1,149 research outputs found
Improved numerical methods for turbulent viscous flows aerothermal modeling program, phase 2
The details of a study to develop accurate and efficient numerical schemes to predict complex flows are described. In this program, several discretization schemes were evaluated using simple test cases. This assessment led to the selection of three schemes for an in-depth evaluation based on two-dimensional flows. The scheme with the superior overall performance was incorporated in a computer program for three-dimensional flows. To improve the computational efficiency, the selected discretization scheme was combined with a direct solution approach in which the fluid flow equations are solved simultaneously rather than sequentially
Bulk and contact-sensitized photocarrier generation in single layer TPD devices
In this paper, we report on the photoelectronic properties of TPD studied in
sandwich geometry. In particular, we have obtained from both forward and
reverse bias measurements the "mew-tau" product for holes in TPD. "mew" is the
hole mobility and "tau" the carrier trapping time. The "mew-tau" product is a
measure of the electronic quality of the material and allows a quantitative
comparison of different samples. We have carried out numerical simulations to
understand the photocurrent in these structures. We show that in reverse bias,
the photocurrent (PC) is due to bulk. The carrier generation is governed by
field assisted exciton dissociation at electric fields greater than 10^6 V/cm.
At lower fields the generation of carriers occurs spontaneously in the bulk of
the sample. In forward bias, the photocurrent is due to exciton dissociation at
the ITO contact. We also obtain a "mew-tau" product for holes from forward bias
PC measurements which is in agreement with the value obtained from reverse bias
measurements. Based on our experiments, we demonstrate that TPD in a sandwich
structure is a good candidate for cheap large area solar blind UV detector
arrays.Comment: Submitted to J. Appl. Phy
Differential Dynamic Microscopy to characterize Brownian motion and bacteria motility
We have developed a lab work module where we teach undergraduate students how
to quantify the dynamics of a suspension of microscopic particles, measuring
and analyzing the motion of those particles at the individual level or as a
group. Differential Dynamic Microscopy (DDM) is a relatively recent technique
that precisely does that and constitutes an alternative method to more
classical techniques such as dynamics light scattering (DLS) or video particle
tracking (VPT). DDM consists in imaging a particle dispersion with a standard
light microscope and a camera. The image analysis requires the students to code
and relies on digital Fourier transform to obtain the intermediate scattering
function, an autocorrelation function that characterizes the dynamics of the
dispersion. We first illustrate DDM on the textbook case of colloids where we
measure the diffusion coefficient. Then we show that DDM is a pertinent tool to
characterize biologic systems such as motile bacteria i.e.bacteria that can
self propel, where we not only determine the diffusion coefficient but also the
velocity and the fraction of motile bacteria. Finally, so that our paper can be
used as a tutorial to the DDM technique, we have joined to this article movies
of the colloidal and bacterial suspensions and the DDM algorithm in both Matlab
and Python to analyze the movies
Photoconduction in Alq3
Photoelectronic properties of Alq3 were studied by photoconductivity
measurements in thin film, sandwich (ITO/Alq3/LiF/Al) devices. We find that the
photocurrent is dominated by bulk generation of carriers for incident photon
energies greater than 2.75 eV. The quantum efficiency of photocarrier
generation has been measured from carrier collection measurements to be about
10%. The quantum efficiency is largely independent of electric field. This
enables a direct measurement of the electric field dependence of mobility using
photoconductivity measurements, which is used for quantitative analysis of the
dark forward current in these devices. Photoconductivity measurements were also
used to obtain (\mu_{0n} \tau_n) product which can be used as a measure of
material quality. For Alq3, we find that the value of (\mu_{0n} \tau_n) product
was between 3x10^{-15} cm^2/V to 8x10^{-15} cm^2/V for different samples. In
forward bias, at high field the photocurrent shows saturation accompanied by a
phase shift. These effects are attributed to space charge effects in the
device.Comment: 12 figure
Blue and white light electroluminescence in a multilayer OLED using a new aluminium complex
Synthesis, structure, optical absorption, emission and electroluminescence properties of a new blue emitting Al complex, namely, bis-(2-amino-8-hydroxyquinolinato), acetylacetonato Al(III) are reported. Multilayer OLED using the Al complex showed blue emission at 465 nm, maximum brightness of ~ 425 cd/m2 and maximum current efficiency of 0.16 cd/A. Another multilayer OLED using the Al complex doped with phosphorescent Ir complex showed 'white' light emission, CIE coordinate (0.41, 0.35), maximum brightness of ~ 970 cd/m2 and maximum current efficiency of 0.53 cd/A
Self-sustained oscillations in whiskers without vortex shedding
Sensing the flow of water or air disturbance is critical for the survival of
many animals: flow information helps them localize food, mates, and prey and to
escape predators. Across species, many flow sensors take the form of long,
flexible cantilevers. These cantilevers are known to exhibit sustained
oscillations when interacting with fluid flow. In the presence of vortex
shedding, the oscillations occur through mechanisms such as wake- or
vortex-induced vibrations. There is, however, no clear explanation for the
mechanisms governing the sustained oscillation of flexible cantilevers without
vortex shedding. In recent work, we showed that a flexible cylindrical
cantilever could experience sustained oscillations in its first natural
vibration mode in water at Reynolds numbers below the critical Reynolds number
of vortex shedding. The oscillations were shown to be driven by a frequency
match (synchronization) between the flow frequency and the cantilever's
first-mode natural frequency. Here, we use a body-fitted fluid-structure solver
based on the Navier-Stokes and nonlinear structural equations to simulate the
dynamics of a cantilevered whisker in the air at a subcritical value of
Reynolds number. Results show that second-mode synchronization governs the
whisker's sustained oscillation. Wavy patterns in the shear layer dominate the
whisker's wake during the vibrations, indicating that parallel shear layers
synchronize with the whisker's motion. As a result of this synchronization,
oval-shaped motion trajectories, with matching streamwise and cross-flow
vibration frequencies, are observed along the whisker. The outcomes of this
study suggest possible directions for designing artificial bio-inspired flow
sensors
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