Morphological instability of the solid-liquid interface in crystal growth under supercooled liquid film flow and natural convection airflow

Ring-like ripples on the surface of icicles are an example of morphological instability of the ice-water interface during ice growth under supercooled water film flow. The surface of icicles is typically covered with ripples of about 1 cm in wavelength, and the wavelength appears to be almost independent of external temperature, icicle radius, and volumetric water flow rate. One side of the water layer consists of the water-air surface and growing ice is the other. This is one of the more complicated moving phase boundary problems with two interfaces. A recent theoretical work [K. Ueno, Phys. Rev. E 68, (2003) 021603] to address the underlying instability that produces ripples is based on the assumption of the absence of airflow around icicles. In this paper, we extend the previous theoretical framework to include a natural convection airflow ahead of the water-air surface and consider whether the effect of natural convection airflow on the wavelength of ripples produced on an ice surface is essential or not.Comment: 19 pages, 5 figure

Simulation of fluid flow in hydrophobic rough microchannels

Surface effects become important in microfluidic setups because the surface to volume ratio becomes large. In such setups the surface roughness is not any longer small compared to the length scale of the system and the wetting properties of the wall have an important influence on the flow. However, the knowledge about the interplay of surface roughness and hydrophobic fluid-surface interaction is still very limited because these properties cannot be decoupled easily in experiments. We investigate the problem by means of lattice Boltzmann (LB) simulations of rough microchannels with a tunable fluid-wall interaction. We introduce an ``effective no-slip plane'' at an intermediate position between peaks and valleys of the surface and observe how the position of the wall may change due to surface roughness and hydrophobic interactions. We find that the position of the effective wall, in the case of a Gaussian distributed roughness depends linearly on the width of the distribution. Further we are able to show that roughness creates a non-linear effect on the slip length for hydrophobic boundaries.Comment: 10 pages, 5 figure

Constraints on the Growth and Spin of the Supermassive Black Hole in M32 From High Cadence Visible Light Observations

We present 1-second cadence observations of M32 (NGC221) with the CHIMERA instrument at the Hale 200-inch telescope of the Palomar Observatory. Using field stars as a baseline for relative photometry, we are able to construct a light curve of the nucleus in the g-prime and r-prime band with 1sigma=36 milli-mag photometric stability. We derive a temporal power spectrum for the nucleus and find no evidence for a time-variable signal above the noise as would be expected if the nuclear black hole were accreting gas. Thus, we are unable to constrain the spin of the black hole although future work will use this powerful instrument to target more actively accreting black holes. Given the black hole mass of (2.5+/-0.5)*10^6 Msun inferred from stellar kinematics, the absence of a contribution from a nuclear time-variable signal places an upper limit on the accretion rate which is 4.6*10^{-8} of the Eddington rate, a factor of two more stringent than past upper limits from HST. The low mass of the black hole despite the high stellar density suggests that the gas liberated by stellar interactions was primarily at early cosmic times when the low-mass black hole had a small Eddington luminosity. This is at least partly driven by a top-heavy stellar initial mass function at early cosmic times which is an efficient producer of stellar mass black holes. The implication is that supermassive black holes likely arise from seeds formed through the coalescence of 3-100 Msun mass black holes that then accrete gas produced through stellar interaction processes.Comment: 8 pages, 3 figures, submitted to the Astrophysical Journal, comments welcom

Using GIS to identify clusters of potential donors to colleges and universities

As tuition at colleges and universities continues to rise, many development offices face increased pressure to raise additional funds from alumni and friends. This pressure has intensified, in part due to costs associated with the investment in the latest computer technology. But these costly computer tools also can be used by schools to increase philanthropic giving. This paper explores ways in which development offices can use one computer-based research tool, Geographic Information Systems, to better identify potential donors. GIS allows a researcher to overlay data on a map and then search for patterns that might not be otherwise apparent. The paper offers a brief history of GIS and explores its diverse uses. The paper focuses on several current uses of GIS at colleges and universities and explores initial efforts by schools to use the technology in philanthropic giving. Finally, the paper demonstrates how GIS can work in a university capital campaign. Using data from one school, the paper shows how geocoding can help a development office focus on such questions as 1) whether alumni and friends who currently contribute are geographically “clustered” in identifiable neighborhoods; 2) what the wealth of these neighborhoods is and whether the wealth correlates with the level of giving; and 3) for alumni and potential donors who are not contributors, what their giving potential is

