Research on space-time structure of turbulence in surf zone

通过测量冲浪区内多个点的速度，研究了冲浪区内的湍流。波浪区的大范围运动，包括水平涡状运动，在产生和对流湍流的意义上是重要的。本文采用流动显示和数值模拟相结合的方法，研究了波浪区的大尺度运动，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向分布的时间变化。假定波峰处的水团下沉产生了与湍流尾迹和混合层中的速度相对应的速度亏缺，并由平均流产生，在下沉的破碎器中，水团下沉的规模很大。下沉射流和槽水之间的大速度梯度所产生的大的水平涡状运动和湍流是主要的。它们是通过在波浪水池中的流动可视化来观察的。结果表明，该模型重复了水团下沉、水槽水面飞溅和水平涡状运动的过程，并用MAC方法对波浪区的运动进行了数值分析。该模型为二维模型，单元尺寸为0.5 cm。不可能模拟出所谓的斜涡以及水滴和气泡的存在。结果表明，射流中的水颗粒具有很高的速度，水平涡状运动的环流值随时间变化，涡量和压力在空间上的分布也得到了解释。

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.