501,949 research outputs found
Estimation of orographically induced wave drag in the stable boundary layer during the CASES-99 experimental campaign
This paper addresses the quantification of gravity wave drag due to small hills in the stable boundary layer. A single column atmospheric model is used to forecast wind and temperature profiles in the boundary layer. Next, these profiles are used to calculate vertical profiles of gravity wave drag. Climatology of wave drag magnitude and ¿wave drag events¿ is presented for the CASES-99 experimental campaign. It is found that gravity wave drag events occur for several relatively calm nights, and that the wave drag is then of equivalent magnitude as the turbulent drag. We also illustrate that wave drag events modify the wind speed sufficiently to substantially change the surface sensible heat flu
Theory of Concentration Dependence in Drag Reduction by Polymers and of the MDR asymptote
A simple model of the effect of polymer concentration on the amount of drag
reduction in turbulence is presented, simulated and analyzed. The qualitative
phase diagram of drag coefficient vs. Reynolds number (Re) is recaptured in
this model, including the theoretically elusive onset of drag reduction and the
Maximum Drag Reduction (MDR) asymptote. The Re-dependent drag and the MDR are
analytically explained, and the dependence of the amount of drag on material
parameters is rationalized
Correlated Coulomb drag in capacitively coupled quantum-dot structures
We study theoretically Coulomb drag in capacitively coupled quantum dots
(CQDs) -- a biasdriven dot coupled to an unbiased dot where transport is due to
Coulomb mediated energy transfer drag. To this end, we introduce a
master-equation approach which accounts for higher-order tunneling
(cotunneling) processes as well as energy-dependent lead couplings, and
identify a mesoscopic Coulomb drag mechanism driven by nonlocal multi-electron
cotunneling processes. Our theory establishes the conditions for a nonzero drag
as well as the direction of the drag current in terms of microscopic system
parameters. Interestingly, the direction of the drag current is not determined
by the drive current, but by an interplay between the energy-dependent lead
couplings. Studying the drag mechanism in a graphene-based CQD heterostructure,
we show that the predictions of our theory are consistent with recent
experiments on Coulomb drag in CQD systems.Comment: 6 pages, 4 figures + supplementary. Published versio
Coulomb drag in the mesoscopic regime
We present a theory for Coulomb drag between two mesoscopic systems which
expresses the drag in terms of scattering matrices and wave functions. The
formalism can be applied to both ballistic and disordered systems and the
consequences can be studied either by numerical simulations or analytic means
such as perturbation theory or random matrix theory. The physics of Coulomb
drag in the mesoscopic regime is very different from Coulomb drag between
extended electron systems. In the mesoscopic regime we in general find
fluctuations of the drag comparable to the mean value. Examples are the
vanishing average drag for chaotic 2D-systems and the dominating fluctuations
of drag between quasi-ballistic wires with almost ideal transmission.Comment: 4 pages including 2 figures. Proceedings of 19NSM, to apear in Phys.
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Bubbly Turbulent Drag Reduction Is a Boundary Layer Effect
In turbulent Taylor-Couette flow, the injection of bubbles reduces the overall drag. On the other hand, rough walls enhance the overall drag. In this work, we inject bubbles into turbulent Taylor-Couette flow with rough walls (with a Reynolds number up to 4×105), finding an enhancement of the dimensionless drag as compared to the case without bubbles. The dimensional drag is unchanged. As in the rough-wall case no smooth boundary layers can develop, the results demonstrate that bubbly drag reduction is a pure boundary layer effec
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