62,081 research outputs found
Preliminary design of an intermittent smoke flow visualization system
A prototype intermittent flow visualization system that was designed to study vortex flow field dynamics has been constructed and tested through its ground test phase. It produces discrete pulses of dense white smoke consisting of particles of terephthalic acid by the pulsing action of a fast-acting three-way valve. The trajectories of the smoke pulses can be tracked by a video imaging system without intruding in the flow around in flight. Two methods of pulsing the smoke were examined. The simplest and safest approach is to simply divert the smoke between the two outlet ports on the valve; this approach should be particularly effective if it were desired to inject smoke at two locations during the same test event. The second approach involves closing off one of the outlet ports to momentarily block the flow. The second approach requires careful control of valve dwell times to avoid excessive pressure buildup within the cartridge container. This method also increases the velocity of the smoke injected into the flow. The flow of the smoke has been blocked for periods ranging from 30 to 80 milliseconds, depending on the system volume and the length of time the valve is allowed to remain open between valve closings
Numerical Approximation of Asymptotically Disappearing Solutions of Maxwell's Equations
This work is on the numerical approximation of incoming solutions to
Maxwell's equations with dissipative boundary conditions whose energy decays
exponentially with time. Such solutions are called asymptotically disappearing
(ADS) and they play an importarnt role in inverse back-scatering problems. The
existence of ADS is a difficult mathematical problem. For the exterior of a
sphere, such solutions have been constructed analytically by Colombini, Petkov
and Rauch [7] by specifying appropriate initial conditions. However, for
general domains of practical interest (such as Lipschitz polyhedra), the
existence of such solutions is not evident.
This paper considers a finite-element approximation of Maxwell's equations in
the exterior of a polyhedron, whose boundary approximates the sphere. Standard
Nedelec-Raviart-Thomas elements are used with a Crank-Nicholson scheme to
approximate the electric and magnetic fields. Discrete initial conditions
interpolating the ones chosen in [7] are modified so that they are (weakly)
divergence-free. We prove that with such initial conditions, the approximation
to the electric field is weakly divergence-free for all time. Finally, we show
numerically that the finite-element approximations of the ADS also decay
exponentially with time when the mesh size and the time step become small.Comment: 15 pages, 3 figure
Human exposure limits to hypergolic fuels
Over the past four decades, many studies have been conducted on the toxicities of the rocket propellants hydrazine (HZ) and monomethylhydrazine (MH). Numerous technical challenges have made it difficult to unambiguously interpret the results of these studies, and there is considerable divergence between results obtained by different investigators on the inhalation concentrations (MAC's) for each toxic effect inducible by exposure to hypergolic fuels in spacecraft atmospheres, NASA undertook a critical review of published and unpublished investigations on the toxicities of these compounds. The current state of the art practices for similar studies. While many questions remain unanswered, MAC's were determined using the best available data for a variety of toxic endpoints for potential continuous exposure durations ranging from 1 hour to 180 days. Spacecraft MAC's (SMAC's) were set for each compound based on the most sensitive toxic endpoint at each exposure duration
Exploratory study of hypergolic ignition spike phenomena, phase 2, part 2, July 1 to September 30, 1966
Chemical analysis, physical properties, and combustion characteristics of hydrazine nitrat
Antenna-coupled millimeter-wave LiNbO_3 electro-optic modulator
The phase-velocity mismatch due to material dispersion in traveling-wave LiNbO_3 optical
waveguide modulators may be greatly reduced by breaking the modulation transmission line into short
segments and connecting each segment to its own surface antenna. The array of antennas is then
illuminated by the modulation signal at an angle which produces a delay from antenna to antenna to
match the optical waveguide's delay
60 GHz and 94 GHz antenna-coupled LiNbO_3 electrooptic modulators
Antenna-coupled LiBbO_3 electrooptic modulators can overcome the material dispersion which would otherwise prevent sensitive high-frequency operation. The authors previously demonstrated the concept with a phase modulator at X-band. They have extended this demonstration to a narrowband 60-GHz phase modulator and broadband amplitude modulator designs at 60 and 94 GHz, respectively
- …