Active versus passive scalar turbulence

Active and passive scalars transported by an incompressible two-dimensional conductive fluid are investigated. It is shown that a passive scalar displays a direct cascade towards the small scales while the active magnetic potential builds up large-scale structures in an inverse cascade process. Correlations between scalar input and particle trajectories are found to be responsible for those dramatic differences as well as for the behavior of dissipative anomalies.     

Generation of tunable far-infrared radiation with a quantum cascade laser

We demonstrate the generation of cw tunable far-infrared radiation by mixing a quantum cascade laser and a CO>(2) laser in a W-Ni metal-insulator-metal diode. The first known spectroscopic application to the recording of an H(79)Br transition near 4.47 THz is reported.     

Scaling and universality in turbulent convection

Anomalous correlation functions of the temperature field in two-dimensional turbulent convection are shown to be universal with respect to the choice of external sources. Moreover, they are equal to the anomalous correlations of the concentration field of a passive tracer advected by the convective flow itself. The statistics of velocity differences is found to be universal, self-similar, and close to Gaussian. These results point to the conclusion that temperature intermittency in two-dimensional turbulent convection may be traced back to the existence of statistically preserved structures, as it is in passive scalar turbulence.     

Functional Calculus in Hölder-Zygmund Spaces

In this paper we characterize those functions of the real line to itself such that the nonlinear superposition operator defined by maps the Hölder-Zygmund space to itself, is continuous, and is times continuously differentiable. Our characterizations cover all cases in which is real and 0$">, and seem to be novel when 0$"> is an integer.

     

An approximated 3-d model of the Langevin transducer and its experimental validation

In this work, an approximated 3-D analytical model of the Langevin transducer is proposed. The model, improving the classical 1-D approach describing the thickness extensional mode, allows us to predict also the radial modes of both the piezoelectric ceramic disk and the loading masses; furthermore, it is able to describe the coupling between radial and thickness extensional modes. In order to validate the model, the computed frequency spectrum is compared with that obtained by measurements carried out on 13 manufactured samples of different thicknesses to diameter ratios. The comparison shows that the model predicts with quite good accuracy the resonance frequencies of the two lowest frequency modes, i.e., those of practical interest, all over the explored range. Finally, the coupling effect between thickness and radial modes on the frontal displacement is measured and discussed.     

Reflection of drill-string extensional waves at the bit-rock contact

Downward propagating extensional waves are partially reflected at the bit-rock contact. The evaluation of the reflection coefficient is important to obtain while drilling information about the acoustic properties of the formations. The scope of this work is to estimate the bit-rock reflection coefficient, assuming a flat drill bit in perfect contact with the formation. Using the low-frequency approximation, which holds when the wavelength is much larger than the lateral dimensions of the borehole, the drill-string is assumed to be a laterally free rod, and the formation an homogeneous and isotropic medium. This work shows that the reflection coefficient of the extensional waves depends, along with the elastic properties of the formation, on the ratio of the cross sections of the drill-string and borehole. The impedance of the drilled rock can be calculated from the measured reflection coefficient, which is related to the amplitude of waves produced in the string and in the formation by a working drill-bit.     

Size Functions from a Categorical Viewpoint

A new categorical approach to size functions is given. Using this point of view, it is shown that size functions of a Morse map, f: M can be computed through the 0-dimensional homology. This result is extended to the homology of arbitrary degree in order to obtain new invariants of the shape of the graph of the given map.