5,663 research outputs found
Damping of quasi-2D internal wave attractors by rigid-wall friction
The reflection of internal gravity waves at sloping boundaries leads to
focusing or defocusing. In closed domains, focusing typically dominates and
projects the wave energy onto 'wave attractors'. For small-amplitude internal
waves, the projection of energy onto higher wave numbers by geometric focusing
can be balanced by viscous dissipation at high wave numbers. Contrary to what
was previously suggested, viscous dissipation in interior shear layers may not
be sufficient to explain the experiments on wave attractors in the classical
quasi-2D trapezoidal laboratory set-ups. Applying standard boundary layer
theory, we provide an elaborate description of the viscous dissipation in the
interior shear layer, as well as at the rigid boundaries. Our analysis shows
that even if the thin lateral Stokes boundary layers consist of no more than 1%
of the wall-to-wall distance, dissipation by lateral walls dominates at
intermediate wave numbers. Our extended model for the spectrum of 3D wave
attractors in equilibrium closes the gap between observations and theory by
Hazewinkel et al. (2008)
A luminosity monitor for the A4 parity violation experiment at MAMI
A water Cherenkov luminosity monitor system with associated electronics has
been developed for the A4 parity violation experiment at MAMI. The detector
system measures the luminosity of the hydrogen target hit by the MAMI electron
beam and monitors the stability of the liquid hydrogen target. Both is required
for the precise study of the count rate asymmetries in the scattering of
longitudinally polarized electrons on unpolarized protons. Any helicity
correlated fluctuation of the target density leads to false asymmetries. The
performance of the luminosity monitor, investigated in about 2000 hours with
electron beam, and the results of its application in the A4 experiment are
presented.Comment: 22 pages, 12 figures, submitted to NIM
Inertial waves and modes excited by the libration of a rotating cube
We report experimental measurements of the flow in a cubic container
submitted to a longitudinal libration, i.e. a rotation modulated in time.
Velocity fields in a vertical and a horizontal plane are measured in the
librating frame using a corotating particle image velocimetry system. When the
libration frequency is smaller than twice the mean rotation rate
, inertial waves can propagate in the interior of the fluid. At
arbitrary excitation frequencies , the oscillating flow
shows two contributions: (i) a basic flow induced by the libration motion, and
(ii) inertial wave beams propagating obliquely upward and downward from the
horizontal edges of the cube. In addition to these two contributions, inertial
modes may also be excited at some specific resonant frequencies. We
characterize in particular the resonance of the mode of lowest order compatible
with the symmetries of the forcing, noted [2,1,+]. By comparing the measured
flow fields to the expected inviscid inertial modes computed numerically
[L.R.M. Maas, Fluid Dyn. Res. \textbf{33}, 373 (2003)], we show that only a
subset of inertial modes, matching the symmetries of the forcing, can be
excited by the libration.Comment: Phys. Fluids (in press
Partner Selection Criteria for Participation-based Cooperation
The Dutch construction industry is under pressure. In order to answer the expectations set by society, innovations are necessary. The industry stands in need of clients and contractors who find common objectives and work in participation-based cooperation (such as partnering or alliance) in order to stimulate innovation development. On paper, public clients strive for innovative cooperation with their contractors. Yet, many clients and contractors still try to realise their conflicting objectives, which leads to difficulties in the relationship and counteract innovation. Underlying this lack of support for participation-based cooperation is the inadequate knowledge on the criteria public clients must and may set in order to select a suitable partner. This study describes and analyses the selection of a partner for participation based cooperation by a public client. Criteria for partner selection were assessed with a literature review in relation to participation-based cooperation. A hierarchical case study on five project alliances verified these criteria. The study shows that public clients have to reconsider the specification phase in the tendering process. The research indicates that certain additional criteria must be set if the selection of a partner is to succeed. In particular, criteria concerning the potential employees in the participation-based relationship are necessary. It is recognized by the Regieraad Bouw that public clients need wise applications in order to make clear decisions in the tendering process. The result of this study is a practical set of criteria, which can simplify the selection of a suitable partner and which can contribute to the support of participation-based cooperation in the Dutch construction industry
Color-superconductivity in the strong-coupling regime of Landau gauge QCD
The chirally unbroken and the superconducting 2SC and CFL phases are
investigated in the chiral limit within a Dyson-Schwinger approach for the
quark propagator in QCD. The hierarchy of Green's functions is truncated such
that at vanishing density known results for the vacuum and at asymptotically
high densities the corresponding weak-coupling expressions are recovered. The
anomalous dimensions of the gap functions are analytically calculated. Based on
the quark propagator the phase structure is studied, and results for the gap
functions, occupation numbers, coherence lengths and pressure differences are
given and compared with the corresponding expressions in the weak-coupling
regime. At moderate chemical potentials the quasiparticle pairing gaps are
several times larger than the extrapolated weak-coupling results.Comment: 14 pages, 9 figures; v2: one reference adde
Two infrared Yang-Mills solutions in stochastic quantization and in an effective action formalism
Three decades of work on the quantum field equations of pure Yang-Mills
theory have distilled two families of solutions in Landau gauge. Both coincide
for high (Euclidean) momentum with known perturbation theory, and both predict
an infrared suppressed transverse gluon propagator, but whereas the solution
known as "scaling" features an infrared power law for the gluon and ghost
propagators, the "massive" solution rather describes the gluon as a vector
boson that features a finite Debye screening mass.
