Search results for "Sound"
showing 10 items of 1845 documents
Optical frequency combs for space applications
2016
Optical frequency comb-based high resolution laser spectroscopy has been demonstrated in space under micro-gravity on two sounding rocket based experiments. The comb has been used to simultaneously measure two different atomic transitions.
A Method Based on Amplitude Probability Density Representation for Sounding High Frequency Noise in Ionospheric Channels
2021
High Frequency (HF) communications efficiency require a precise characterization of the ionospheric channel’s noise. We present a rapid and accurate method to sound the HF ionospheric channels that enables tracing of the time-availability of the channel based on imposed electric field strength thresholds. The method makes use of the amplitude probability density implemented in a real-time spectrum analyzer. Sounding of 3, 10 and 20 kHz bandwidth channels in the 4.8 – 8.8 MHz range is exemplified and specific observations are presented.
Dissipative terms of thermal nature in the theory of an ideal monoatomic superfluid
1996
A dissipative model of helium II was built up in previous works, using a 13-field extended thermodynamic theory formulated by Liu and Muller. In this work a generalization of such model is presented, where an extended thermodynamics with 14 fields due to Kremer is used. It is shown that the fourteenth field is able to account for the experimental data concerning the second sound attenuation. Further, the proposed theory is able to explain the Osborne experiment. Finally, a comparison with the two-fluid model is performed, emphasizing the different ways in which the dissipative phenomena are explained by the two theories.
Propagation of fourth sound in turbulent superfluids via extended thermodynamics
2011
The work deals with further developments of a study previously initiated, in which a macroscopic one-fluid model of inhomogeneous turbulent superfluids, based on extended thermodynamics, had been formulated. In this work the study is carried on. First the influence of the remnant vortices on the propagation of the first and second sound is studied. Then a boundary condition able to explain the reversible flow of superfluid flowing through a thin capillary is postulated and two vector fields, which have the dimensions of velocity and can be interpreted as the velocities of normal and superfluid components, are introduced. By using these new fields, a comparison between this model and the Hal…
Measuring longitudinal wave speed in solids: two methods and a half
2006
Three methods to analyse longitudinal wave propagation in metallic rods are discussed. Two of these methods also prove to be useful for measuring the sound propagation speed. The experimental results, as well as some interpretative models built in the context of a workshop on mechanical waves at the Graduate School for Pre-Service Physics Teacher Education, Palermo University, are described. Some considerations about observed modifications in trainee teachers' attitudes to utilizing physics experiments to build pedagogical activities are discussed.
Extended thermodynamics of polymers and superfluids
2008
Abstract Polymer solutions and turbulent superfluids have in common the presence of a complex tangle of lines – macromolecules in the former, quantized vortex lines in the latter – which contribute to the internal friction and viscous pressure of the system and make them typical non-Newtonian fluids. Here we briefly review some recent studies on such tangles and their consequences on the dynamics and thermodynamics of the whole system, using the framework of extended irreversible thermodynamics. For polymer solutions, we deal with the coupling of diffusion and viscous pressure and its effects on the stability of the solution and shear-induced phase separation; for superfluids, we focus our …
A thermodynamical model of inhomogeneous superfluid turbulence
2007
In this paper we perform a thermodynamical derivation of a nonlinear hydrodynamical model of inhomogeneous superfluid turbulence. The theory chooses as fundamental fields the density, the velocity, the energy density, the heat flux and the averaged vortex line length per unit volume. The restrictions on the constitutive quantities are derived from the entropy principle, using the Liu method of Lagrange multipliers. The mathematical and physical consequences deduced by the theory are analyzed both in the linear and in the nonlinear regime. Field equations are written and the wave propagation is studied with the aim to describe the mutual interactions between the second sound and the vortex t…
A mathematical model of counterflow superfluid turbulence describing heat waves and vortex-density waves
2008
The interaction between vortex density waves and high-frequency second sound in counterflow superfluid turbulence is examined, incorporating diffusive and elastic contributions of the vortex tangle. The analysis is based on a set of evolution equations for the energy density, the heat flux, the vortex line density, and the vortex flux, the latter being considered here as an independent variable, in contrast to previous works. The latter feature is crucial in the transition from diffusive to propagative behavior of vortex density perturbations, which is necessary to interpret the details of high-frequency second sound.
Rollvection versus linearvection: Comparison of brain activations in PET
2004
We conducted a PET study to directly compare the differential effects of visual motion stimulation that induced either rollvection about the line of sight or forward linearvection along this axis in the same subjects. The main question was, whether the areas that respond to vection are identical or separate and distinct for rollvection and linearvection. Eleven healthy volunteers were exposed to large-field (100 degrees x 60 degrees ) visual motion stimulation consisting of (1) dots accelerating from a focus of expansion to the edge of the screen (forward linearvection) and (2) dots rotating counterclockwise in the frontal plane (clockwise rollvection). These two stimuli, which induced appa…
Thermal Excitation of Fourth Sound in Liquid Helium II
1974
In narrow channels filled with helium II two wave modes propagate, fourth sound1–3 and the fifth wave mode.4–6 According to new results of theoretical studies,7–8 it has been predicted that it should be possible to excite both wave modes mechanically by vibrating the diaphragm of a condenser microphone as well as thermally by periodically heating the surface of a solid body (e.g., a resistance layer). Shapiro and Rudnick3 have produced and detected fourth-sound signals mechanically. In the present contribution it will be experimentally verified that it is also possible to excite fourth sound thermally. Since the theory7,8 gives the result that fourth sound can be generated thermally only if…