21,773 research outputs found
Investigation of a drag reduction on a circular cylinder in rotary oscillation
Drag reduction in two-dimensional flow over a circular cylinder, achieved using rotary oscillation, was investigated with computational simulations. In the experiments of Tokumaru & Dimotakis (1991), this mechanism was observed to yield up to 80% drag reduction at Re = 15 000 for certain ranges of frequency and amplitude of sinusoidal rotary oscillation. Simulations with a high-resolution viscous vortex method were carried out over a range of Reynolds numbers (150â15 000) to explore the effects of oscillatory rotational forcing. Significant drag reduction was observed for a rotational forcing which had been very effective in the experiments. The impact of the forcing is strongly Reynolds number dependent. The cylinder oscillation appears to trigger a distinctive shedding pattern which is related to the Reynolds number dependence of the drag reduction. It appears that the source of this unusual shedding pattern and associated drag reduction is vortex dynamics in the boundary layer initiated by the oscillatory cylinder rotation. The practical efficiency of the drag reduction procedure is also discussed
Fluid-structure interaction of two bodies in an inviscid fluid
The interaction of two arbitrary bodies immersed in a two-dimensional inviscid fluid is investigated.
Given the linear and angular velocities of the bodies, the solution of the potential flow problem with
zero circulation around both bodies is reduced to the determination of a suitable Laurent series in a
conformally mapped domain that satisfies the boundary conditions. The potential flow solution is
then used to determine the force and moment acting on each body by using generalized Blasius
formulas. The current formulation is applied to two examples. First, the case of two rigid circular
cylinders interacting in an unbounded domain is investigated. The forces on two cylinders with
prescribed motion forced-forced is determined and compared to previous results for validation
purposes. We then study the response of a single âfreeâ cylinder due to the prescribed motion of the
other cylinder forced-free. This forced-free situation is used to justify the hydrodynamic benefits
of drafting in aquatic locomotion. In the case of two neutrally buoyant circular cylinders, the aft
cylinder is capable of attaining a substantial propulsive force that is the same order of magnitude of
its inertial forces. Additionally, the coupled interaction of two cylinders given an arbitrary initial
condition free-free is studied to show the differences of perfect collisions with and without the
presence of an inviscid fluid. For a certain range of collision parameters, the fluid acts to deflect the
cylinder paths just enough before the collision to drastically affect the long time trajectories of the
bodies. In the second example, the flapping of two plates is explored. It is seen that the interactions
between each plate can cause a net force and torque at certain instants in time, but for idealized
sinusoidal motions in irrotational potential flow, there is no net force and torque acting at the system
center
Photocurrents in nanotube junctions
Photocurrents in nanotube p-n junctions are calculated using a
non-equilibrium Green function quantum transport formalism. The short-circuit
photocurrent displays band-to-band transitions and photon-assisted tunneling,
and has multiple sharp peaks in the infrared, visible, and ultraviolet ranges.
