102 research outputs found
Slow sound laser in lined flow ducts
We consider the propagation of sound in a waveguide with an impedance wall.
In the low frequency regime, the first effect of the impedance is to decrease
the propagation speed of acoustic waves. Therefore, a flow in the duct can
exceed the wave propagation speed at low Mach numbers, making it effectively
supersonic. We analyze a setup where the impedance along the wall varies such
that the duct is supersonic then subsonic in a finite region and supersonic
again. In this specific configuration, the subsonic region act as a resonant
cavity, and triggers a laser-like instability. We show that the instability is
highly subwavelength. Besides, if the subsonic region is small enough, the
instability is static. We also analyze the effect of a shear flow layer near
the impedance wall. Although its presence significantly alter the instability,
its main properties are maintained.Comment: 20 pages, 13 figures. V2: several clarifications added and Fig. 4
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Perfect absorption of water waves by linear or nonlinear critical coupling
We report on experiments of perfect absorption for surface gravity waves
impinging a wall structured by a subwavelength resonator. By tuning the
geometry of the resonator, a balance is achieved between the radiation damping
and the intrinsic viscous damping, resulting in perfect absorption by critical
coupling. Besides, it is shown that the resistance of the resonator, hence the
intrinsic damping, can be controlled by the wave amplitude, which provides a
way for perfect absorption tuned by nonlinear mechanisms. The perfect absorber
that we propose, without moving parts or added material, is simple, robust and
it presents a deeply subwavelength ratio wavelength/size
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