5,264 research outputs found
Strong wave-mean-flow coupling in baroclinic acoustic streaming
The interaction of an acoustic wave with a stratified fluid can drive strong
streaming flows owing to the baroclinic production of fluctuating vorticity, as
recently demonstrated by Chini et al. (J. Fluid Mech., 744, 2014, pp. 329). In
the present investigation, a set of wave/mean-flow interaction equations is
derived that governs the coupled dynamics of a standing acoustic wave mode of
characteristic (small) amplitude {\epsilon} and the streaming flow it drives in
a thin channel with walls maintained at differing temperatures. Unlike
classical Rayleigh streaming, the resulting mean flow arises at O({\epsilon})
rather than at O({\epsilon^2}). Consequently, fully two-way coupling between
the waves and the mean flow is possible: the streaming is sufficiently strong
to induce O(1) rearrangements of the imposed background temperature and density
fields, which modifies the spatial structure and frequency of the acoustic mode
on the streaming time scale. A novel Wentzel-Kramers-Brillouin-Jeffreys
analysis is developed to average over the fast wave dynamics, enabling the
coupled system to be integrated strictly on the slow time scale of the
streaming flow. Analytical solutions of the reduced system are derived for weak
wave forcing and are shown to reproduce results from prior direct numerical
simulations (DNS) of the compressible Navier Stokes and heat equations with
remarkable accuracy. Moreover, numerical simulations of the reduced system are
performed in the regime of strong wave mean flow coupling for a fraction of the
computational cost of the corresponding DNS. These simulations shed light on
the potential for baroclinic acoustic streaming to be used as an effective
means to enhance heat transfer.Comment: 29 pages, 7 figure
Recommendations for hygiene of masks and circuits in mechanically home ventilated patients
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Network connectivity between the winter Arctic Oscillation and summer sea ice in CMIP6 models and observations
The indirect effect of winter Arctic Oscillation (AO) events on the following summer Arctic sea ice extent suggests an inherent winter-to-summer mechanism for sea ice predictability. On the other hand, operational regional summer sea ice forecasts in a large number of coupled climate models show a considerable drop in predictive skill for forecasts initialised prior to the date of melt onset in spring, suggesting that some drivers of sea ice variability on longer timescales may not be well represented in these models. To this end, we introduce an unsupervised learning approach based on cluster analysis and complex networks to establish how well the latest generation of coupled climate models participating in phase 6 of the World Climate Research Programme Coupled Model Intercomparison Project (CMIP6) are able to reflect the spatio-temporal patterns of variability in Northern Hemisphere winter sea-level pressure and Arctic summer sea ice concentration over the period 1979â2020, relative to ERA5 atmospheric reanalysis and satellite-derived sea ice observations, respectively. Two specific global metrics are introduced as ways to compare patterns of variability between models and observations/reanalysis: the adjusted Rand index â a method for comparing spatial patterns of variability â and a network distance metric â a method for comparing the degree of connectivity between two geographic regions. We find that CMIP6 models generally reflect the spatial pattern of variability in the AO relatively well, although they overestimate the magnitude of sea-level pressure variability over the north-western Pacific Ocean and underestimate the variability over northern Africa and southern Europe. They also underestimate the importance of regions such as the Beaufort, East Siberian, and Laptev seas in explaining pan-Arctic summer sea ice area variability, which we hypothesise is due to regional biases in sea ice thickness. Finally, observations show that historically, winter AO events (negatively) covary strongly with summer sea ice concentration in the eastern Pacific sector of the Arctic, although now under a thinning ice regime, both the eastern and western Pacific sectors exhibit similar behaviour. CMIP6 models however do not show this transition on average, which may hinder their ability to make skilful seasonal to inter-annual predictions of summer sea ice
Energy-efficient domain wall motion governed by the interplay of helicity-dependent optical effect and spin-orbit torque
Spin-orbit torque provides a powerful means of manipulating domain walls
along magnetic wires. However, the current density required for domain wall
motion is still too high to realize low power devices. Here we experimentally
demonstrate helicity-dependent domain wall motion by combining synchronized
femtosecond laser pulses and short current pulses in Co/Ni/Co ultra-thin film
wires with perpendicular magnetization. Domain wall can remain pinned under one
laser circular helicity while depinned by the opposite circular helicity.
Thanks to the all-optical helicity-dependent effect, the threshold current
density due to spin-orbit torque can be reduced by more than 50%. Based on this
joint effect combining spin-orbit torque and helicity-dependent laser pulses,
an optoelectronic logic-in-memory device has been experimentally demonstrated.
This work enables a new class of low power spintronic-photonic devices beyond
the conventional approach of all-optical switching or all-current switching for
data storage.Comment: 21 pages, 5 figure
A Search for Water in the Atmosphere of HAT-P-26b Using LDSS-3C
The characterization of a physically-diverse set of transiting exoplanets is
an important and necessary step towards establishing the physical properties
linked to the production of obscuring clouds or hazes. It is those planets with
identifiable spectroscopic features that can most effectively enhance our
understanding of atmospheric chemistry and metallicity. The newly-commissioned
LDSS-3C instrument on Magellan provides enhanced sensitivity and suppressed
fringing in the red optical, thus advancing the search for the spectroscopic
signature of water in exoplanetary atmospheres from the ground. Using data
acquired by LDSS-3C and the Spitzer Space Telescope, we search for evidence of
water vapor in the transmission spectrum of the Neptune-mass planet HAT-P-26b.
Our measured spectrum is best explained by the presence of water vapor, a lack
of potassium, and either a high-metallicity, cloud-free atmosphere or a
solar-metallicity atmosphere with a cloud deck at ~10 mbar. The emergence of
multi-scale-height spectral features in our data suggests that future
observations at higher precision could break this degeneracy and reveal the
planet's atmospheric chemical abundances. We also update HAT-P-26b's transit
ephemeris, t_0 = 2455304.65218(25) BJD_TDB, and orbital period, p =
4.2345023(7) days.Comment: 9 pages, 8 figures, Accepted for publication in Ap
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