In this work we examine the gauge dependence of these solutions by adopting
stochastic quantization. What we find, in four dimensions and in a rainbow
approximation, is that stochastic quantization supports both solutions in
Landau gauge but the scaling solution abruptly disappears when the parameter
controlling the drift force is separated from zero (soft gauge-fixing),
recovering only the perturbative propagators; the massive solution seems to
survive the extension outside Landau gauge. These results are consistent with
the scaling solution being related to the existence of a Gribov horizon, with
the massive one being more general.
We also examine the effective action in Faddeev-Popov quantization that
generates the rainbow and we find, for a bare vertex approximation, that the
the massive-type solutions minimise the quantum effective action.Comment: 13 pages, 7 figures. Change of title to reflect version accepted for
publicatio
Mean flow generation by three-dimensional non-linear internal wave beams
We study the generation of strong mean flow by weakly non-linear internal
wave beams. With a perturbational expansion, we construct analytic solutions
for 3D internal wave beams, exact up to first order accuracy in the viscosity
parameter. We specifically focus on the subtleties of wave beam generation by
oscillating boundaries, such as wave makers in laboratory set-ups. The exact
solutions to the linearized equations allow us to derive an analytic expression
for the mean vertical vorticity production term, which induces a horizontal
mean flow. Whereas mean flow generation associated with viscous beam
attenuation - known as streaming - has been described before, we are the first
to also include a peculiar inviscid mean flow generation in the vicinity of the
oscillating wall, resulting from line vortices at the lateral edges of the
oscillating boundary. Our theoretical expression for the mean vertical
vorticity production is in good agreement with earlier laboratory experiments,
for which the previously unrecognized inviscid mean flow generation mechanism
turns out to be significant
Waves attractors in rotating fluids: a paradigm for ill-posed Cauchy problems
In the limit of low viscosity, we show that the amplitude of the modes of
oscillation of a rotating fluid, namely inertial modes, concentrate along an
attractor formed by a periodic orbit of characteristics of the underlying
hyperbolic Poincar\'e equation. The dynamics of characteristics is used to
elaborate a scenario for the asymptotic behaviour of the eigenmodes and
eigenspectrum in the physically relevant r\'egime of very low viscosities which
are out of reach numerically. This problem offers a canonical ill-posed Cauchy
problem which has applications in other fields.Comment: 4 pages, 5 fi
Observations on the vertical structure of tidal and inertial currents in the central North Sea
Tidal and inertial current ellipses, measured at several locations and depths in the central North Sea during a number of monthly periods in 1980, 1981 and 1982, are decomposed into counterrotating, circular components to which Ekman dynamics are applied to determine Ekman layer depths and vertical phase differences, from which are inferred overall values of the eddy viscosity and drag coefficient. Stratification effects produce an additional vertical phase shift of the anticyclonic rotary component, indicative of an inverse proportionality of the eddy viscosity to the vertical density gradient. From the time variations of the Ekman layer depths of the semidiurnal tidal components, as well as from the vertical structure of the inertial current component, we infer variations in the relative vorticity of the low-frequency flow
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