RESUMEN
We report high-speed, large dynamic range spectral domain interrogation of fiber-optic Fabry-Perot (FP) interferometric sensors. An optical interrogation system employing a piezoelectric FP tunable filter and an array of fiber-Bragg gratings for wavelength referencing is developed to acquire the reflection spectrum of FP sensors at a high interrogation speed with a wide wavelength range. A 98 nm wavelength interrogation range was obtained at the resonance frequency of â¼110kHz of the FP tunable filter. At this frequency, the resolution of the FP cavity length measurement was 1.8 nm. To examine the performance of the proposed high-speed spectral domain interrogation scheme, two diaphragm-based fiber-tip FP sensors (a pressure sensor and acoustic sensor) were interrogated. The pressure measurement results show that the high-speed spectral domain interrogation method has the advantages of being robust to light intensity fluctuations and having a much larger dynamic range compared with the conventional intensity-based interrogation method. Moreover, owing to its capability of measuring the absolute FP cavity length, the proposed interrogation system mitigates the sensitivity drift that intensity-based interrogation often suffers from. The acoustic measurement results demonstrate that the high-speed spectral domain interrogation method is capable of high-frequency acoustic measurements of up to 20 kHz. This work will benefit many applications that require high-speed interrogation of fiber-optic FP interferometric sensors.
RESUMEN
The nonlinear wave pattern generated by a localized pressure source moving over a liquid free surface at speeds below the minimum phase speed (c{min}) of linear gravity-capillary waves is investigated experimentally and theoretically. At these speeds, freely propagating fully localized solitary waves, or "lumps," are known theoretically to be possible. For pressure-source speeds far below c{min}, the surface response is a local depression similar to the case with no forward speed. As the speed is increased, a critical value is reached c{c} approximately 0.9c{min} where there is an abrupt transition to a wavelike state that features a steady disturbance similar to a steep lump behind the pressure forcing. As the speed approaches c{min}, a second transition is found; the new state is unsteady and is characterized by continuous shedding of lumps from the tips of a V-shaped pattern.
RESUMEN
Mechanically generated weakly breaking waves were studied experimentally in clean water and water with a soluble surfactant whose bulk concentration was above the critical micelle concentration (CMC). For the surfactant case, the breaker, which forms a surface-tension-dominated spilling breaker in clean water [wave frequencies 1.42 to 1.15Hz , see Duncan, J. Fluid Mech. 379, 191 (1999)], ranges from a spiller at the highest frequency to an overturning wave with a plunging microjet at the lowest frequency. It is shown that this behavior is consistent with that of a wave in a pure liquid with a lower surface tension than water rather than water with a surfactant monolayer. The analysis of the geometrical characteristics of the breaking surface generated by the jet impact on the front face of the wave crest indicates a patch of more violent turbulence suggesting an increase of air-sea transfer compared to the clean water case.
RESUMEN
Breaking waves markedly increase the rates of air-sea transfer of momentum, energy and mass. In light to moderate wind conditions, spilling breakers with short wavelengths are observed frequently. Theory and laboratory experiments have shown that, as these waves approach breaking in clean water, a ripple pattern that is dominated by surface tension forms at the crest. Under laboratory conditions and in theory, the transition to turbulent flow is triggered by flow separation under the ripples, typically without leading to overturning of the free surface. Water surfaces in nature, however, are typically contaminated by surfactant films that alter the surface tension and produce surface elasticity and viscosity. Here we present the results of laboratory experiments in which spilling breaking waves were generated mechanically in water with a range of surfactant concentrations. We find significant changes in the breaking process owing to surfactants. At the highest concentration of surfactants, a small plunging jet issues from the front face of the wave at a point below the wave crest and entraps a pocket of air on impact with the front face of the wave. The bubbles and turbulence created during this process are likely to increase air-sea transfer.