Research on space-time structure of turbulence in surf zone

通过对碎波带内多点流速的测量，研究了碎波带内的湍流。碎波带中的大尺度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。本文通过流场显示和数值模拟研究了碎波带的大尺度运动，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向分布的时间变化。本文假定，在波浪中，由于水团从波峰处的下沉，在平均流的作用下，产生了一个与湍流尾流和混合层中的速度亏损相对应的速度亏损，在下沉式破碎机中，水团的下沉尺度较大。大的水平涡状运动和湍流产生的大的速度梯度之间的俯冲射流和槽水是占主导地位的。它们是通过波浪水槽中的流动可视化来观察的。结果表明，碎波带内的运动是一个水团的倾翻、槽面飞溅和水平涡状运动的重复过程，并采用MAC方法对碎波带内的运动进行了数值分析。该模型为二维模型，单元尺寸为0.5cm。不可能模拟所谓的斜涡以及水滴和气泡的存在。然而，射流中的水粒子的很高的速度，水平涡状运动的环流值的时间变化和涡量和压力的空间分布澄清。

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.