Knot undulator to generate linearly polarized photons with low on-axis power density

Heat load on beamline optics is a serious problem to generate pure linearly polarized photons in the third generation synchrotron radiation facilities. For permanent magnet undulators, this problem can be overcome by a figure-8 operating mode. But there is still no good method to tackle this problem for electromagnetic elliptical undulators. Here, a novel operating mode is suggested, which can generate pure linearly polarized photons with very low on-axis heat load. Also the available minimum photon energy of linearly polarized photons can be extended much by this method

A manifold learning approach for integrated computational materials engineering

Image-based simulation is becoming an appealing technique to homogenize properties of real microstructures of heterogeneous materials. However fast computation techniques are needed to take decisions in a limited time-scale. Techniques based on standard computational homogenization are seriously compromised by the real-time constraint. The combination of model reduction techniques and high performance computing contribute to alleviate such a constraint but the amount of computation remains excessive in many cases. In this paper we consider an alternative route that makes use of techniques traditionally considered for machine learning purposes in order to extract the manifold in which data and fields can be interpolated accurately and in real-time and with minimum amount of online computation. Locallly Linear Embedding is considered in this work for the real-time thermal homogenization of heterogeneous microstructures

The energy budget in Rayleigh-Benard convection

It is shown using three series of Rayleigh number simulations of varying aspect ratio AR and Prandtl number Pr that the normalized dissipation at the wall, while significantly greater than 1, approaches a constant dependent upon AR and Pr. It is also found that the peak velocity, not the mean square velocity, obeys the experimental scaling of Ra^{0.5}. The scaling of the mean square velocity is closer to Ra^{0.46}, which is shown to be consistent with experimental measurements and the numerical results for the scaling of Nu and the temperature if there are strong correlations between the velocity and temperature.Comment: 5 pages, 3 figures, new version 13 Mar, 200

Cavitation pressure in liquid helium

Recent experiments have suggested that, at low enough temperature, the homogeneous nucleation of bubbles occurs in liquid helium near the calculated spinodal limit. This was done in pure superfluid helium 4 and in pure normal liquid helium 3. However, in such experiments, where the negative pressure is produced by focusing an acoustic wave in the bulk liquid, the local amplitude of the instantaneous pressure or density is not directly measurable. In this article, we present a series of measurements as a function of the static pressure in the experimental cell. They allowed us to obtain an upper bound for the cavitation pressure P_cav (at low temperature, P_cav < -2.4 bar in helium 3, P_cav < -8.0 bar in helium 4). From a more precise study of the acoustic transducer characteristics, we also obtained a lower bound (at low temperature, P_cav > -3.0 bar in helium 3, P_cav > - 10.4 bar in helium 4). In this article we thus present quantitative evidence that cavitation occurs at low temperature near the calculated spinodal limit (-3.1 bar in helium 3 and -9.5 bar in helium 4). Further information is also obtained on the comparison between the two helium isotopes. We finally discuss the magnitude of nonlinear effects in the focusing of a sound wave in liquid helium, where the pressure dependence of the compressibility is large.Comment: 11 pages, 9 figure

Cavitation of Electrons Bubbles in Liquid Helium Below saturation Pressure

We have used a Hartree-type electron-helium potential together with a density functional description of liquid $^4$He and $^3$He to study the explosion of electron bubbles submitted to a negative pressure. The critical pressure at which bubbles explode has been determined as a function of temperature. It has been found that this critical pressure is very close to the pressure at which liquid helium becomes globally unstable in the presence of electrons. It is shown that at high temperatures the capillary model overestimates the critical pressures. We have checked that a commonly used and rather simple electron-helium interaction yields results very similar to those obtained using the more accurate Hartree-type interaction. We have estimated that the crossover temperature for thermal to quantum nucleation of electron bubbles is very low, of the order of 6 mK for $^4$He.Comment: 22 pages, 9 figure

Wall roughness induces asymptotic ultimate turbulence

Turbulence is omnipresent in Nature and technology, governing the transport of heat, mass, and momentum on multiple scales. For real-world applications of wall-bounded turbulence, the underlying surfaces are virtually always rough; yet characterizing and understanding the effects of wall roughness for turbulence remains a challenge, especially for rotating and thermally driven turbulence. By combining extensive experiments and numerical simulations, here, taking as example the paradigmatic Taylor-Couette system (the closed flow between two independently rotating coaxial cylinders), we show how wall roughness greatly enhances the overall transport properties and the corresponding scaling exponents. If only one of the walls is rough, we reveal that the bulk velocity is slaved to the rough side, due to the much stronger coupling to that wall by the detaching flow structures. If both walls are rough, the viscosity dependence is thoroughly eliminated in the boundary layers and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of transport, whose existence had been predicted by Robert Kraichnan in 1962 (Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be extrapolated to arbitrarily large Reynolds numbers

The joy of ruling: an experimental investigation on collective giving

We analyse team dictator games with different voting mechanisms in the laboratory. Individuals vote to select a donation for all group members. Standard Bayesian analysis makes the same prediction for all three mechanisms: participants should cast the same vote regardless of the voting mechanism used to determine the common donation level. Our experimental results show that subjects fail to choose the same vote. We show that their behaviour is consistent with a joy of ruling: individuals get an extra utility when they determine the voting outcome

Mesoscale physical–biological–biogeochemical linkages in the open ocean : an introduction to the results of the E-Flux and EDDIES programs

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1133-1138, doi:10.1016/j.dsr2.2008.03.001.Mesoscale currents, fronts, and eddies are ubiquitous and energetic features of ocean circulation. These phenomena, sometimes referred to as the “internal weather of the sea,” accommodate a diverse set of physical, chemical, and biological interactions that influence marine biogeochemistry on a wide range of timescales. These biogeochemical processes include the “biological pump”, i.e. the transfer or flux of biologically produced organic matter and associated elements from the surface ocean to depth (Ducklow et al., 2001; Volk and Hoffert, 1985). Within ~ 80% of the world’s oceans, the productivity and species composition of the autotrophic organisms that contribute to the biological pump are typically limited by major nutrients (e.g. nitrogen, phosphorus, and silica), or trace metals (e.g. iron). Primary production in such oligotrophic regions therefore depends mostly on intense recycling of nutrients within the surface sunlit waters, with only a small fraction supported by that entering from the atmosphere, or from the physical transport of nutrients from nutrient-rich deep waters below. Evidence that mesoscale and submesoscale phenomena play a role in the latter process dates back more than two decades (Angel and Fasham, 1983; Franks et al., 1986; Ring Group, 1981; Tranter et al., 1980; Venrick, 1990; Woods, 1988).E-Flux and EDDIES were supported by the National Science Foundation Chemical, Biological, and Physical Oceanography Programs. Additional support for the EDDIES project was provided by NASA