal convection in glycerol in an aspect ration one cubic cell. The kinematic viscosity of the fluid strongly depends upon the temperature. The symmetry between the top and bottom boundary layers is thus broken, the so-called non-Boussinesq regime. In a previous paper Wu and(More)
The swimming of a sheet, originally treated by G.I. Taylor (1951) for the case of Stokes flow, is considered at moderate and high Reynolds numbers using matched asymptotic expansions. It is shown that for propagatingwaves with frequency ω, wavenumber k, and amplitude b, the swimming speed must be deduced from a dual expansion in powers of the small(More)
Small flexible bodies are observed to hover in an oscillating air column. The air is driven by a large speaker at frequencies in the range 10–65 Hz at amplitudes 1–5 cm. The bodies are made of stiffened tissue paper, bent to form an array of four wings, symmetric about a vertical axis. The flapping of the wings, driven by the oscillating flow, leads to(More)
The advection and diffusion of a passive scalar is investigated for a map of the 2-torus. The map is chaotic, and the limit of almost-uniform stretching is considered. This allows an analytic understanding of the transition from a phase of constant scalar variance (for short times) to exponential decay (for long times). This transition is embodied in a(More)
We construct a valveless pump consisting of a section of elastic tube and a section of rigid tube, connected in a closed loop and filled with water. By periodically squeezing the elastic tube at an asymmetric location, a persistent flow around the tubes is created. This effect, called the Liebau phenomenon or valveless pumping, has been known for some time,(More)
Ornithopters, or flapping-wing aircraft, offer an alternative to helicopters in achieving manoeuvrability at small scales, although stabilizing such aerial vehicles remains a key challenge. Here, we present a hovering machine that achieves self-righting flight using flapping wings alone, without relying on additional aerodynamic surfaces and without(More)
Erosion of solid material by flowing fluids plays an important role in shaping land-forms, and in this natural context is often dictated by processes of high complexity. Here, we examine the coupled evolution of solid shape and fluid flow within the idealized setting of a cylindrical body held against a fast, unidirectional flow, and eroding under the(More)