685 research outputs found
The Relative Hydrodynamic Resistance of Various Types of Rivet Heads from Tests of Planning Surfaces, Special Report
The Committee was requested to investigate the effect of various types of rivet heads on hydrodynamic resistance. The proposal was made to obtain the resistance of the various types of rivets by tests of planing surfaces on which the full size rivets would be arranged. The testing methods, results and conclusions are given
The Increase in Frictional Resistance Caused by Various Types of Rivet Heads as Determined by Tests of Planing Surfaces
The increase in the frictional resistance of a surface caused by the presence of rivet heads was determined by towing four planing surfaces of the same dimensions. One surface was smooth and represented a surface without rivet heads or one with perfectly flush countersunk rivets. The other three surfaces were each fitted with the same number of full-size rivet heads but of a different type arranged in the same pattern on each surface. The surfaces were towed at speeds representative of the high water speeds encountered by seaplanes during take-off and the range of Reynolds Number covered by the test was from 4 x 10(exp 6) to 18 x 10(exp 6). The rivet heads investigated were oval countersunk, brazier, and round for rivets having shanks 5/32 inch in diameter. The oval countersunk heads were sunk below the surface by dimpling the plating around them. The results of the tests showed that, for the rivet heads investigated, the increase in the friction coefficient of the surface is directly proportional to the height of the rivet head. The order of merit in regard to low resistance is flush countersunk, oval countersunk (whether sunk below the surface or not), brazier, and round
Automation of a flow injection system for the determination of dissolved silver at picomolar concentrations in seawater with inductively coupled plasma mass spectrometry
An automated flow injection system for the determination of dissolved silver at ultratrace concentrations in seawater, and controlled under LabVIEWâ„¢, is described. The flow injection system allows online processing of seawater samples before their analysis using a magnetic sector inductively coupled plasma mass spectrometry (MS-ICP-MS) instrument. Samples were analysed with a minimum amount of manipulation, thereby reducing the risk of contamination. In addition, the flow injection approach with incorporation of an anion exchange minicolumn allowed ready removal of analytical interferences caused by the saline matrix. The software allowed full control of all flow injection components (valves and pumps) and removed manual time control and, therefore, operator errors. The optimized system was capable of five sample injections per h, including preconcentration and wash steps. The limit of detection was 0.5 pM for a 240-s sample load time, which allowed the determination of dissolved silver in open ocean waters, where picomolar concentration levels are typically encountered
Bright matter wave solitons in Bose-Einstein condensates
We review recent experimental and theoretical work on the creation
of bright matter wave solitons in Bose–Einstein condensates. In two recent experiments,
solitons are formed from Bose–Einstein condensates of 7Li by utilizing
a Feshbach resonance to switch from repulsive to attractive interactions.
The solitons are made to propagate in a one-dimensional potential formed by a
focused laser beam. For repulsive interactions, the wavepacket undergoes dispersivewavepacket
spreading, while for attractive interactions, localized solitons are
formed. In our experiment, a multi-soliton train containing up to ten solitons is
observed to propagate without spreading for a duration of 2 s. Adjacent solitons
are found to interact repulsively, in agreement with a calculation based on the
nonlinear Schr¨odinger equation assuming that the soliton train is formed with an
alternating phase structure. The origin of this phase structure is not entirely clear
Cooper pairing and single particle properties of trapped Fermi gases
We calculate the elementary excitations and pairing of a trapped atomic Fermi
gas in the superfluid phase. The level spectra and pairing gaps undergo several
transitions as the strength of the interactions between and the number of atoms
are varied. For weak interactions, the Cooper pairs are formed between
particles residing in the same harmonic oscillator shell. In this regime, the
nature of the paired state is shown to depend critically on the position of the
chemical potential relative to the harmonic oscillator shells and on the size
of the mean field. For stronger interactions, we find a region where pairing
occur between time-reversed harmonic oscillator states in different shells
also.Comment: Slightly revised version: Mistakes in equation references in figures
corrected. Accepted for Phys. Rev.
- …