The electromagnetically driven flow in the wide gap of a concentric spheres system is studied experimentally and numerically in the laminar regime ($Re \leq 1540$). The mainly azimuthal driving Lorentz force is promoted by the interaction of a direct current and a dipolar magnetic field. The current is injected through two ring-shaped copper electrodes located at the equatorial zone of each sphere and the magnetic field is produced by a permanent magnet located inside the inner sphere. Velocity profiles for the azimuthal component in the equatorial plane were obtained with Particle Image Velocimetry and the radial velocity component of the flow was recorded using Ultrasonic Doppler Velocimetry. Laser-fluorescein technique was used for flow visualization. It was found that for a critical electric current ($Re=1140$), an instability occurs and the flow becomes time-dependent. We found, theoretically and experimentally a vortex breakdown structure at each of the polar zones of the spherical gap, which to the best knowledge of the authors, this is the first time it is reported with electromagnetic forcing. A full three-dimensional numerical simulation reproduces the experimental observations qualitatively and quantitatively.
Abstract:In this paper, we address theoretically and experimentally the optimization problem of the heat transfer occurring in two coupled thermoelectric devices. A simple experimental set up is used. The optimization parameters are the applied electric currents. When one thermoelectric is analysed, the temperature difference ∆T between the thermoelectric boundaries shows a parabolic profile with respect to the applied electric current. This behaviour agrees qualitatively with the corresponding experimental measurement. The global entropy generation shows a monotonous increase with the electric current. In the case of two coupled thermoelectric devices, elliptic isocontours for ∆T are obtained in applying an electric current through each of the thermoelectrics. The isocontours also fit well with measurements. Optimal figure of merit is found for a specific set of values of the applied electric currents. The entropy generation-thermal figure of merit relationship is studied. It is shown that, given a value of the thermal figure of merit, the device can be operated in a state of minimum entropy production.
An experimental and theoretical study of the spectral response of coupled viscoelastic bars subject to axial oscillations is performed. Novel closed formulas for the envelope function and its width are derived. These formulas explicitly show the role played by energy dissipation. They show that the internal friction does not affect the width of the envelope of the individual resonances. The formulation is based on the equations of classical mechanics combined with Voigt’s viscoelastic model. The systems studied consist of a sequence of one, two, or three coupled bars, with their central axes collinear. One of the bars is assumed to be much longer than the others. We discuss the connection between our results and the concept of the strength function phenomenon discovered for the first time in nuclear physics. Our formulation is an alternative and exact approach to the approximated studies based on the fuzzy structure theory that has been used by other authors to describe systems consisting of coupled bars. The analytical expressions describe the measurements in the laboratory very well.
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