Wind power fluctuations for an individual turbine and plant have been widely reported to follow the Kolmogorov spectrum of atmospheric turbulence; both vary with a fluctuation time scale τ as ...τ^{2/3}. Yet, this scaling has not been explained through turbulence theory. Using turbines as probes of turbulence, we show the τ^{2/3} scaling results from a large scale influence of atmospheric turbulence. Owing to this long-range influence spanning 100s of kilometers, when power from geographically distributed wind plants is summed into aggregate power at the grid, fluctuations average (geographic smoothing) and their scaling steepens from τ^{2/3}→τ^{4/3}, beyond which further smoothing is not possible. Our analysis demonstrates grids have already reached this τ^{4/3} spectral limit to geographic smoothing.
A two-dimensional system of photoelastic disks subject to vertical tapping against gravity was experimentally monitored from ordered to disordered configurations by varying bidispersity. The packing ...fraction ϕ, coordination number Z, and an appropriately defined force-chain orientational order parameter S all exhibit as similar sharp transition with a small increase in disorder. A measurable change in S, but not ϕ and Z, was detected under tapping. We find disorder-induced metastability does not show configurational relaxation, but can be detected via force-chain reorientations.
Price and volatility co-jumps Bandi, F.M.; Renò, R.
Journal of financial economics,
January 2016, 2016-01-00, 20160101, Letnik:
119, Številka:
1
Journal Article
Recenzirano
The nature of the dependence between discontinuities in prices and contemporaneous discontinuities in volatility (co-jumps) has been reported by many as being elusive, in terms of sign, magnitude, ...and statistical significance. Using a novel identification strategy in continuous time relying on trade-level information for spot variance estimation, as well as infinitesimal cross-moments, we document that a sizeable proportion of discontinuous changes in prices are associated with strongly anti-correlated, contemporaneous, discontinuous changes in volatility. Assuming a possibly nonmonotonic pricing kernel, we illustrate the equilibrium implications of price and volatility co-jumps for return and variance risk premia.
Juvenile songbirds learn vocal communication from adult tutors of the same species but not from adults of other species. How species-specific learning emerges from the basic features of song prosody ...remains unknown. In the zebra finch auditory cortex, we discovered a class of neurons that register the silent temporal gaps between song syllables and are distinct from neurons encoding syllable morphology. Behavioral learning and neuronal coding of temporal gap structure resisted song tutoring from other species: Zebra finches fostered by Bengalese finch parents learned Bengalese finch song morphology transposed onto zebra finch temporal gaps. During the vocal learning period, temporal gap neurons fired selectively to zebra finch Neuroi. 508, 840-866 (2008). song. The innate temporal coding of intersyllable silent gaps suggests a neuronal barcode for conspecific vocal learning and social communication in acoustically diverse environments.
The scale of predictability Bandi, F.M.; Perron, B.; Tamoni, A. ...
Journal of econometrics,
January 2019, 2019-01-00, 20190101, Letnik:
208, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We introduce a new stylized fact: the hump-shaped behavior of slopes and coefficients of determination as a function of the aggregation horizon when running (forward/backward) predictive regressions ...of future excess market returns onto past economic uncertainty (as proxied by market variance, consumption variance, or economic policy uncertainty). To justify this finding formally, we propose a novel modeling framework in which predictability is specified as a property of components of both excess market returns and economic uncertainty. We dub this property scale-specific predictability. We show that classical predictive systems imply restricted forms of scale-specific predictability. We conclude that for certain predictors, like economic uncertainty, the restrictions imposed by classical predictive systems may be excessively strong.
Tension grips the flow Bandi, M. M.
Journal of fluid mechanics,
07/2018, Letnik:
846
Journal Article
Recenzirano
Odprti dostop
Surface tension plays a dominant role in the formation and stability of soap films. It renders them both a quasi-two-dimensional fluid and an elastic membrane at the same time. The techniques for ...measuring the surface tension of the soap solution may very well apply to the static soap film, but how can the surface tension of a soap film be unintrusively measured, and what value would it assume? The answer, being at the intersection of physical chemistry, non-equilibrium physics and interfacial fluid dynamics, is not amenable to deduction via established methods. In a joint theoretical and experimental study, Sane et al. (J. Fluid Mech., vol. 841, 2018, R2) exploit elasticity theory to glean the answer through a simple, yet elegant framework.
We experimentally study the temporal second-order structure functions for integer powers of turbulent fluid velocity fluctuations , in three dimensional (3D) and two dimensional (2D) turbulence. Here ...is a composite time-series constructed by averaging the concurrent time-series () sampled at N spatially distributed Eulerian points. The N = 1 case has been extensively studied for velocity fluctuations (m = 1) and to a lesser extent for m > 1. The averaging method in case of N > 1 diverges from the Kolmogorov framework and has not been studied because fluctuations in are expected to smooth with increasing N leaving behind uninteresting large-scale mean flow information, but we find this is not so. We report the evolution of scaling exponents for in going from a single (N = 1) to a spatial average over several Eulerian points (). Our 3D experiments in a tank with rotating jets at the floor show for all m-values in agreement with prior results and evolves to an asymptotic value of . The evolution of follows the functional form , where points is the only fit parameter representing the convergence rate constant. Results for the 2D experiments conducted in a gravity assisted soap film in the enstrophy cascade regime are in sharp contrast with their 3D counterparts. Firstly varies polynomially with m and asymptotes to a constant value at m = 5. Secondly, the evolution of is logarithmic , where A and B are fit parameters and eventually deviates at large N and asymptotes to for all m. The starkly different convergence forms (exponential in 3D versus logarithmic in 2D) may be interpreted as a signature of inter-scale couplings in the respective turbulent flows by decomposing the two-point correlator for into a self-correlation and cross-correlation term. In addition to aiding in the theoretical development, the results may also have implications for determination of resolution in 2D turbulence experiments and simulations, wind energy and atmospheric boundary layer turbulence.
The stiff human foot enables an efficient push-off when walking or running, and was critical for the evolution of bipedalism
. The uniquely arched morphology of the human midfoot is thought to ...stiffen it
, whereas other primates have flat feet that bend severely in the midfoot
. However, the relationship between midfoot geometry and stiffness remains debated in foot biomechanics
, podiatry
and palaeontology
. These debates centre on the medial longitudinal arch
and have not considered whether stiffness is affected by the second, transverse tarsal arch of the human foot
. Here we show that the transverse tarsal arch, acting through the inter-metatarsal tissues, is responsible for more than 40% of the longitudinal stiffness of the foot. The underlying principle resembles a floppy currency note that stiffens considerably when it curls transversally. We derive a dimensionless curvature parameter that governs the stiffness contribution of the transverse tarsal arch, demonstrate its predictive power using mechanical models of the foot and find its skeletal correlate in hominin feet. In the foot, the material properties of the inter-metatarsal tissues and the mobility of the metatarsals may additionally influence the longitudinal stiffness of the foot and thus the curvature-stiffness relationship of the transverse tarsal arch. By analysing fossils, we track the evolution of the curvature parameter among extinct hominins and show that a human-like transverse arch was a key step in the evolution of human bipedalism that predates the genus Homo by at least 1.5 million years. This renewed understanding of the foot may improve the clinical treatment of flatfoot disorders, the design of robotic feet and the study of foot function in locomotion.
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