Atmosphere Loss by Aerial Bursts

We present a simple analytic description of atmospheric mass loss by aerial bursts and demonstrate that mass loss from aerial bursts becomes significant when the maximum impactor size that leads to an aerial burst rather than a ground explosion, $r_o$, is larger than the minimum impactor size needed to achieve atmospheric loss, $r_{min}$. For vertical trajectories, which give the most stringent limit, this condition is approximately satisfied when $\rho_o/\rho_i \gtrsim 0.4 v_e/v_\infty$, which implies atmospheric densities need to be comparable to impactor densities for impactor velocities that are a few times the escape velocity of the planet. The range of impactor radii resulting in aerial burst-induced mass loss, $r_o-r_{min}$, increases with the ratio of the atmosphere to the impactor density and with the trajectory angle of the impactor. The range of impactor radii that result in aerial burst-induced mass loss and the atmospheric mass lost is larger in adiabatic atmospheres than isothermal atmospheres of equivalent total mass, scale height, and atmospheric surface density. Our results imply that aerial bursts are not expected to significantly contribute to the atmospheric mass-loss history of Earth, but are expected to play an important role for planets and exoplanets similar to Neptune with significant atmospheres. For Neptune-like atmospheres, the atmospheric mass ejected per impactor mass by aerial bursts is comparable to that lost by ground explosions, which implies that, for impactors following a Dohnanyi size distribution, overall loss by aerial busts is expected to exceed that by ground explosions by a factor of $(r_{ground}/r_{aerial})^{0.5}$.Comment: 10 pages, 9 figure

Theory of pressure acoustics with boundary layers and streaming in curved elastic cavities

The acoustic fields and streaming in a confined fluid depend strongly on the acoustic boundary layer forming near the wall. The width of this layer is typically much smaller than the bulk length scale set by the geometry or the acoustic wavelength, which makes direct numerical simulations challenging. Based on this separation in length scales, we extend the classical theory of pressure acoustics by deriving a boundary condition for the acoustic pressure that takes boundary-layer effects fully into account. Using the same length-scale separation for the steady second-order streaming, and combining it with time-averaged short-range products of first-order fields, we replace the usual limiting-velocity theory with an analytical slip-velocity condition on the long-range streaming field at the wall. The derived boundary conditions are valid for oscillating cavities of arbitrary shape and wall motion as long as the wall curvature and displacement amplitude are both sufficiently small. Finally, we validate our theory by comparison with direct numerical simulation in two examples of two-dimensional water-filled cavities: The well-studied rectangular cavity with prescribed wall actuation, and the more generic elliptical cavity embedded in an externally actuated rectangular elastic glass block.Comment: 18 pages, 5 figures, pdfLatex, RevTe

Out of equilibrium dynamics of coherent non-abelian gauge fields

We study out-of-equilibrium dynamics of intense non-abelian gauge fields. Generalizing the well-known Nielsen-Olesen instabilities for constant initial color-magnetic fields, we investigate the impact of temporal modulations and fluctuations in the initial conditions. This leads to a remarkable coexistence of the original Nielsen-Olesen instability and the subdominant phenomenon of parametric resonance. Taking into account that the fields may be correlated only over a limited transverse size, we model characteristic aspects of the dynamics of color flux tubes relevant in the context of heavy-ion collisions.Comment: 12 pages, 10 figures; PRD version, minor change

Measuring memory integration: A metric tapping memory representation ratherthan inference

Our ability to link related events could be supported either byconnecting their representations in memory, or by storing themseparately but integrating their content when later drawinginferences. Here, we adapted classic memory contingencyanalyses to develop and validate an integration index designedto tap stored representations. We conducted three pre-registered experiments adopting this metric. We found positiverecall dependency for associations experienced both within thesame and across different events. Compared to a conventionalinference test, we found that recall dependency was moresensitive to a manipulation of memory integration. Leveragingrecall dependency to investigate individual differencesrevealed that better memory for contextual detail wasassociated with faster inference judgments, consistent withhigh-fidelity representations of related memories—but only forpeople who tended to store memories separately. Ourapproach, thus, provides an important tool to illuminate howrelated events are represented in memory

Shear instability and coherent structures in shallow flow adjacent to a porous layer

Author Posting. © Cambridge University Press, 2007. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 593 (2007): 1-32, doi:10.1017/S0022112007008415.Results are presented from an experimental study of shallow flow in a channel partially obstructed by an array of circular cylinders. The cylinder array is a model for emergent vegetation in an open channel, but also represents a simple sparse porous medium. A shear layer with regular vortex structures forms at the edge of the array, evolving downstream to an equilibrium width and vortex size. The vortices induce nearly periodic oscillations with a frequency that matches the most unstable linear mode for a parallel shear flow. The shear layer is asymmetric about the array interface and has a two-layer structure. An inner region of maximum shear near the interface contains a velocity inflection point and establishes the penetration of momentum into the array. An outer region, resembling a boundary layer, forms in the main channel, and establishes the scale of the vortices. The vortex structure, educed by conditional sampling, shows strong crossflows with sweeps from the main channel and ejections from the array, which create significant momentum and mass fluxes across the interface. The sweeps maintain the coherent structures by enhancing shear and energy production at the interface. A linear stability analysis is consistent with the experimental results and demonstrates that the instability is excited by the differential drag between the channel and the array.This material is based upon work supported by the National Science Foundation under Grant 0125056