Study on Large Scale Turbulent Structure for Vertical Mixing Phenomena in Density Stratified Layer

本研究的目的是阐明密度流垂直混合过程中发生大尺度湍流结构的机理，并建立数值模拟方法。利用“MIRAI”海洋观测船获得的三维密度分层水箱实验测量数据、大村湾潮流数据以及大气天气和海洋数据，与数值模拟结果进行了比较。结果表明：(1)建立了两层中性密度粒子密度电流测量技术，实测速度分布与计算结果吻合较好。(2)密度流前端速度不含强混合湍流，但在前鼻后部发生分离。在分离区域的湍流是显著的。(3)密度混合现象受分离的影响较大，主要发生在底岸后方。(4)密度分层稳定层阻止了上升流混合，特别是涡对上升流不能破坏大密度跳变层。(5)通过数值模拟地表冷却混合过程，明确了小尺度羽流的诞生是密度混合的起源。(6)将数值模拟器“MEG模型”修正为边界拟合坐标网格，计算大村湾地区的流型。结果与实测潮汐资料吻合较好。验证了该方案的有效性。(7)通过云的移动来评价大气密度和流速。结果与探空数据进行了比较。结果表明，云的运动速度与对流层的大气速度分布基本一致。对流活动处于低洼期时存在分层层，而在活跃期不存在分层层。

Objectives of this research work are to clarify the mechanism of large-scale turbulent structure, which occurs at vertical mixing process in density current, and to develop the numerical simulating method. Experimental measurements in three-dimensional density stratified water tank, tidal current data Ohmura-bay and atmospheric weather and ocean data obtained by Ocean observation ship "MIRAI" are used to compare with the numerical simulated results.Obtained results are as follows,(1) Measurement technique for density current using neutral density particles in two layers is developed and measured velocity distributions give good agreement with the calculated results.(2) Front-end speed of the density current does not include strong mixture turbulence, but separations are occurred at the rear part of front nose. The turbulence at that separated regions are significant.(3) Density mixing phenomena is strongly affected by the separation, which occurs at the behind of bottom bank.(4) Density stratified stable layer prevents upwelling mixing, especially the upwelling by vortex pair cannot break the large density jump layer.(5) Mixing process by surface cooling is simulated by numerically and it is clarified that the birth of small-scale plumes are the origin of the density mixing.(6) Numerical simulator "MEG model" is modified to the boundary fitted coordinates grid and the flow pattern is calculated in the Omura-Bay area. Results show good agreement with the measured tidal data. The availability of this scheme is confirmed.(7) Atmospheric density current velocity is evaluated by cloud movement. The results are compared with data of radiosonde. It shows that the cloud movement velocity is almost coinciding with the atmospheric velocity distribution in the convection layer. The facts that there is a stratified layer when the convective activity is in depressed period, but no layer in the active period.