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

本研究的目的是阐明异重流垂向混合过程中大尺度湍流结构的形成机理，发展数值模拟方法。利用三维密度分层水槽的实验测量、大村湾的潮流资料和海洋观测船“米拉伊”的大气、海洋资料，与数值模拟结果进行了比较，得到如下结果：（1）发展了两层中性密度粒子异重流测量技术，测量得到的异重流流速分布与计算结果吻合较好。(2)异重流的前端速度不包含强烈的混合气湍流，但在前头部后部有分离。在分离区域处的湍流是显著的。(3)底岸后方的分离对密度混合现象有很大的影响。(4)密度分层稳定层阻止了上涌混合，特别是涡对的上涌不能打破大的密度跳跃层。(5)对地面冷却引起的混合过程进行了数值模拟，阐明了小尺度羽流的产生是密度混合的起源。(6)将数值模拟器“MEG模式”修改为边界拟合坐标网格，计算了大村湾海域的流态。计算结果与实测潮汐资料吻合较好。验证了该方案的有效性。(7)大气密度流速度是由云的运动来估算的。并与探空资料进行了比较。结果表明，对流层中云的移动速度与大气速度分布基本一致。对流活动处于低气压期时有层结，而在活跃期则无层结。

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.