990 research outputs found
Simulations of a weakly conducting droplet under the influence of an alternating electric field
We investigate the electrohydrodynamics of an initially spherical droplet
under the influence of an external alternating electric field by conducting
axisymmetric numerical simulations using a charge-conservative volume-of-fluid
based finite volume flow solver. The mean amplitude of shape oscillations of a
droplet subjected to an alternating electric field for leaky dielectric fluids
is the same as the steady-state deformation under an equivalent root mean
squared direct electric field for all possible electrical conductivity ratio
and permittivity ratio of the droplet to the surrounding fluid.
In contrast, our simulations for weakly conducting media show that this
equivalence between alternating and direct electric fields does not hold for
. Moreover, for a range of parameters, the deformation obtained
using the alternating and direct electric fields is qualitatively different,
i.e. for low and high , the droplet becomes prolate under alternating
electric field but deforms to an oblate shape in the case of the equivalent
direct electric field. A parametric study is conducted by varying the time
period of the applied alternating electric field, the permittivity and the
electrical conductivity ratios. It is observed that while increasing has
a negligible effect on the deformation dynamics of the droplet for , it
enhances the deformation of the droplet when for both alternating and
direct electric fields. We believe that our results may be of immense
consequence in explaining the morphological evolution of droplets in a plethora
of scenarios ranging from nature to biology.Comment: 10 pages, 8 figure
Development of a Diode-Laser Absorption-Spectroscopy Sensor for Real-Time Control of Combustion Systems
Tunable diode-laser absorption spectroscopy (TDLAS) sensors are widely used for measuring gas properties. These sensors offer several advantages including: small footprint, affordability, applicability to harsh environments, rapid time response, and calibration-free operation. As a result, diode-laser sensors can be integrated into control-systems and have previously been used to control gas-turbine combustors. In this study, high-frequency sine waves were generated continuously by a LabVIEW program to simultaneously scan and modulate the wavelength and intensity of a diode laser. The modulated laser light was transmitted 20 cm through the air and measured on a photodetector. Custom-built lock-in software was used to acquire the photodetector signal and extract the corresponding 1st- and 2nd-harmonic wavelength-modulation absorption spectroscopy signals (WMS-1f and -2f) resulting from H2O absorption. The WMS-2f/1f signal was then calculated to enable calibration-free monitoring of gases in real time. During future work, newly developed WMS signal-processing techniques will be used to convert the measured WMS-2f/1f signals into measurements of temperature and H2O concentration, thereby enabling monitoring and control of real combustion systems
Collective excitations and low temperature transport properties of bismuth
We examine the influence of collective excitations on the transport
properties (resistivity, magneto- optical conductivity) for semimetals,
focusing on the case of bismuth. We show, using an RPA approximation, that the
properties of the system are drastically affected by the presence of an
acoustic plasmon mode, consequence of the presence of two types of carriers
(electrons and holes) in this system. We found a crossover temperature T*
separating two different regimes of transport. At high temperatures T > T* we
show that Baber scattering explains quantitatively the DC resistivity
experiments, while at low temperatures T < T* interactions of the carriers with
this collective mode lead to a T^5 behavior of the resistivity. We examine
other consequences of the presence of this mode, and in particular predict a
two plasmon edge feature in the magneto-optical conductivity. We compare our
results with the experimental findings on bismuth. We discuss the limitations
and extensions of our results beyond the RPA approximation, and examine the
case of other semimetals such as graphite or 1T-TiSe_2
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