813 research outputs found
Three-dimensional structure of the flow inside the left ventricle of the human heart
The laboratory models of the human heart left ventricle developed in the last
decades gave a valuable contribution to the comprehension of the role of the
fluid dynamics in the cardiac function and to support the interpretation of the
data obtained in vivo. Nevertheless, some questions are still open and new ones
stem from the continuous improvements in the diagnostic imaging techniques.
Many of these unresolved issues are related to the three-dimensional structure
of the left-ventricular flow during the cardiac cycle. In this paper we
investigated in detail this aspect using a laboratory model. The ventricle was
simulated by a flexible sack varying its volume in time according to a
physiologically shaped law. Velocities measured during several cycles on series
of parallel planes, taken from two orthogonal points of view, were combined
together in order to reconstruct the phase averaged, three-dimensional velocity
field. During the diastole, three main steps are recognized in the evolution of
the vortical structures: i) straight propagation in the direction of the long
axis of a vortex-ring originated from the mitral orifice; ii) asymmetric
development of the vortex-ring on an inclined plane; iii) single vortex
formation. The analysis of three-dimensional data gives the experimental
evidence of the reorganization of the flow in a single vortex persisting until
the end of the diastole. This flow pattern seems to optimize the cardiac
function since it directs velocity towards the aortic valve just before the
systole and minimizes the fraction of blood residing within the ventricle for
more cycles
The dispersal of dense water formed in an idealized coastal polynya on a shallow sloping shelf
Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 1563–1581, doi:10.1175/JPO-D-13-0188.1.This study examines the dispersal of dense water formed in an idealized coastal polynya on a sloping shelf in the absence of ambient circulation and stratification. Both numerical and laboratory experiments reveal two separate bottom pathways for the dense water: an offshore plume moving downslope into deeper ambient water and a coastal current flowing in the direction of Kelvin wave propagation. Scaling analysis shows that the velocity of the offshore plume is proportional not only to the reduced gravity, bottom slope, and inverse of the Coriolis parameter, but also to the ratio of the dense water depth to total water depth. The dense water coastal current is generated by the along-shelf baroclinic pressure gradient. Its dynamics can be separated into two stages: (i) near the source region, where viscous terms are negligible, its speed is proportional to the reduced gravity wave speed and (ii) in the far field, where bottom drag becomes important and balances the pressure gradient, the velocity is proportional to Hc[g′/(LCd)]1/2 in which Hc is the water depth at the coast, g′ the reduced gravity, Cd the quadratic bottom drag coefficient, and L the along-shelf span of the baroclinic pressure gradient. The velocity scalings are verified using numerical and laboratory sensitivity experiments. The numerical simulations suggest that only 3%–23% of the dense water enters the coastal pathway, and the percentage depends highly on the ratio of the velocities of the offshore and coastal plumes. This makes the velocity ratio potentially useful for observational studies to assess the amount of dense water formed in coastal polynyas.WGZ was sponsored by the WHOI
Arctic Research Initiative program. CC received support
from the National Science Foundation Project OCE-1130008.2014-12-0
Correcting Coherent Errors by Random Operation on Actual Quantum Hardware
Characterizing and mitigating errors in current noisy intermediate-scale devices is important to improve the performance of the next generation of quantum hardware. To investigate the importance of the different noise mechanisms affecting quantum computation, we performed a full quantum process tomography of single qubits in a real quantum processor in which echo experiments are implemented. In addition to the sources of error already included in the standard models, the obtained results show the dominant role of coherent errors, which we practically corrected by inserting random single-qubit unitaries in the quantum circuit, significantly increasing the circuit length over which quantum computations on actual quantum hardware produce reliable results
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Foreign exchange risk and the predictability of carry trade returns
This paper provides an empirical investigation of the time-series predictive ability of foreign exchange risk measures on the return to the carry trade, a popular investment strategy that borrows in low-interest currencies and lends in high-interest currencies. Using quantile regressions, we find that higher market variance is significantly related to large future carry trade losses, which is consistent with the unwinding of the carry trade in times of high volatility. The decomposition of market variance into average variance and average correlation shows that the predictive power of market variance is primarily due to average variance since average correlation is not significantly related to carry trade returns. Finally, a new version of the carry trade that conditions on market variance generates performance gains net of transaction costs
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What Do Stock Markets Tell Us about Exchange Rates?
