Showing papers by "David S. Cannell published in 1999"
TL;DR: In this article, the authors present experimental results for early transients near the onset of convection of an ethanol-water mixture in cylindrical containers heated from below, where they found transient radially traveling waves whose amplitude grew in time.
Abstract: We present experimental results for early transients near the onset of convection of an ethanol-water mixture in cylindrical containers heated from below. The separation ratio of the mixture was $\ensuremath{\psi}\ensuremath{\approx}\ensuremath{-}0.08,$ and the aspect ratios $\ensuremath{\Gamma}\ensuremath{\equiv}r/d (r$ is the radius and d the height of the sample cell) of two different containers were 10.91 and 11.53. For this system the onset of convection occurs via a subcritical Hopf bifurcation to traveling waves. Beyond the bifurcation we found transient radially traveling waves whose amplitude grew in time. We decomposed the transient patterns into azimuthal modes of the form $\mathrm{cos}m\ensuremath{\theta}.$ The azimuthal symmetry of the pattern depended strongly on $\ensuremath{\Gamma}.$ For $\ensuremath{\Gamma}=10.91$ odd azimuthal modes were preferred, while for $\ensuremath{\Gamma}=11.53$ even modes dominated. We measured the spatial and temporal growth rates at various $\ensuremath{\epsilon}\ensuremath{\equiv}\ensuremath{\Delta}T/\ensuremath{\Delta}{T}_{c}\ensuremath{-}1$ for different azimuthal modes and compared the results for the two aspect ratios. We found the temporal growth rates to be proportional to $\ensuremath{\epsilon},$ but the spatial growth rates were essentially independent of $\ensuremath{\epsilon}.$ Reflection coefficients deduced from the spatial growth rates agree with theory reasonably well. As convection evolved, the patterns collapsed onto one or more diameters, during which time higher-order azimuthal modes grew significantly in amplitude.
7 citations
11 Jan 1999
TL;DR: Meyer et al. as mentioned in this paper proposed a method for characterizing particles in both turbid and transparent fluids by cross-correlating the scattered intensity fluctuations at two nearby points in the far field.
Abstract: Laser light scattering is a standard laboratory technique used for particle sizing, critical fluid studies, and many other areas of interest. We describe a method for characterizing particles in both turbid and transparent fluids by crosscorrelating the scattered intensity fluctuations at two nearby points in the far field. This approach is simple, easy to use, and works over a wide range of angles. Using the technique presented we have extended the concentration range of dynamic light scattering measurements to samples which are so turbid as to be visually opaque. Additionally, we are developing fiber optic homodyning as a technique for suppressing the detrimental effects of stray light, which can act as a local oscillator of unknown strength and corrupt otherwise good dynamic light scattering data. This should allow the use of much smaller sample cells, offering an improvement both for high concentrations and protein solutions. It should also allow for the acquisition of good signals from dynamic light scattering at forward and backward scattering angles, where stray light is often severe, and, where using a microscope for example, many lenses are present. By using fiber optics to mix a local oscillator with the scattered light, perfect wavefront matching is achievable. Such mixing efficiency is almost impossible to attain with bulk optics. By using a fiber coupler to add sufficient local oscillator, the beating effect of any stray light can be lowered to the well-characterized asymptotic limit. This technique should also be capable of better particle size resolution when several particle sizes are present. Copyright Q 1999 by William V. Meyer. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. (c)l999 American Institute of Aeronautics & Astronautics
3 citations