18,807 research outputs found
Improved pH buffering agent for sodium hypochlorite
Sodium citrate/citric acid was found to be an effective buffer for pH control when used with sodium hypochlorite. The mixture does not corrode aluminum. The buffer appears to form a type of conversion coating that may provide corrosion-resistant properties to aluminum in other applications
Study of EVA operations associated with satellite services
Extravehicular mobility unit (EMU) factors associated with satellite servicing activities are identified and the EMU improvements necessary to enhance satellite servicing operations are outlined. Areas of EMU capabilities, equipment and structural interfaces, time lines, EMU modifications for satellite servicing, environmental hazards, and crew training are vital to manned Eva/satellite services and as such are detailed. Evaluation of EMU capabilities indicates that the EMU can be used in performing near term, basic satellite servicing tasks; however, satellite servicing is greatly enhanced by incorporating key modifications into the EMU. The servicing missions involved in contamination sensitive payload repair are illustrated. EVA procedures and equipment can be standardized, reducing both crew training time and in orbit operations time. By standardizing and coordinating procedures, mission cumulative time lines fall well within the EMU capability
Analysis of dynamic stall using unsteady boundary-layer theory
The unsteady turbulent boundary layer and potential flow about a pitching airfoil are analyzed using numerical methods to determine the effect of pitch rate on the delay in forward movement of the rear flow reversal point. An explicit finite difference scheme is used to integrate the unsteady boundary layer equations, which are coupled at each instant of time to a fully unsteady and nonlinear potential flow analysis. A substantial delay in forward movement of the reversal point is demonstrated with increasing pitch rate, and it is shown that the delay results partly from the alleviation of the gradients in the potential flow, and partly from the effects of unsteadiness in the boundary layer itself. The predicted delay in flow-reversal onset, and its variation with pitch rate, are shown to be in reasonable agreement with experimental data relating to the delay in dynamic stall. From the comparisons it can be concluded (a) that the effects of time-dependence are sufficient to explain the failure of the boundary layer to separate during the dynamic overshoot, and (b) that there may be some link between forward movement of the reversal point and dynamic stall
The utility of unmanned probes in lunar exploration
Utility of unmanned probes of Ranger or Surveyor class in Apollo exploration program - Lunar scientific exploratio
Run-and-tumble particles with hydrodynamics: sedimentation, trapping and upstream swimming
We simulate by lattice Boltzmann the nonequilibrium steady states of
run-and-tumble particles (inspired by a minimal model of bacteria), interacting
by far-field hydrodynamics, subject to confinement. Under gravity, hydrodynamic
interactions barely perturb the steady state found without them, but for
particles in a harmonic trap such a state is quite changed if the run length is
larger than the confinement length: a self-assembled pump is formed. Particles
likewise confined in a narrow channel show a generic upstream flux in
Poiseuille flow: chiral swimming is not required
Relaxation of strained silicon on Si0.5Ge0.5 virtual substrates
Strain relaxation has been studied in tensile strained silicon layers grown on Si0.5Ge0.5 virtual substrates, for layers many times the critical thickness, using high resolution x-ray diffraction. Layers up to 30 nm thick were found to relax less than 2% by the glide of preexisting 60° dislocations. Relaxation is limited because many of these dislocations dissociate into extended stacking faults that impede the dislocation glide. For thicker layers, nucleated microtwins were observed, which significantly increased relaxation to 14%. All these tensile strained layers are found to be much more stable than layers with comparable compressive strain
Quantum Stability of (2+1)-Spacetimes with Non-Trivial Topology
Quantum fields are investigated in the (2+1)-open-universes with non-trivial
topologies by the method of images. The universes are locally de Sitter
spacetime and anti-de Sitter spacetime. In the present article we study
spacetimes whose spatial topologies are a torus with a cusp and a sphere with
three cusps as a step toward the more general case. A quantum energy momentum
tensor is obtained by the point stripping method. Though the cusps are no
singularities, the latter cusps cause the divergence of the quantum field. This
suggests that only the latter cusps are quantum mechanically unstable. Of
course at the singularity of the background spacetime the quantum field
diverges. Also the possibility of the divergence of topological effect by a
negative spatial curvature is discussed. Since the volume of the negatively
curved space is larger than that of the flat space, one see so many images of a
single source by the non-trivial topology. It is confirmed that this divergence
does not appear in our models of topologies. The results will be applicable to
the case of three dimensional multi black hole\cite{BR}.Comment: 17 pages, revtex, 3 uuencoded figures containe
Average features of the muon component of EAS or = 10(17) eV
Three 10 sq m liquid scintillators were situated at approximately 0 m, 150 m and 250 m from the center of the Haverah Park array. The detectors were shielded by lead/barytes giving muon detection thresholds of 317 MeV, 431 MeV and 488 MeV respectively. During part of the operational period the 431 MeV threshold was lowered to 313 MeV for comparison purposes. For risetime measurement fast phototubes were used and the 10% to 70% amplitude time interval was parameterized by T sub 70. A muon lateral density distribution of the form rho mu (R theta) = krho(500)0.94 1/R(1 + R/490)-eta has been fitted to the data for 120 m R 600 m and 0.27 (500) 2.55. The shower size parameter (500) is the water Cerenkov response at 500 m from the core of the extensive air showers (EAS) and is relatable to the primary energy. The results show general consistency
Topological Phase Transitions and Holonomies in the Dimer Model
We demonstrate that the classical dimer model defined on a toroidal hexagonal
lattice acquires holonomy phases in the thermodynamic limit. When all
activities are equal the lattice sizes must be considered mod 6 in which case
the finite size corrections to the bulk partition function correspond to a
massless Dirac Fermion in the presence of a flat connection with nontrivial
holonomy. For general bond activities we find that the phase transition in this
model is a topological one, where the torus degenerates and its modular
parameter becomes real at the critical temperature. We argue that these
features are generic to bipartite dimer models and we present a more general
lattice whose continuum partition function is that of a massive Dirac Fermion.Comment: 7 pages, 4 figures. Minor corrections with additional figure
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