杭州湾潮致余流数值研究

NUMERICAL MODELING OF RESIDUAL CURRENTS IN HANGZHOU BAY

运用1959年10月-1992年5月在杭州湾250余测次海流周日连续观测资料，运用σ坐标系下的三维潮波控制方程，模拟该湾的欧拉余流，进行欧拉余流产生机制的数值试验；并根据欧拉流动的数值计算结果，采用拉格朗日速度在欧拉流场的近似展开，求得水质点运动的轨迹和速度。结果表明，杭州湾潮致余流的最大余流速度为46．0cm/s，惯性效应是杭州湾潮致余流产生的主要原因；杭州湾拉格朗日余流场主要被逆时针的大涡旋控制，表层和底层的拉格朗日流有较大差别。一个潮周期中，水质点大致运行了一个极扁的椭圆轨迹，其长轴约10－20km，而标识水质点净位移约1－3km。

After using a 3-dimensional hydrodynamic numerical model on σ-coordinate to simulate tidal movements in Hangzhou Bay, the tide-induced Eulerian residual currents were calculated (by averaging current vectors in M2 tidal periods) for use in computer test runs on their mechanism and the effects of river runoff. Maximum Eulerian residual current of about 46.0cm/s was obtained at the bay head where the topography was quite complex. Normally the velocities were less than 5cm/s and directed northeastward to the outer sea. In the south of the bay mouth, they were mainly southward. There were several small residual eddies along the northern coast, and a strong clockwise circulation at the bay head. Test runs indicate that residual currents are mainly due to the non-linear advection terms in the momentum equations, namely inertia effects induced by topographic variation. The Changjiang River runoff affects significantly the residual currents at the bay mouth and near the northern shore of the Bay. The Qiantang River runoff can cause a 10cm/s increase (from the normal 1cm/s), in residual current velocities during flood period and maximum discharge- Lagrangian residual currents, residual drifts and tracks of water particles were also calculated in M2 tidal periods. Water particles ran in unclosed elliptical tracks. The tidal excursions were about 12-20km, nearly one order of magnitude greater than drifting distance. Maximum Lagrangian velocity of about 25 cm/s was obtained at the bay head and in the south of bay mouth. Normally the speeds were 1-2cm/s. Test runs indicate that Lagrangian currents in Hangzhou Bay are controlled by a big counterclockwise vorticity, and are quite different from Eulerian currents.