| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 105 === Small island wakes such as the Green Island (~7 km) wakes are simulated via a high-resolution model, MITgcm (500 m). The effect of variety of island geometry, horizontal diffusion, advection scheme and the horizontal resolution have been investigated in our model, which help us to acquire the most appropriate setup for further analyses.
Previous numerical and laboratory experiments for a flow pass a cylinder have similar results to our model with the Strouhal number vs. Reynolds number diagram. The comparison between the numerical model and the field observations are similar in terms of the shedding period ~12 hours, and wake patterns at near field. The surface signatures of the wake, including the near field recirculation to the shed eddies, are in accordance with the satellite imagery.
Our model suggests that the mechanism of Green Island wakes is primarily a phenomenon of von Kármán vortex street. Investigation of the shedding frequency as a function of the horizontal explicit eddy viscosity shows analogous trend with previous water tank experiments. Furthermore, even though rotation and stratification are considered in our island wake scenario, the transition regime is still measurable. The island wake behaviors also greatly depend on Reynolds number (Re). In addition, the aspect ratio in our simulation is similar to the Kármán’s ratio. It indicates that Green Island wakes have analogous features to the von Kármán vortex street.
The asymmetry of island wakes is a result of the inertial instability, which tends to destabilize the anticyclonic vorticity. Consequently, strong temperature drop associating with upwelling is more substantial in the cyclonic recirculation than the anticyclonic recirculation. In the scenario of small island (Green Island), strong upstream flow magnitude (~1 ms-1), the normalized vorticity (relative vorticity divided by planetary vorticity) tends to be larger than 10. As a result, the growth rate is large, meaning the instability will grow rapidly. However, the distortion of the anticyclonic recirculation is not as significant as previous studies. The defect of the inertial instability is presumably due to (1) stratification and (2) strong relative vorticity magnitude, which the restoring force may compensate the anticyclonic recirculation and consolidate the shed eddy.
The wake generation of the von Kármán-like island wakes is studied by evaluating the eddy kinetic energy budget. The current shear in the lateral boundary is the major energy source to generate eddy kinetic energy via the horizontal Reynolds stress to eddy kinetic energy. Therefore, the importance of the barotropic conversion term prevails the baroclinic conversion term.
Our model results suggest the hotspot of the strong vertical shear, which may result in turbulent mixing and is co-located with strong horizontal shear layer. We found the vertical shear is primarily sourced from (1) the island-shelf effect and (2) the tilting of the vertical vorticity component. Evidence from previous observations shows active overturning and high Turbulence Kinetic Energy (TKE) dissipation rate at the shear layer.
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