1,647 research outputs found
Interactions within the turbulent boundary layer at high Reynolds number
Simultaneous streamwise velocity measurements across the vertical direction obtained in the atmospheric surface layer (Re_τ ≃ 5 × 10^5) under near thermally neutral conditions are used to outline and quantify interactions between the scales of turbulence, from the very-large-scale motions to the dissipative scales. Results from conditioned spectra, joint probability density functions and conditional averages show that the signature of very-large-scale oscillations can be found across the whole wall region and that these scales interact with the near-wall turbulence from the energy-containing eddies to the dissipative scales, most strongly in a layer close to the wall, z^+ ≲ 10^3. The scale separation achievable in the atmospheric surface layer appears to be a key difference from the low-Reynolds-number picture, in which structures attached to the wall are known to extend through the full wall-normal extent of the boundary layer. A phenomenological picture of very-large-scale motions coexisting and interacting with structures from the hairpin paradigm is provided here for the high-Reynolds-number case. In particular, it is inferred that the hairpin-packet conceptual model may not be exhaustively representative of the whole wall region, but only of a near-wall layer of z^+ = O(10^3), where scale interactions are mostly confined
Arlis/ANZ 2.0
We've all been hearing about Web 2.0 and Library 2.0 lately. But just what are they all about, and is there anything there that Arlis/ANZ can take advantage of in order to better go about its business? Can there, should there, be an Arlis/ANZ 2.0? Arlis/ANZ, the Arts Libraries Society of Australia and New Zealand, is preparing to review it's online prescence. The Arlis/ANZ website is now three years old, and has been well maintained during that time. However it has not yet been systematically reviewed in terms of content, functionality, and strategic direction. The Arlis/ANZ Website version 'One' will be redeveloped into Arlis/ANZ Website version 'Two'. Although much of that review will concentrate on the structure, look and feel of the site, the review process offers an opportunity to incorporate Web 2.0 developments. By incorporating that 'best' of Web 2.0 in a strategic manner, Arlis/ANZ 2.0 - the website - has powerful potential to contribute toward the strength, cohesion and ongoing development of Arlis/ANZ 2.0 - the Society
On the design of optimal compliant walls for turbulence control
This paper employs the theoretical framework developed by Luhar et al. (J.
Fluid Mech., 768, 415-441) to consider the design of compliant walls for
turbulent skin friction reduction. Specifically, the effects of simple
spring-damper walls are contrasted with the effects of more complex walls
incorporating tension, stiffness and anisotropy. In addition, varying mass
ratios are tested to provide insight into differences between aerodynamic and
hydrodynamic applications. Despite the differing physical responses, all the
walls tested exhibit some important common features. First, the effect of the
walls (positive or negative) is greatest at conditions close to resonance, with
sharp transitions in performance across the resonant frequency or phase speed.
Second, compliant walls are predicted to have a more pronounced effect on
slower-moving structures because such structures generally have larger
wall-pressure signatures. Third, two-dimensional (spanwise constant) structures
are particularly susceptible to further amplification. These features are
consistent with many previous experiments and simulations, suggesting that
mitigating the rise of such two-dimensional structures is essential to
designing performance-improving walls. For instance, it is shown that further
amplification of such large-scale two-dimensional structures explains why the
optimal anisotropic walls identified by Fukagata et al. via DNS (J. Turb., 9,
1-17) only led to drag reduction in very small domains. The above observations
are used to develop design and methodology guidelines for future research on
compliant walls
Off-Diagonal Elements of the DeWitt Expansion from the Quantum Mechanical Path Integral
The DeWitt expansion of the matrix element M_{xy} = \left\langle x \right|
\exp -[\case{1}{2} (p-A)^2 + V]t \left| y \right\rangle, in
powers of can be made in a number of ways. For (the case of interest
when doing one-loop calculations) numerous approaches have been employed to
determine this expansion to very high order; when (relevant for
doing calculations beyond one-loop) there appear to be but two examples of
performing the DeWitt expansion. In this paper we compute the off-diagonal
elements of the DeWitt expansion coefficients using the Fock-Schwinger gauge.