The operation of such devices in the nanoscale regime leads to unusual size
effects, where the photocurrent scales linearly and oscillates with device
length. The oscillations can be related to the density of states in the valence
band, a factor that also determines the relative magnitude of the photoresponse
for different bands.Comment: 5 pages, 4 figures, submitte
Electro-expulsive separation system
An electro-expulsive system has one or more overlapped conductors, each comprising a flexible ribbon conductor, which is folded back on itself. The conductors are embedded in an elastomeric material. Large current pulses are fed to the conductors from power storage units. As a result of the antiparallel currents, the opposed segments of a conductor are forcefully separated and the elastomeric material is distended. Voids in the elastomer aid the separation of the conductor segments. The distention is almost instantaneous when a current pulse reaches the conductor and the distention tends to remove any solid body on the surface of the elastomeric material
Numerical simulation of separated flows
A new numerical method, based on the Vortex Method, for the simulation of two-dimensional separated flows, was developed and tested on a wide range of gases. The fluid is incompressible and the Reynolds number is high. A rigorous analytical basis for the representation of the Navier-Stokes equation in terms of the vorticity is used. An equation for the control of circulation around each body is included. An inviscid outer flow (computed by the Vortex Method) was coupled with a viscous boundary layer flow (computed by an Eulerian method). This version of the Vortex Method treats bodies of arbitrary shape, and accurately computes the pressure and shear stress at the solid boundary. These two quantities reflect the structure of the boundary layer. Several versions of the method are presented and applied to various problems, most of which have massive separation. Comparison of its results with other results, generally experimental, demonstrates the reliability and the general accuracy of the new method, with little dependence on empirical parameters. Many of the complex features of the flow past a circular cylinder, over a wide range of Reynolds numbers, are correctly reproduced
Symbol synchronization in convolutionally coded systems
Alternate symbol inversion is sometimes applied to the output of convolutional encoders to guarantee sufficient richness of symbol transition for the receiver symbol synchronizer. A bound is given for the length of the transition-free symbol stream in such systems, and those convolutional codes are characterized in which arbitrarily long transition free runs occur
Optical polarimetric monitoring of the type II-plateau SN 2005af
Aims. Core-collapse supernovae may show significant polarization that implies
non-spherically symmetric explosions. We observed the type II-plateau SN 2005af
using optical polarimetry in order to verify whether any asphericity is present
in the supernova temporal evolution. Methods. We used the IAGPOL imaging
polarimeter to obtain optical linear polarization measurements in R (five
epochs) and V (one epoch) broadbands. Interstellar polarization was estimated
from the field stars in the CCD frames. The optical polarimetric monitoring
began around one month after the explosion and lasted ~30 days, between the
plateau and the early nebular phase. Results. The weighted mean observed
polarization in R band was [1.89 +/- 0.03]% at position angle (PA) 54 deg.
After foreground subtraction, the level of the average intrinsic polarization
for SN 2005af was ~0.5% with a slight enhancement during the plateau phase and
a decline at early nebular phase. A rotation in PA on a time scale of days was
also observed. The polarimetric evolution of SN 2005af in the observed epochs
is consistent with an overall asphericity of ~20% and an inclination of ~30
deg. Evidence for a more complex, evolving asphericity, possibly involving
clumps in the SN 2005af envelope, is found.Comment: 6 pages, 5 figures, to be published A&
A "partitioned leaping" approach for multiscale modeling of chemical reaction dynamics
We present a novel multiscale simulation approach for modeling stochasticity
in chemical reaction networks. The approach seamlessly integrates
exact-stochastic and "leaping" methodologies into a single "partitioned
leaping" algorithmic framework. The technique correctly accounts for stochastic
noise at significantly reduced computational cost, requires the definition of
only three model-independent parameters and is particularly well-suited for
simulating systems containing widely disparate species populations. We present
the theoretical foundations of partitioned leaping, discuss various options for
its practical implementation and demonstrate the utility of the method via
illustrative examples.Comment: v4: 12 pages, 5 figures, final accepted version. Error found and
fixed in Appendi
ECONOMIC EFFECTS OF COPPER-NICKEL DEVELOPMENT IN NORTHEAST MINNESOTA
Computer simulations of industry gross output, employment and earnings changes associated with alternative copper-nickel development scenarios are presented in this report. The direct and indirect economic effects of seven development scenarios are projected for a mining impact Study Area in St. Louis County, Minnesota.Community/Rural/Urban Development, Resource /Energy Economics and Policy,
The historical development and basis of human factors guidelines for automated systems in aeronautical operations
In order to derive general design guidelines for automated systems a study was conducted on the utilization and acceptance of existing automated systems as currently employed in several commercial fields. Four principal study area were investigated by means of structured interviews, and in some cases questionnaires. The study areas were aviation, a both scheduled airline and general commercial aviation; process control and factory applications; office automation; and automation in the power industry. The results of over eighty structured interviews were analyzed and responses categoried as various human factors issues for use by both designers and users of automated equipment. These guidelines address such items as general physical features of automated equipment; personnel orientation, acceptance, and training; and both personnel and system reliability
- âŠ