107 research outputs found
Thermal Detection of Turbulent and Laminar Dissipation in Vortex Front Motion
We report on direct measurements of the energy dissipated in the spin-up of
the superfluid component of 3He-B. A vortex-free sample is prepared in a
cylindrical container, where the normal component rotates at constant angular
velocity. At a temperature of 0.20Tc, seed vortices are injected into the
system using the shear-flow instability at the interface between 3He-B and
3He-A. These vortices interact and create a turbulent burst, which sets a
propagating vortex front into motion. In the following process, the free energy
stored in the initial vortex-free state is dissipated leading to the emission
of thermal excitations, which we observe with a bolometric measurement. We find
that the turbulent front contains less than the equilibrium number of vortices
and that the superfluid behind the front is partially decoupled from the
reference frame of the container. The final equilibrium state is approached in
the form of a slow laminar spin-up as demonstrated by the slowly decaying tail
of the thermal signal.Comment: 12 pages, 5 figures, to appear in Journal of Low Temperature Physic
Superfluid vortex front at T -> 0: Decoupling from the reference frame
Steady-state turbulent motion is created in superfluid 3He-B at low
temperatures in the form of a turbulent vortex front, which moves axially along
a rotating cylindrical container of 3He-B and replaces vortex-free flow with
vortex lines at constant density. We present the first measurements on the
thermal signal from dissipation as a function of time, recorded at 0.2 Tc
during the front motion, which is monitored using NMR techniques. Both the
measurements and the numerical calculations of the vortex dynamics show that at
low temperatures the density of the propagating vortices falls well below the
equilibrium value, i.e. the superfluid rotates at a smaller angular velocity
than the container. This is the first evidence for the decoupling of the
superfluid from the container reference frame in the zero-temperature limit.Comment: 4 pages, 4 figure
Self-localization of magnon Bose-Einstein condensates in the ground state and on excited levels: from harmonic to box-like trapping potential
Long-lived coherent spin precession of 3He-B at low temperatures around 0.2
Tc is a manifestation of Bose-Einstein condensation of spin-wave excitations or
magnons in a magnetic trap which is formed by the order-parameter texture and
can be manipulated experimentally. When the number of magnons increases, the
orbital texture reorients under the influence of the spin-orbit interaction and
the profile of the trap gradually changes from harmonic to a square well, with
walls almost impenetrable to magnons. This is the first experimental example of
Bose condensation in a box. By selective rf pumping the trap can be populated
with a ground-state condensate or one at any of the excited energy levels. In
the latter case the ground state is simultaneously populated by relaxation from
the exited level, forming a system of two coexisting condensates.Comment: 4 pages, 5 figure
Quasiparticle scattering measurements of laminar and turbulent vortex flow in the spin-down of superfluid 3He-B
The dynamics of quantized vortices is studied in superfluid 3He-B after a
rapid stop of rotation. We use Andreev reflection of thermal excitations to
monitor vortex motion with quartz tuning fork oscillators in two different
experimental setups at temperatures below 0.2Tc. Deviations from ideal
cylindrical symmetry in the flow environment cause the early decay to become
turbulent. This is identified from a rapid initial overshoot in the vortex
density above the value before the spin-down and its subsequent decay with a
t^(-3/2) time dependence. The high polarization of the vortices along the
rotation axis significantly suppresses the effective turbulent kinematic
viscosity below the values reported for more homogeneous turbulence and leads
to a laminar late-time response. The vortex dissipation down to T < 0.15Tc is
determined from the precession frequency of the polarized vortex configuration.
In the limit of vanishing normal component density, the laminar dissipation is
found to approach a temperature-independent value, whose origin is currently
under discussion.Comment: 8 pages, 5 figure
Super Stability of Laminar Vortex Flow in Superfluid 3He-B
Vortex flow remains laminar up to large Reynolds numbers (Re~1000) in a
cylinder filled with 3He-B. This is inferred from NMR measurements and
numerical vortex filament calculations where we study the spin up and spin down
responses of the superfluid component, after a sudden change in rotation
velocity. In normal fluids and in superfluid 4He these responses are turbulent.
In 3He-B the vortex core radius is much larger which reduces both surface
pinning and vortex reconnections, the phenomena, which enhance vortex bending
and the creation of turbulent tangles. Thus the origin for the greater
stability of vortex flow in 3He-B is a quantum phenomenon. Only large flow
perturbations are found to make the responses turbulent, such as the walls of a
cubic container or the presence of invasive measuring probes inside the
container.Comment: 4 pages, 6 figure
Correlating Pedestrian Flows and Search Engine Queries
An important challenge for ubiquitous computing is the development of
techniques that can characterize a location vis-a-vis the richness and
diversity of urban settings. In this paper we report our work on correlating
urban pedestrian flows with Google search queries. Using longitudinal data we
show pedestrian flows at particular locations can be correlated with the
frequency of Google search terms that are semantically relevant to those
locations. Our approach can identify relevant content, media, and
advertisements for particular locations.Comment: 4 pages, 1 figure, 1 tabl
Discovering the right place to check-in using web-based proximate selection
With information technology becoming increasingly embedded in our everyday physical world, there is a growing set of mobile applications that involve a connection with the digital representation of physical places. This association is normally initiated with a check-in procedure, through which a person asserts her presence at a particular place and determines the context for subsequent interactions. The common assumption is that a mobile application will be able to search the surrounding environment and present the user with the intended check-in target; however, in a world of ubiquitous place-based services, this assumption may no longer hold. A person in an urban environment would, at any moment, be surrounded by a large number of places, all of which could be regarded as possible interaction contexts for that person. In this work, we investigate the real-word challenges associated with wide-scale place selection and how the process can be affected by the place environment, by the position of the person in relation to the target place and by positioning errors. To study this reality, we used Google Places as a directory of georeferenced places. We conducted 14,400 nearby place queries structured around different combinations of our three independent variables. The results suggest that the overall performance is poor, except for low-density scenarios, and that this discovery process, albeit relevant, should always be combined with other place discovery approaches. The results also help to understand how this performance is affected by check-in positions and by the properties of the place environment.- (undefined
Propagation of thermal excitations in a cluster of vortices in superfluid 3He-B
We describe the first measurement on Andreev scattering of thermal
excitations from a vortex configuration with known density, spatial extent, and
orientations in 3He-B superfluid. The heat flow from a blackbody radiator in
equilibrium rotation at constant angular velocity is measured with two quartz
tuning fork oscillators. One oscillator creates a controllable density of
excitations at 0.2Tc base temperature and the other records the thermal
response. The results are compared to numerical calculations of ballistic
propagation of thermal quasiparticles through a cluster of rectilinear
vortices.Comment: 6 pages, 4 figure
Gamification of Mobile Experience Sampling Improves Data Quality and Quantity
The Experience Sampling Method is used to capture high-quality in situ data from study participants. This method has become popular in studies involving smartphones, where it is often adapted to motivate participation through the use of gamification techniques. However, no work to date has evaluated whether gamification actually affects the quality and quantity of data collected through Experience Sampling. Our study systematically investigates the effect of gamification on the quantity and quality of experience sampling responses on smartphones. In a field study, we combine event contingent and interval contingent triggers to ask participants to describe their location. Subsequently, participants rate the quality of these entries by playing a game with a purpose. Our results indicate that participants using the gamified version of our ESM software provided significantly higher quality responses, slightly increased their response rate, and provided significantly more data on their own accord. Our findings suggest that gamifying experience sampling can improve data collection and quality in mobile settings
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