12:00 〜 12:15
[ACG39-12] 氷縁域における波浪減衰の実験的研究
キーワード：波浪減衰、波浪・海氷相互作用、造波氷海水槽、はす葉氷
The coverage of sea ice in the Arctic sea is decreasing. This leads to an increase in wave height and sea failure. Between the open water and solid ice, the wave propagates to the Marginal Ice Zone (MIZ) and ice floes attenuate waves. However, the mechanism of wave attenuation under sea ice is not clear.
Wave attenuation under ice cover is investigated. Waves generated in the open-water area attenuate while propagating over the ice-covered area, but the mechanism of attenuation is still unclear, and various formulations are proposed. For example, in some models, wave energy is dissipated in the viscous boundary layer right beneath the horizontally immobile thin ice layer (e.g., Weber, 1987). In such cases, the spatial decay scale is proportional to f -3.5, where f is the wave frequency. In other models, the ice is represented as a viscous layer with a finite thickness (e.g., Lamb, 1932), and wave energy is dissipated throughout the ice layer. This leads to the decay scale proportional to f -5. Furthermore, these models consider ice as a continuous layer and do not take floe size into account. Unclear is a floe-to-wavelength ratio at which this approximation breaks down.
The experiment facility was built in the Kashiwa campus, the University of Tokyo. A wave tank with a dimension of 8m x 1.5m x 1.0m (0.6m water depth), with a plunger-type wavemaker and a beach at each end, is located in a freezer room. Surface displacement was measured with low-cost ultrasonic wave gauges, which were developed using modules of microcomputers and rangefinders. The experiments were conducted with freshwater, supposing that the difference in material and thermodynamic properties from seawater would not qualitatively alter the processes.
Using the ice floes produced under various wave forcings, we measured the propagating waves with various frequencies using an array of ultrasonic wave gauges. The results suggest that waves do not exponentially decay when the wavelength is shorter than or similar to floe sizes. It is suggested that the inertia of ice floes can no longer be neglected and that wave reflection and scattering become important at this scale. On the other hand, when waves are longer than floe sizes, the attenuation property is insensitive to floe sizes, suggesting the validity of representing ice as a continuous layer. In such cases, the decay scale followed f -3 to f -3.5, which suggests the applicability of the dissipation model through the viscous boundary layer. However, the effective viscosity estimated from the measurements was greater than the molecular viscosity by more than one order of magnitude, so further investigation of the dissipation process is needed.
Wave attenuation under ice cover is investigated. Waves generated in the open-water area attenuate while propagating over the ice-covered area, but the mechanism of attenuation is still unclear, and various formulations are proposed. For example, in some models, wave energy is dissipated in the viscous boundary layer right beneath the horizontally immobile thin ice layer (e.g., Weber, 1987). In such cases, the spatial decay scale is proportional to f -3.5, where f is the wave frequency. In other models, the ice is represented as a viscous layer with a finite thickness (e.g., Lamb, 1932), and wave energy is dissipated throughout the ice layer. This leads to the decay scale proportional to f -5. Furthermore, these models consider ice as a continuous layer and do not take floe size into account. Unclear is a floe-to-wavelength ratio at which this approximation breaks down.
The experiment facility was built in the Kashiwa campus, the University of Tokyo. A wave tank with a dimension of 8m x 1.5m x 1.0m (0.6m water depth), with a plunger-type wavemaker and a beach at each end, is located in a freezer room. Surface displacement was measured with low-cost ultrasonic wave gauges, which were developed using modules of microcomputers and rangefinders. The experiments were conducted with freshwater, supposing that the difference in material and thermodynamic properties from seawater would not qualitatively alter the processes.
Using the ice floes produced under various wave forcings, we measured the propagating waves with various frequencies using an array of ultrasonic wave gauges. The results suggest that waves do not exponentially decay when the wavelength is shorter than or similar to floe sizes. It is suggested that the inertia of ice floes can no longer be neglected and that wave reflection and scattering become important at this scale. On the other hand, when waves are longer than floe sizes, the attenuation property is insensitive to floe sizes, suggesting the validity of representing ice as a continuous layer. In such cases, the decay scale followed f -3 to f -3.5, which suggests the applicability of the dissipation model through the viscous boundary layer. However, the effective viscosity estimated from the measurements was greater than the molecular viscosity by more than one order of magnitude, so further investigation of the dissipation process is needed.