958 research outputs found
The effect of small-amplitude time-dependent changes to the surface morphology of a sphere
Typical approaches to manipulation of flow separation employ passive means or active techniques such as blowing and suction or plasma acceleration. Here it is
demonstrated that the flow can be significantly altered by making small changes to the shape of the surface. A proof of concept experiment is performed using a very simple time-dependent perturbation to the surface of a sphere: a roughness element of 1% of the sphere diameter is moved azimuthally around a sphere surface upstream of the uncontrolled laminar separation point, with a rotational frequency as large as the vortex shedding frequency. A key finding is that the non-dimensional time to observe
a large effect on the lateral force due to the perturbation produced in the sphere boundary layers as the roughness moves along the surface is ˆt =tU_(∞)/D ≈4. This slow
development allows the moving element to produce a tripped boundary layer over an extended region. It is shown that a lateral force can be produced that is as large as the
drag. In addition, simultaneous particle image velocimetry and force measurements reveal that a pair of counter-rotating helical vortices are produced in the wake, which
have a significant effect on the forces and greatly increase the Reynolds stresses in the wake. The relatively large perturbation to the flow-field produced by the small
surface disturbance permits the construction of a phase-averaged, three-dimensional (two-velocity component) wake structure from measurements in the streamwise/radial
plane. The vortical structure arising due to the roughness element has implications for flow over a sphere with a nominally smooth surface or distributed roughness. In
addition, it is shown that oscillating the roughness element, or shaping its trajectory, can produce a mean lateral force
Forces on a Sphere in the Presence of Static and Dynamic Roughness Elements
Though the effect of distributed roughness on
flow over a sphere has been examined in detail, there have been few observations as to the effect of an isolated roughness element on the forces induced on a sphere that is in uniform flow. In this experimental study, we
examine how the forces are altered due to both a stationary and dynamic three-dimensional roughness element in the Reynolds number range of 5 x 104 to 5 x 105. It is found that even a small change to the geometry of the sphere, by adding a cylindrical roughness element with a width and height of 1% the sphere diameter, dramatically alters the drag and lateral forces over a wide range of Reynolds numbers. Of particular interest is that the mean of
the lateral force magnitude can be increased by a factor of about seven, compared with a stationary stud, by moving the isolated roughness at a constant angular velocity about
the sphere. These results can be applied to tripping a laminar boundary layer, steering a bluff body, and increasing the mixing of two fluids, using a minimal amount of energy input. This research is a first step towards understanding the interaction between time dependent surface motion and the subsequent alteration of the location of the boundary layer separation line and wake development
Spinors in Weyl Geometry
We consider the wave equation for spinors in -dimensional Weyl
geometry. By appropriately coupling the Weyl vector as well as
the spin connection to the spinor field, conformal
invariance can be maintained. The one loop effective action generated by the
coupling of the spinor field to an external gravitational field is computed in
two dimensions. It is found to be identical to the effective action for the
case of a scalar field propagating in two dimensions.Comment: 13 pages, REVTEX, no figure
Wall-bounded turbulent flows at high Reynolds numbers: Recent advances and key issues
Wall-bounded turbulent flows at high Reynolds numbers have become an increasingly active area of
research in recent years. Many challenges remain in theory, scaling, physical understanding,
experimental techniques, and numerical simulations. In this paper we distill the salient advances of
recent origin, particularly those that challenge textbook orthodoxy. Some of the outstanding
questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the
principal model parameters such as the von Kármán “constant,” the parametrization of roughness
effects, and the scaling of mean flow and Reynolds stresses, are highlighted. Research avenues that
may provide answers to these questions, notably the improvement of measuring techniques and the
construction of new facilities, are identified. We also highlight aspects where differences of opinion
persist, with the expectation that this discussion might mark the beginning of their resolution
Peculiarities of the Canonical Analysis of the First Order Form of the Einstein-Hilbert Action in Two Dimensions in Terms of the Metric Tensor or the Metric Density
The peculiarities of doing a canonical analysis of the first order
formulation of the Einstein-Hilbert action in terms of either the metric tensor
or the metric density along with the affine connection are discussed. It is shown that the
difference between using as opposed to
appears only in two spacetime dimensions. Despite there being a different
number of constraints in these two approaches, both formulations result in
there being a local Poisson brackets algebra of constraints with field
independent structure constants, closed off shell generators of gauge
transformations and off shell invariance of the action. The formulation in
terms of the metric tensor is analyzed in detail and compared with earlier
results obtained using the metric density. The gauge transformations, obtained
from the full set of first class constraints, are different from a
diffeomorphism transformation in both cases.Comment: 13 page
Lamb Wave Tomography for Corrosion Mapping
As the world-wide civil aviation fleet continues to age, methods for accurately predicting the presence of structural flaws-such as hidden corrosion-that compromise airworthiness become increasingly necessary. Ultrasonic guided waves, Lamb waves, allow large sections of aircraft structures to be rapidly inspected. However, extracting quantitative information from Lamb wave data has always involved highly trained personnel with a detailed knowledge of mechanical-waveguide physics. Our work focuses on using a variety of different tomographic reconstruction techniques to graphically represent the Lamb wave data in images that can be easily interpreted by technicians. Because the velocity of Lamb waves depends on thickness, we can convert the travel times of the fundamental Lamb modes into a thickness map of the inspection region. In this paper we show results for the identification of single or multiple back-surface corrosion areas in typical aluminum aircraft skin structures
Nonlinear Dirac and diffusion equations in 1 + 1 dimensions from stochastic considerations
We generalize the method of obtaining the fundamental linear partial
differential equations such as the diffusion and Schrodinger equation, Dirac
and telegrapher's equation from a simple stochastic consideration to arrive at
certain nonlinear form of these equations. The group classification through one
parameter group of transformation for two of these equations is also carried
out.Comment: 18 pages, Latex file, some equations corrected and group analysis in
one more case adde
Exact One Loop Running Couplings in the Standard Model
Taking the dominant couplings in the standard model to be the quartic scalar
coupling, the Yukawa coupling of the top quark, and the SU(3) gauge coupling,
we consider their associated running couplings to one loop order. Despite the
non-linear nature of the differential equations governing these functions, we
show that they can be solved exactly. The nature of these solutions is
discussed and their singularity structure is examined. It is shown that for a
sufficiently small Higgs mass, the quartic scalar coupling decreases with
increasing energy scale and becomes negative, indicative of vacuum instability.
This behavior changes for a Higgs mass greater than 168 GeV, beyond which this
couplant increases with increasing energy scales and becomes singular prior to
the ultraviolet (UV) pole of the Yukawa coupling. Upper and lower bounds on the
Higgs mass corresponding to new physics at the TeV scale are obtained and
compare favourably with the numerical results of the one-loop and two-loop
analyses with inclusion of electroweak couplings.Comment: 5 pages, LaTeX, additional references and further discussion in this
version. Accepted for publication in Canadian Journal of Physic
Invariant measure in hot gauge theories
We investigate properties of the invariant measure for the gauge field
in finite temperature gauge theories both on the lattice and in the continuum
theory. We have found the cancellation of the naive measure in both cases. The
result is quite general and holds at any finite temperature. We demonstrate,
however, that there is no cancellation at any temperature for the invariant
measure contribution understood as Z(N) symmetrical distribution of gauge field
configurations. The spontaneous breakdown of Z(N) global symmetry is entirely
due to the potential energy term of the gluonic interaction in the effective
potential. The effects of this measure on the effective action, mechanism of
confinement and condensation are discussed.Comment: Latex file, 65.5kB, no figure
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