Research on space-time structure of turbulence in surf zone

通过测量波浪带中若干点的速度，研究了波浪带中的湍流。包括水平涡状运动在内的大尺度运动在产生和对流湍流的意义上是重要的。本文采用流动可视化和数值模拟相结合的方法研究了波浪带的大尺度运动。试图用一个简单的定性模型来解释一个波浪周期内湍流垂直分布的时间变化。假设湍流尾迹和混合层中相应的速度缺陷是由波峰的水团俯冲产生的，并被平均流对流。在下沉破碎机中，水团的下沉规模较大。大的水平涡状运动和俯冲射流与槽水之间的大速度梯度所产生的湍流是主要的。它们是通过波浪槽内的流动显示来观察的。研究发现，水团俯冲、槽水面飞溅和水平涡状运动的过程是反复发生的。为了定量地阐明冲浪区的运动，采用MAC方法进行了数值分析。模型为二维，单元尺寸为0.5cm。不可能模拟出所谓的斜涡和水滴、气泡的存在。阐明了射流中水粒子的极高速度、水平涡状运动环流值的时间变化以及涡度和压力的空间分布。

The turbulence in surf zone has been investigated by measuring a velocity at several points in the surf zone. Large-scale motion in surf zone including a horizontal vortex-like motion is important in the sense that it generates and convect the turbulence. Here the large-scale motion in surf zone is investigated by a flow-visualization and a numerical simulation.It is tried to explain the time variation of vertical distribution of turbulence during one wave period by using one simple qualitative model. It is assumed that a velocity defect corresponding to that in the turbulent wake and mixing layer is generated by the plunging of water mass from wave crest and is convected by the mean flow.In the plunging breaker, the scale of plunging of water mass is large. The large horizontal vortex-like motion and the turbulence generated by a large velocity gradient between the plunging jet and the trough water is predominant. They are observed by a flow-visualization in a wave tank. It is found that a process of the plunging of water mass, splash from trough water surface and horizontal vortex-like motion is repeated.A numerical analysis is done to clarify the motion in surf zone quantitatively by using a MAC method. The model is two-dimensional, and the size of cell is 0.5cm. It is impossible to simulate the so-called slanting vortex and the existence of water drops and air bubbles. Nevertheless, a very high speed of water particle in the jet, time variation of circulation value of the horizontal vortex-like motion and spacial distributions of a vorticity and pressure are clarified.