11,208 research outputs found
Helical structure of longitudinal vortices embedded in turbulent wall-bounded flow
Embedded vortices in turbulent wall-bounded flow over a flat plate, generated
by a passive rectangular vane-type vortex generator with variable angle
to the incoming flow in a low-Reynolds number flow ( based on the
inlet grid mesh size m and free stream velocity m s) have been studied with respect to helical symmetry. The
studies were carried out in a low-speed closed-circuit wind tunnel utilizing
Stereoscopic Particle Image Velocimetry (SPIV). The vortices have been shown to
possess helical symmetry, allowing the flow to be described in a simple
fashion. Iso-contour maps of axial vorticity revealed a dominant primary vortex
and a weaker secondary one for . For
angles outside of this range, the helical symmetry was impaired due to the
emergence of additional flow effects. A model describing the flow has been
utilized, showing strong concurrence with the measurements, even though the
model is decoupled from external flow processes that could perturb the helical
symmetry. The pitch, vortex core size, circulation and the advection velocity
of the vortex all vary linearly with the device angle . This is
important for flow control, since one thereby can determine the axial velocity
induced by the helical vortex as well as the swirl redistributing the axial
velocity component for a given device angle . This also simplifies
theoretical studies, \eg to understand and predict the stability of the vortex
and to model the flow numerically
Message passing for vertex covers
Constructing a minimal vertex cover of a graph can be seen as a prototype for
a combinatorial optimization problem under hard constraints. In this paper, we
develop and analyze message passing techniques, namely warning and survey
propagation, which serve as efficient heuristic algorithms for solving these
computational hard problems. We show also, how previously obtained results on
the typical-case behavior of vertex covers of random graphs can be recovered
starting from the message passing equations, and how they can be extended.Comment: 25 pages, 9 figures - version accepted for publication in PR
Intersubband Electron Interaction in 1D-2D Junctions
We have shown that the electron transport through junctions of
one-dimensional and two-dimensional systems, as well as through quantum point
contacts, is considerably affected by the interaction of electrons of different
subbands. The interaction mechanism is caused by Friedel oscillations, which
are produced by electrons of the closed subbands even in smooth junctions.
Because of the interaction with these oscillations, electrons of the open
subbands experience a backscattering. The electron reflection coefficient,
which describes the backscattering, has a sharp peak at the energy equal to the
Fermi energy and may be as high as about 0.1. This result allows one to explain
a number of available experimental facts.Comment: 5 pages, 3 figure
A hard-sphere model on generalized Bethe lattices: Statics
We analyze the phase diagram of a model of hard spheres of chemical radius
one, which is defined over a generalized Bethe lattice containing short loops.
We find a liquid, two different crystalline, a glassy and an unusual
crystalline glassy phase. Special attention is also paid to the close-packing
limit in the glassy phase. All analytical results are cross-checked by
numerical Monte-Carlo simulations.Comment: 24 pages, revised versio
Validation of a model for estimating the strength of the vortex created by a Vortex Generator from its Bound Circulation
A hypothesis is tested and validated for predicting the vortex strength
induced by a vortex generator in wall-bounded flow by combining the knowledge
of the Vortex Generator (VG) geometry and the approaching boundary layer
velocity distribution. In this paper, the spanwise distribution of bound
circulation on the vortex generator is computed from integrating the pressure
force along the VG height calculated using CFD. It is then assumed that all
this bound circulation is shed into the wake to fulfill Helmholtz's theorem and
then curl up into one primary tip vortex. To validate this, the trailed
circulation estimated from the distribution of the bound circulation is
compared to the one in the wake behind the vortex generator determined directly
from the wake velocities at some downstream distance. In practical situations,
the pressure distribution on the vane is unknown and consequently other
estimates of the spanwise force distribution on the VG must instead be applied,
such as using 2D airfoil data corresponding to the VG geometry at each
wall-normal distance. Such models have previously been proposed and used as an
engineering tool to aid preliminary VG design and it is not the purpose of this
paper to refine such engineering models, but to validate their assumptions such
as applying a lifting line model on a VG that has a very low aspect ratio and
placed in wall boundary layer. Herein, high Reynolds number boundary layer
measurements of VG induced flow were used to validate the Reynolds Averaged
Navier-Stokes (RANS) modeled circulation results and are used for further
illustration and validation of the hypothesis.Comment: Accepted for publicatio
Dynamical models for sand ripples beneath surface waves
We introduce order parameter models for describing the dynamics of sand
ripple patterns under oscillatory flow. A crucial ingredient of these models is
the mass transport between adjacent ripples, which we obtain from detailed
numerical simulations for a range of ripple sizes. Using this mass transport
function, our models predict the existence of a stable band of wavenumbers
limited by secondary instabilities. Small ripples coarsen in our models and
this process leads to a sharply selected final wavenumber, in agreement with
experimental observations.Comment: 9 pages. Shortened version of previous submissio
Ownership and control in a competitive industry
We study a differentiated product market in which an investor initially owns a controlling stake in one of two competing firms and may acquire a non-controlling or a controlling stake in a competitor, either directly using her own assets, or indirectly via the controlled firm. While industry profits are maximized within a symmetric two product monopoly, the investor attains this only in exceptional cases. Instead, she sometimes acquires a noncontrolling stake. Or she invests asymmetrically rather than pursuing a full takeover if she acquires a controlling one. Generally, she invests indirectly if she only wants to affect the product market outcome, and directly if acquiring shares is profitable per se. --differentiated products,separation of ownership and control,private benefits of control
Flat galaxies with dark matter halos - existence and stability
We consider a model for a flat, disk-like galaxy surrounded by a halo of dark
matter, namely a Vlasov-Poisson type system with two particle species, the
stars which are restricted to the galactic plane and the dark matter particles.
These constituents interact only through the gravitational potential which
stars and dark matter create collectively. Using a variational approach we
prove the existence of steady state solutions and their nonlinear stability
under suitably restricted perturbations.Comment: 39 page
Strange Meson Enhancement in PbPb Collisions
The NA44 Collaboration has measured yields and differential distributions of
K+, K-, pi+, pi- in transverse kinetic energy and rapidity, around the
center-of-mass rapidity in 158 A GeV/c Pb+Pb collisions at the CERN SPS. A
considerable enhancement of K+ production per pi is observed, as compared to
p+p collisions at this energy. To illustrate the importance of secondary hadron
rescattering as an enhancement mechanism, we compare strangeness production at
the SPS and AGS with predictions of the transport model RQMD.Comment: 11 pages, including 4 figures, LATE
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