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The report concerns the possible amplification of waves in fluids due to fluctuations with time of some parameter which defines in part the natural frequencies of the system. Resonance is most likely to occur when some characteristic frequency is half of the frequency of parametric variation. First, the fluid is taken to be within a container which oscillates in the vertical direction so that the gravitational acceleration, relative to the container, varies with time. Both surface waves, in the case of an interface, and internal waves, in the case of continuous variation of density, are considered. In the latter case, the possible resonant frequencies have an upper bond. For an interface, rotation tends to eliminate the resonance of surface waves but to promote the resonance of inertial waves. For a continuous variation of density, rotation tends to place a lower bond on the possible resonant frequencies. Secondly, two fluids of different densities are taken to flow parallel to each other in an oscillatory manner and with different mean values of velocity. (Author).
This work is to explore the time-reversal symmetry breaking employing an acoustic wave platform through space-time parametric modulations, for the purpose of developing nonlinear devices such as amplifiers and mixers. The strength of acoustic wave platform is that it can obtain great leverage on the slow velocity and small wavelength of acoustic waves at RF to minimize the footprint of transmission lines. To realize parametric amplification, nonlinear Surface Acoustic Wave(SAW) grating is employed to pump a dynamic biasing electric field into the piezoelectric material to couple the pump electromagnetic wave to the propagating surface acoustic waves in the condition of phase matched. Two different ways to achieve nonlinear change of piezoelectric coefficient and elastic stiffness are proposed: 1) Material nonlinearity using ferromagnetic material like BST; 2) Geometrical nonlinearity using the coupling between vertical and parallel stiffness. The fractional modulation of elastic stiffness is approximate 4% in the first approach and can reach 10% or even higher in the second approach. Simulation validation using COMSOL demonstrates the parametric fixing effects produced on the Surface Acoustic Wave devices. Fabrication and characterization of Surface Acoustic Wave delay lines are also provided.