The sign of the correlation between equity returns and exchange rate returns can be positive or negative in theory. Using data for a broad set of forty-two countries, we find that exchange rate movements are in fact unrelated to differentials in country-level equity returns. Consequently, a trading strategy that invests in countries with the highest expected equity returns and shorts those with the lowest generates substantial returns and Sharpe ratios. These returns partially reflect compensation for global equity volatility risk, but significant excess returns remain after controlling for exposure to standard risk factors
Turbulence investigation in a laboratory model of the ascending aorta
This study aims to investigate turbulence inside a model of the
human ascending aorta as a function of the main flow control
parameters. For this purpose, we performed a two-dimensional
in vitro investigation of the pulsatile flow inside a laboratory
model of a healthy aorta by varying both the Reynolds and
Womersley numbers. Our findings indicate that the velocity
fluctuations become significant particularly during the
deceleration phase of the flow, reach the maximum near the
systolic peak and then decay during the rest of the diastole
phase. Higher levels of turbulence were recovered for
increasing Stroke Volumes, in particular maxima of Turbulent
Kinetic Energy occurred in the bulk region while higher
values of Reynolds shear stresses were found in
correspondence of the sinus of Valsalva
Interactions between permeation and gating in the TMEM16B/anoctamin2 calcium-activated chloride channel
At least two members of the TMEM16/anoctamin family, TMEM16A (also known as anoctamin1) and TMEM16B (also known as anoctamin2), encode Ca2+-activated Cl- channels (CaCCs), which are found in various cell types and mediate numerous physiological functions. Here, we used whole-cell and excised inside-out patch-clamp to investigate the relationship between anion permeation and gating, two processes typically viewed as independent, in TMEM16B expressed in HEK 293T cells. The permeability ratio sequence determined by substituting Cl- with other anions (PX/PCl) was SCN- > I- > NO3- > Br- > Cl- > F- > gluconate. When external Cl- was substituted with other anions, TMEM16B activation and deactivation kinetics at 0.5 μM Ca2+ were modified according to the sequence of permeability ratios, with anions more permeant than Cl- slowing both activation and deactivation and anions less permeant than Cl- accelerating them. Moreover, replacement of external Cl- with gluconate, or sucrose, shifted the voltage dependence of steady-state activation (G-V relation) to more positive potentials, whereas substitution of extracellular or intracellular Cl- with SCN- shifted G-V to more negative potentials. Dose-response relationships for Ca2+ in the presence of different extracellular anions indicated that the apparent affinity for Ca2+ at +100 mV increased with increasing permeability ratio. The apparent affinity for Ca2+ in the presence of intracellular SCN- also increased compared with that in Cl-. Our results provide the first evidence that TMEM16B gating is modulated by permeant anions and provide the basis for future studies aimed at identifying the molecular determinants of TMEM16B ion selectivity and gating. © 2014 Betto et al
Offshore transport of shelf waters through interaction of vortices with a shelfbreak current
Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 905–919, doi:10.1175/JPO-D-12-0150.1.Interactions between vortices and a shelfbreak current are investigated, with particular attention to the exchange of waters between the continental shelf and slope. The nonlinear, three-dimensional interaction between an anticyclonic vortex and the shelfbreak current is studied in the laboratory while varying the ratio ε of the maximum azimuthal velocity in the vortex to the maximum alongshelf velocity in the shelfbreak current. Strong interactions between the shelfbreak current and the vortex are observed when ε > 1; weak interactions are found when ε < 1. When the anticyclonic vortex comes in contact with the shelfbreak front during a strong interaction, a streamer of shelf water is drawn offshore and wraps anticyclonically around the vortex. Measurements of the offshore transport and identification of the particle trajectories in the shelfbreak current drawn offshore from the vortex allow quantification of the fraction of the shelfbreak current that is deflected onto the slope; this fraction increases for increasing values of ε. Experimental results in the laboratory are strikingly similar to results obtained from observations in the Middle Atlantic Bight (MAB); after proper scaling, measurements of offshore transport and offshore displacement of shelf water for vortices in the MAB that span a range of values of ε agree well with laboratory predictions.Laboratory work was supported by the
National Science Foundation through Grant OCE-
0081756. Glider observations in March–April 2006 were
supported by the National Science Foundation through
Grant OCE-0220769. Glider observations in July–
October 2007 were supported by a grant from Raytheon.
RET was supported by the Postdoctoral Scholar Program
at the Woods Hole Oceanographic Institution,
with funding provided by the Cooperative Institute for
the North Atlantic Region. The REMUS observations
were funded by the Office of Naval Research. GGG was
supported by the National Science Foundation through
Grant OCE-1129125 for analysis and writing.2013-11-0
Structure and dynamics of a subglacial discharge plume in a Greenlandic fjord
Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 8670–8688, doi:10.1002/2016JC011764.Discharge of surface-derived meltwater at the submerged base of Greenland's marine-terminating glaciers creates subglacial discharge plumes that rise along the glacier/ocean interface. These plumes impact submarine melting, calving, and fjord circulation. Observations of plume properties and dynamics are challenging due to their proximity to the calving edge of glaciers. Therefore, to date information on these plumes has been largely derived from models. Here we present temperature, salinity, and velocity data collected in a plume that surfaced at the edge of Saqqarliup Sermia, a midsized Greenlandic glacier. The plume is associated with a narrow core of rising waters approximately 20 m in diameter at the ice edge that spreads to a 200 m by 300 m plume pool as it reaches the surface, before descending to its equilibrium depth. Volume flux estimates indicate that the plume is primarily driven by subglacial discharge and that this has been diluted in a ratio of 1:10 by the time the plume reaches the surface. While highly uncertain, meltwater fluxes are likely 2 orders of magnitude smaller than the subglacial discharge flux. The overall plume characteristics agree with those predicted by theoretical plume models for a convection-driven plume with limited influence from submarine melting.National Science Foundation (NSF) Grant Numbers: PLR-1418256 , OCE-1434041;
Woods Hole Oceanographic Institution (WHOI) Ocean and Climate Change Institute (OCCI) Arctic Research Initiative
OCCI;
National Aeronautics and Space Administration Grant Number: NNX10AN83H2017-06-1
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