15,086 research outputs found
Homogeneity and isotropy in a laboratory turbulent flow
We present a new design for a stirred tank that is forced by two parallel
planar arrays of randomly actuated synthetic jets. This arrangement creates
turbulence at high Reynolds number with low mean flow. Most importantly, it
exhibits a region of 3D homogeneous isotropic turbulence that is significantly
larger than the integral lengthscale. These features are essential for enabling
laboratory measurements of turbulent suspensions. We use quantitative imaging
to confirm isotropy at large, small, and intermediate scales by examining one--
and two--point statistics at the tank center. We then repeat these same
measurements to confirm that the values measured at the tank center are
constant over a large homogeneous region. In the direction normal to the
symmetry plane, our measurements demonstrate that the homogeneous region
extends for at least twice the integral length scale cm. In the
directions parallel to the symmetry plane, the region is at least four times
the integral lengthscale, and the extent in this direction is limited only by
the size of the tank. Within the homogeneous isotropic region, we measure a
turbulent kinetic energy of ms, a dissipation
rate of ms, and a Taylor--scale Reynolds
number of . The tank's large homogeneous region, combined with
its high Reynolds number and its very low mean flow, provides the best
approximation of homogeneous isotropic turbulence realized in a laboratory flow
to date. These characteristics make the stirred tank an optimal facility for
studying the fundamental dynamics of turbulence and turbulent suspensions.Comment: 18 pages, 9 figure
Discrimination of low-frequency tones employs temporal fine structure
An auditory neuron can preserve the temporal fine structure of a
low-frequency tone by phase-locking its response to the stimulus. Apart from
sound localization, however, little is known about the role of this temporal
information for signal processing in the brain. Through psychoacoustic studies
we provide direct evidence that humans employ temporal fine structure to
discriminate between frequencies. To this end we construct tones that are based
on a single frequency but in which, through the concatenation of wavelets, the
phase changes randomly every few cycles. We then test the frequency
discrimination of these phase-changing tones, of control tones without phase
changes, and of short tones that consist of a single wavelets. For carrier
frequencies below a few kilohertz we find that phase changes systematically
worsen frequency discrimination. No such effect appears for higher carrier
frequencies at which temporal information is not available in the central
auditory system.Comment: 12 pages, 3 figure
Rebels, Conformists, Contrarians and Momentum Traders
We develop a model of optimal investment with two types of agents with different beliefs about the market dynamics. Market conformists agree with the true log-normal price distribution and rebels believe in price predictability. Depending on their exact beliefs, the rebels may follow either a momentum or a contrarian strategy. It is difficult to detect rebels' beliefs that are not far-fetched from the market perspective. The long-run investment portfolios of both conformist and rebels need not be biased towards equities.
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