8,855 research outputs found
Speed control with low armature loss for very small sensorless brushed DC motors
A method for speed control of brushed dc motors is presented. It is particularly applicable to motors with armatures of less than 1 cm3. Motors with very small armatures are difficult to control using the usual pulsewidth-modulation (PWM) approach and are apt to overheat if so driven. The technique regulates speed via the back electromotive force but does not require current-discontinuous drives. Armature heating in small motors under PWM drive is explained and quantified. The method is verified through simulation and measurement. Control is improved, and armature losses are minimized. The method can expect to find application in miniature mechatronic equipment
Fluid thrust control system
A pure fluid thrust control system is described for a pump-fed, regeneratively cooled liquid propellant rocket engine. A proportional fluid amplifier and a bistable fluid amplifier control overshoot in the starting of the engine and take it to a predetermined thrust. An ejector type pump is provided in the line between the liquid hydrogen rocket nozzle heat exchanger and the turbine driving the fuel pump to aid in bringing the fluid at this point back into the regular system when it is not bypassed. The thrust control system is intended to function in environments too severe for mechanical controls
Orbit-resolved photometry and echelle spectroscopy of the cataclysmic variable ST LMi during a 2007 high state
We present high-resolution echelle spectra and contemporaneous photometry of the polar ST LMi during a high state in 2007 March. Emission lines at Hα, He I λ5876, and He I λ7065 show similar line profiles over orbital phase and have narrow and broad components. These profile changes with phase are very similar to those reported in earlier high-state studies of ST LMi. The radial velocity curves from double Gaussian fits to the line profiles are interpreted as two crossing curves, neither of which is coincident with the orbital motion of the secondary star. We attribute one component to infall motions near the white dwarf and the other to a gas streaming along magnetic field lines connecting the two stars
High spectral resolution time-resolved optical spectroscopy of V893 Sco
We present high resolution time-resolved optical spectra of the high
inclination short orbital period dwarf nova V893 Sco. We performed spectral
analysis through radial velocity measurements, Doppler mapping, and ratioed
Doppler maps. Our results indicate that V893 Sco's accretion disk is dissimilar
to WZ Sge's accretion disk, and does not fit any of the current accretion disk
models. We derive the system parameters M1 and i, and present evidence for V893
Sco as a very young cataclysmic variable and an ER UMa star. We advance the
hypothesis that all ER UMa stars may be newly formed cataclysmic variables.Comment: 23 pages (total), 8 figures, accepted by Ap
A Near-Infrared Spectroscopic Study of the Accreting Magnetic White Dwarf SDSS J121209.31+013627.7 and its Substellar Companion
The nature of the excess near-infrared emission associated with the magnetic
white dwarf commonly known as SDSS 1212 is investigated primarily through
spectroscopy, and also via photometry. The inferred low mass secondary in this
system has been previously detected by the emission and variation of H,
and the m spectral data presented here are consistent with the
presence of a late L or early T dwarf. The excess flux seen beyond 1.5 m
in the phase-averaged spectrum is adequately modeled with an L8 dwarf
substellar companion and cyclotron emission in a 7 MG magnetic field. This
interesting system manifests several observational properties typical of
polars, and is most likely an old interacting binary with a magnetic white
dwarf and a substellar donor in an extended low state.Comment: 28 pages, 5 figures, Accepted to Ap
Computational Model for Microbubble Enhanced Performance of Airlift Bioreactor (ALB)
This paper presents a computational model for microbubble enhanced performance of an airlift bioreactor (ALB). Five different bubble diameters were defined in the model under the same conditions (440 ”m to 1 mm bubble diameter). The computational model parameters and the size of the ALB were defined by referring to experimental work done previously. The main objective of the model is to study the effect of bubble size on the rising velocity and the liquid flow velocity in the airlift reactor (ALB). The results obtained from the computational model shows that microbubbles have a better performance over larger bubbles because microbubbles have better gas hold up due to slow rise velocity and are able to increase the flow velocity due to their high surface area to volume ratio
Time-resolved, multi-color photometry and spectroscopy of Virgo 4 (OU Vir): a high orbital inclination, short orbital period dwarf nova
We present multi-color photometry and time resolved spectroscopy of OU Vir.
The analysis of the quiescent light curve shows that OU Vir is characterized by
i) strong cycle-to-cycle brightness variations, and ii) hot spot modulated
light curve with grazing eclipse of the impact region. Colors are derived both
in- and out- of eclipse. The time-resolved spectroscopy allows us to produce
the radial velocity curve from the H accretion disk emission line which
possibly reveals only weak evidence for hot spot line emission. The hot spot is
believed to be a turbulent optically thick region, producing mostly continuum
emission.Comment: 8 pages (including figures), 7 figures. To Be published in A&
"Dark Matter" in Accretion Disks
Using Spitzer Space Telescope photometric observations of the eclipsing,
interacting binary WZ Sge, we have discovered that the accretion disk is far
more complex than previously believed. Our 4.5 and 8 micron time series
observations reveal that the well known gaseous accretion disk is surrounded by
an asymmetric disk of dusty material with a radius approximately 15 times
larger than the gaseous disk. This dust ring contains only a small amount of
mass and is completely invisible at optical and near-IR wavelengths, hence
consisting of "dark matter". We have produced a model dust ring using 1 micron
spherical particles with a density of 3 g/cm and with a temperature profile
ranging from 700-1500K. Our discovery about the accretion disk structure and
the presence of a larger, outer dust ring have great relevance for accretion
disks in general, including those in other interacting binary systems, pre-main
sequence stars, and active galaxies.Comment: 34 pages, 8 figures (3 in color). Accepted to Ap
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