Laser-Generated Ultrasonic Beams

Now, it is well known that when the radiation from a high-power laser (a Q-switched laser) is focused onto a specimen, very large stresses are generated within the specimen primarily by thermo-elastic means as well as others. Generally, in this focused configuration, damage results. 11 Damage 11 is a bad word in the context of this workshop, so consequently we were concerned with harnessing this potential for lasers to generate large stress waves and thereby produce a stress wave of a more useful nature. In particular, we wanted to generate plane compressive stress pulses and sinusoidal wave trains to be used in subsequent wave propagation experiments from a nondestructive point of view. These waves may be used wherever a compressive stress pulse or a sinusoidal wave train with a very large amplitude might be needed. In particular, they may be used for flaw detection through materials that might be very dissipative where signals from piezoelectric crystals might not get through

On the cascade mechanism of short surface wave modulation

International audienceModulation of short surface ripples by long surface or internal waves by a cascade mechanism is considered. At the first stage, the orbital velocity of the long wave (LW) adiabatically modulates an intermediate length nonlinear gravity wave (GW), which generates a bound (parasitic) capillary wave (CW) near its crest in a wide spatial frequency band. Due to strong dependence of the CW amplitude on that of the GW, the resulting ripple modulation by LW can be strong. Adiabatic modulation at the first stage is calculated for an arbitrarily strong LW current. The CWs are calculated based on the Lonquet-Higgins theory, in the framework of a steady periodic solution, which proves to be sufficient for the cases considered. Theoretical results are compared with data from laboratory experiments. A discussion of related sea clutter data is given in the conclusion

New Kinds of Acoustic Solitons

We find that the modified sine-Gordon equation belonging to the class of the soliton equations describes the propagation of extremely short transverse acoustic pulses through the low-temperature crystal containing paramagnetic impurities with effective spin S=1/2 in the Voigt geometry case. The features of nonlinear dynamics of strain field and effective spins, which correspond to the different kinds of acoustic solitons, are studied.Comment: 9 pages, 1 figur

Nonlinear dispersive waves in repulsive lattices

[EN] The propagation of nonlinear waves in a lattice of repelling particles is studied theoretically and experimentally. A simple experimental setup is proposed, consisting of an array of coupled magnetic dipoles. By driving harmonically the lattice at one boundary, we excite propagating waves and demonstrate different regimes of mode conversion into higher harmonics, strongly influenced by dispersion and discreteness. The phenomenon of acoustic dilatation of the chain is also predicted and discussed. The results are compared with the theoretical predictions of $\alpha$-FPU equation, describing a chain of masses connected by nonlinear quadratic springs and numerical simulations. The results can be extrapolated to other systems described by this equation.The work was supported by Spanish Ministry of Economy and Innovation (MINECO) and European Union FEDER through Project No. FIS2015- 65998-C2-2 and by Project No. AICO/2016/060 by Conselleria de Educacion, Investigacion, Cultura y Deporte de la Generalitat Valenciana. L.J.S.-C. gratefully acknowledge the support of PAID-01-14 at Universitat Politscnica de Valsncia. A. M. gratefully acknowledge to Generalitat Valenciana (Santiago Grisolia program).Mehrem, A.; Jimenez, N.; Salmerón-Contreras, LJ.; García-Andrés, FX.; García-Raffi, LM.; Picó Vila, R.; Sánchez Morcillo, VJ. (2017). Nonlinear dispersive waves in repulsive lattices. Physical Review E. 96(1). https://doi.org/10.1103/PhysRevE.96.012208S00220096