Our technique is based on representing by a quantum mechanical path
integral. We also generalize our method to the case of curved space, allowing
us to determine the DeWitt expansion of \tilde M_{xy} = \langle x| \exp
\case{1}{2} [\case{1}{\sqrt {g}} (\partial_\mu - i
A_\mu)g^{\mu\nu}{\sqrt{g}}(\partial_\nu - i A_\nu) ] t| y \rangle by use of
normal coordinates. By comparison with results for the DeWitt expansion of this
matrix element obtained by the iterative solution of the diffusion equation,
the relative merit of different approaches to the representation of as a quantum mechanical path integral can be assessed. Furthermore, the
exact dependence of on some geometric scalars can be
determined. In two appendices, we discuss boundary effects in the
one-dimensional quantum mechanical path integral, and the curved space
generalization of the Fock-Schwinger gauge.Comment: 16pp, REVTeX. One additional appendix concerning end-point effects
for finite proper-time intervals; inclusion of these effects seem to make our
results consistent with those from explicit heat-kernel method
The Double Slit Experiment With Polarizers
The double slit experiment provides a standard way of demonstrating how
quantum mechanics works. We consider modifying the standard arrangement so that
a photon beam incident upon the double slit encounters a polarizer in front of
either one or both of the slits.Comment: 6 page
A new friction factor relationship for fully developed pipe flow
The friction factor relationship for high-Reynolds-number fully developed turbulent pipe flow is investigated using two sets of data from the Princeton Superpipe in the range 31×10^3 ≤ ReD ≤ 35×10^6. The constants of Prandtl’s ‘universal’ friction factor relationship are shown to be accurate over only a limited Reynolds-number range and unsuitable for extrapolation to high Reynolds numbers. New constants, based on a logarithmic overlap in the mean velocity, are found to represent the high-Reynolds-number data to within 0.5%, and yield a value for the von Kármán constant that is consistent with the mean velocity profiles themselves. The use of a generalized logarithmic law in the mean velocity is also examined. A general friction factor relationship is proposed that predicts all the data to within 1.4% and agrees with the Blasius relationship for low Reynolds numbers to within 2.0%
A reduced-order model of three-dimensional unsteady flow in a cavity based on the resolvent operator
A novel reduced-order model for nonlinear flows is presented. The model
arises from a resolvent decomposition in which the nonlinear advection terms of
the Navier-Stokes equation are considered as the input to a linear system in
Fourier space. Results show that Taylor-G\"ortler-like vortices can be
represented from a low-order resolvent decomposition of a nonlinear lid-driven
cavity flow. The present approach provides an approximation of the fluctuating
velocity given the time-mean and the time history of a single velocity probe
Two-week joint mobilization intervention improves self-reported function, range of motion, and dynamic balance in those with chronic ankle instability
We examined the effect of a 2-week anterior-to-posterior ankle joint mobilization intervention on weight-bearing dorsiflexion
range of motion (ROM), dynamic balance, and self-reported function in subjects with chronic ankle instability (CAI). In this prospective
cohort study, subjects received six Maitland Grade III anterior-to-posterior joint mobilization treatments over 2 weeks. Weightbearing
dorsiflexion ROM, the anterior, posteromedial, and posterolateral reach directions of the Star Excursion Balance Test (SEBT),
and self-reported function on the Foot and Ankle Ability Measure (FAAM) were assessed 1 week before the intervention (baseline),
prior to the first treatment (pre-intervention), 24–48 h following the final treatment (post-intervention), and 1 week later (1-week
follow-up) in 12 adults (6 males and 6 females) with CAI. The results indicate that dorsiflexion ROM, reach distance in all directions of
the SEBT, and the FAAM improved (p < 0.05 for all) in all measures following the intervention compared to those prior to the intervention.
No differences were observed in any assessments between the baseline and pre-intervention measures or between the postintervention
and 1-week follow-up measures (p > 0.05). These results indicate that the joint mobilization intervention that targeted
posterior talar glide was able to improve measures of function in adults with CAI for at least 1 week
Gauge Model With Extended Field Transformations in Euclidean Space
An SO(4) gauge invariant model with extended field transformations is
examined in four dimensional Euclidean space. The gauge field is
where are the SO(4) generators in the fundamental
representation. The SO(4) gauge indices also participate in the Euclidean space
SO(4) transformations giving the extended field transformations. We provide the
decomposition of the reducible field in terms of fields
irreducible under SO(4). The SO(4) gauge transformations for the irreducible
fields mix fields of different spin. Reducible matter fields are introduced in
the form of a Dirac field in the fundamental representation of the gauge group
and its decomposition in terms of irreducible fields is also provided. The
approach is shown to be applicable also to SO(5) gauge models in five
dimensional Euclidean space.Comment: 31 pages, Plain LaTe
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