Results 1 - 10 of 44
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[en] The quasilinearization method is employed to investigate the heat transfer phenomenon from a fin with a non-adiabatic tip. Heat exchange with the surroundings occurs jointly by radiation and convection. Computed results for the fin effectiveness are examined in the light of others obtained by calculus of finite differences. (Auth.)
[en] LAI is a key factor in light and rainfall interception processes in forest stands and, for this reason, is called to play an important role in global change adaptive silviculture. Therefore, it is necessary to develop practical and operative methodologies to measure this parameter as well as simple relationships with other silviculture variables. This work has studied 1) the feasibility of LAI-2000 sensor in estimating LAI-stand when readings are taken under direct sunlight conditions; and 2) the ability of LAI in studying rainfall partitioned into throughfall (T) in an Aleppo pine stand after different thinning intensities, as well as its relationships to basal area, (G), cover (FCC), and tree density (D). Results showed that the angular correction scheme applied to LAI-2000 direct-sunlight readings stabilized them for different solar angles, allowing a better operational use of LAI-2000 in Mediterranean areas, where uniform overcast conditions are difficult to meet and predict. Forest cover showed the highest predictive ability of LAI (R2 = 0.98; S = 0.28), then G (R2 = 0.96; S = 0.43) and D (R2 = 0.50; S = 0.28). In the hydrological plane, T increased with thinning intensity, being G the most explanatory variable (R2 = 0.81; S = 3.07) and LAI the one that showed the poorest relation with it (R2 = 0.69; S = 3.95). These results open a way for forest hydrologic modeling taking LAI as an input variable either estimated form LAI-2000 or deducted from inventory data. (Author) 36 refs.
[en] The ultraviolet -visible, UV-vis, absorption edge behaviour of semiconductor oxides as TiO_2 and ZnO promotes their use as inorganic UV filters for sunscreens. In cosmetics, the use of nanoparticles is favoured because of their higher yield and lower whiteness. However, the particle size effect in the UV absorption is unclear. For this reason, the performances of nano and microsized TiO_2 and ZnO are evaluated in both the UV-vis spectroscopy and the Sun Protection Factor, SPF, value. In order to cover the UV range completely by using inorganic filters a new composite is attempted by dry nano dispersion methodology in which TiO_2 nanoparticles are dispersed onto ZnO microparticles. The new composite shows superior UV absorbing properties and ca. 60% SPF value due to a synergism between components that resulted in extended UV coverage and reduction of the total amount of nanoparticles required in the sunscreens. In addition, nanoparticles are effectively anchored onto microparticles avoiding the presence of free nanoparticles. (Author)
[en] While exponential decay is ubiquitous in nature, deviations at both short and long times are dictated by quantum mechanics. Nonexponential decay is known to arise due to the possibility of reconstructing the initial state from the decaying products. We discuss the quantum decay dynamics by tunneling of a many-particle system, characterizing the long-time nonexponential behavior of the nonescape and survival probabilities. The effects of contact interactions and quantum statistics are described. It is found that, whereas for noninteracting bosons the long-time decay follows a power law with an exponent linear in the number of particles N, the exponent becomes quadratic in N in the fermionic case. The same results apply to strongly interacting many-body systems related by the generalized Bose-Fermi duality. The faster fermionic decay can be traced back to the effective hard-core interactions between particles, which are as well the decaying products, and exhibit spatial antibunching which hinders the reconstruction of the initial unstable state. The results are illustrated with a paradigmatic model of quantum decay from a trap allowing leaks by tunneling, whose dynamics is described exactly by means of an expansion in resonant states.
[en] This paper presents the numerical solution of convective heat transfer in the thermal entrance region of cusped ducts. These ducts are formed contiguously by assembling three, four, or five cylindrical rods having the same diameter next to each other. The ducts are also subjected to a constant wall temperature. The solution to the discretization of these duct geometries is obtained by using the numerically generated boundary fitted coordinate system. According to this method, the complex domain in the physical plane is transformed into a regular square domain in the computational plane. The finite difference method based on the control volume is then used to discretize the transformed governing equation. To prove the validity of the results, the present method is also used to obtain the convective heat transfer solution in the entrance region of square and equilateral triangular ducts. Comparison of the results for square and equilateral triangular ducts with available data published in the open literature gives excellent agreement. The representative curves illustrating variations of the bulk temperature and Nusselt numbers with pertinent parameters are plotted for the three different duct geometries
[en] Diffraction in time of matter waves incident on a shutter which is removed at time t = 0 is studied in the presence of a linear potential. The solution is also discussed in phase space in terms of the Wigner function. An alternative configuration relevant to the current experiments where particles are released from a hard-wall trap is also analysed for single-particle states and for a Tonks-Girardeau gas
[en] A class of exact propagators describing the interaction of an N-level atom with a set of on-resonance δ-lasers is obtained by means of the Laplace transform method. State-selective mirrors are described in the limit of strong lasers. For the two-level case, the transient effects arising as a result of the interaction between both a semi-infinite beam and a wavepacket with the on-resonance laser are examined
[en] While the emergent field of quantum thermodynamics has the potential to impact energy science, the performance of thermal machines is often classical. We ask whether quantum effects can boost the performance of a thermal machine to reach quantum supremacy, i.e., surpassing both the efficiency and power achieved in classical thermodynamics. To this end, we introduce a nonadiabatic quantum heat engine operating an Otto cycle with a many-particle working medium, consisting of an interacting Bose gas confined in a time-dependent harmonic trap. It is shown that thanks to the interplay of nonadiabatic and many-particle quantum effects, this thermal machine can outperform an ensemble of single-particle heat engines with same resources, demonstrating the quantum supremacy of many-particle thermal machines. (paper)
[en] This paper deals with the laminar, steady, two-dimensional natural convection inside iso-flux vertical parallel plates. The main goal is to compare the impact of the variable thermophysical properties on the heat transfer characteristics. The equations are solved with the CFD FLUENT computer code. Results are presented in terms of Nusselt number and wall temperatures, and induced dimensionless flow rates as functions of the main geometric and thermal parameters. (authors)