471 research outputs found
Feasibility study for a Scanning Celestial Attitude Determination System /SCADS/ for three axis attitude determination at a Command and Data Acquisition /CDA/ station Final report
Scanning Celestial Attitude Determination System /SCADS/ for three axis attitude determination at Command and Data Acquisition /CDA/ statio
Blending HF Radar and Model Velocities in Monterey Bay Through Normal Mode Analysis
Nowcasts of the surface velocity field in Monterey Bay are made for the period August 1-9, 1994, using HF radar observations blended with results from a primitive equation model. A spectral method called normal mode analysis was used. Objective spatial and temporal filtering were performed, and stream function, velocity potential, relative vorticity, and horizontal divergence were calculated over the domain. This type of nowcasting permits global spectral analysis of mode amplitudes, calculation of enstrophy, and additional analyses using tools like empirical orthogonal functions. The nowcasts reported here include open boundary flow information from the numerical model. Nowcasts using no open boundary flow information, however, still provide excellent results for locations within the observation footprint. This method, then, is useful for filtering high-resolution data like HF radar observations, even when open boundary flow information is unavailable. Also, since the nowcast velocity gradient fields were much less noisy than the observations, this may be an effective method for preconditioning high-resolution observation sets for assimilation into a numerical model. Copyright 2000 by the American Geophysical Union
Rubber friction on wet and dry road surfaces: the sealing effect
Rubber friction on wet rough substrates at low velocities is typically 20-30%
smaller than for the corresponding dry surfaces. We show that this cannot be
due to hydrodynamics and propose a novel explanation based on a sealing effect
exerted by rubber on substrate "pools" filled with water. Water effectively
smoothens the substrate, reducing the major friction contribution due to
induced viscoelastic deformations of the rubber by surface asperities. The
theory is illustrated with applications related to tire-road friction.Comment: Format Revtex 4; 8 pages, 11 figures (no color); Published on Phys.
Rev. B (http://link.aps.org/abstract/PRB/v71/e035428); previous work on the
same topic: cond-mat/041204
Synoptic Lagrangian maps: Application to surface transport in Monterey Bay
Here we report on an effort to describe in detail the evolution of surface water particles in Monterey Bay from the time they first enter until the time they leave. The data used for this study are objective mappings from hourly surface currents obtained from high frequency (HF) radar measurements in Monterey Bay for the period 2 June through 4 August 1999. The basic concept is simple: compute the origin and fate of a large number of particles for every hour during the analysis period. However, analyzing and displaying the enormous amount of computed trajectory information required a new data compression technique: synoptic Lagrangian maps produced by representing each trajectory by its origin/fate and its residence time. The results show unexpected complexity and variability not apparent in the Eulerian current archive. For example, the fraction of particles that escaped to the open ocean during this period varied from about 17 to more than 92 percent. Mean particle residence times ranged from 4.5 to 11 days. The distribution of particle residence times and transport pathways varied over time scales from hours to weeks, and space scales from 2 to 40 km. The wide range of variability in particle properties reported here shows that surface transport studies in Monterey Bay require detailed wind and tidal current information over the entire bay, as well as information about the flow along the open ocean boundary
Rubber friction on (apparently) smooth lubricated surfaces
We study rubber sliding friction on hard lubricated surfaces. We show that
even if the hard surface appears smooth to the naked eye, it may exhibit short
wavelength roughness, which may give the dominant contribution to rubber
friction. That is, the observed sliding friction is mainly due to the
viscoelastic deformations of the rubber by the substrate surface asperities.
The presented results are of great importance for rubber sealing and other
rubber applications involving (apparently) smooth surfaces.Comment: 7 pages, 15 figure
Particle-in-cell simulations of a lens on an f-plane
International audienceA particle-in-cell ansatz for solving the Euler equations in a rotating frame is described. The approach is ideally suited for "layered" models of flows with sharp density and velocity fronts. The material and Coriolis accelerations in the Euler equations are solved at each particle while the gradient accelerations are evaluated on a grid and interpolated at each time step to the particles. The height of each particle is fixed and, depending on the application may be constant for all particles or may vary from particle to particle. The approach is used here to predict the evolution of a lens in a layered model with lower layer outcropping. The integral invariant of the volume is conserved exactly and total energy and total angular momentum are conserved to within 3% throughout a 30 day simulation. Exceptional resolution of the density and velocity fronts is maintained during the simulation without imposing any numerical viscosity. the model also reproduces essential characteristics of analytic solutions to a parabolic shaped lens. This algorithm is well suited to parallel implementation; all of the calculations reported here were done on an IBM SP2. Performance speedup and execution time as a function of the number of processors is given
Rubber friction: role of the flash temperature
When a rubber block is sliding on a hard rough substrate, the substrate
asperities will exert time-dependent deformations of the rubber surface
resulting in viscoelastic energy dissipation in the rubber, which gives a
contribution to the sliding friction. Most surfaces of solids have roughness on
many different length scales, and when calculating the friction force it is
necessary to include the viscoelastic deformations on all length scales. The
energy dissipation will result in local heating of the rubber. Since the
viscoelastic properties of rubber-like materials are extremely strongly
temperature dependent, it is necessary to include the local temperature
increase in the analysis. At very low sliding velocity the temperature increase
is negligible because of heat diffusion, but already for velocities of order
0.01 m/s the local heating may be very important. Here I study the influence of
the local heating on the rubber friction, and I show that in a typical case the
temperature increase results in a decrease in rubber friction with increasing
sliding velocity for v > 0.01 m/s. This may result in stick-slip instabilities,
and is of crucial importance in many practical applications, e.g., for the
tire-road friction, and in particular for ABS-breaking systems.Comment: 22 pages, 27 figure
The Savvidy ``ferromagnetic vacuum'' in three-dimensional lattice gauge theory
The vacuum effective potential of three-dimensional SU(2) lattice gauge
theory in an applied color-magnetic field is computed over a wide range of
field strengths. The background field is induced by an external current, as in
continuum field theory. Scaling and finite volume effects are analyzed
systematically. The first evidence from lattice simulations is obtained of the
existence of a nontrivial minimum in the effective potential. This supports a
``ferromagnetic'' picture of gluon condensation, proposed by Savvidy on the
basis of a one-loop calculation in (3+1)-dimensional QCD.Comment: 9pp (REVTEX manuscript). Postscript figures appende
Examining the temporal evolution of hypervelocity impact phenomena via high-speed imaging and ultraviolet-visible emission spectroscopy
On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion
Surface roughness has a huge impact on many important phenomena. The most
important property of rough surfaces is the surface roughness power spectrum
C(q). We present surface roughness power spectra of many surfaces of practical
importance, obtained from the surface height profile measured using optical
methods and the Atomic Force Microscope. We show how the power spectrum
determines the contact area between two solids. We also present applications to
sealing, rubber friction and adhesion for rough surfaces, where the power
spectrum enters as an important input.Comment: Topical review; 82 pages, 61 figures; Format: Latex (iopart). Some
figures are in Postscript Level
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