非线性波作用下潜堤周围的流体分离与涡流结构

采用粒子图像测速技术（PIV）和基于雷诺时均纳维尔-斯托克斯方程（RANS）的数值模型研究了斯托克斯二阶波作用下抛石基床矩型潜堤周围的流体分离与涡流结构。越堤前后试验波面发生了非线性变形，迎浪面波形的不对称度和偏度分别为-0.21和0.04，波面前倾，背浪面波形的不对称度和偏度分别为-0.39和0.99，相对于垂直轴和水平轴的不对称性更显著。由PIV数据得到的相平均速度场和涡量场显示，迎浪面和背浪面周期性地生成顺时针涡和逆时针涡，但均未能充分发展，由随后到来的变向水流携带与自由液面或结构表面相互作用而耗散。迎浪区域的涡对运动范围较窄，局限于距离堤面约0.5倍Keulegan-Carpenter数的范围内，波能损耗小，背浪区域的涡对扩散范围较大，局限于距离堤背约1倍Keulegan-Carpenter数范围内，波能损耗大。迎浪基肩上存在一个小型环流系统，影响范围约2个水质点运动轨迹，可能引起局部冲刷。基于RANS-VOF（Volume of Fluid）格式构建了数值波浪水槽，借助试验数据探讨了不同造波方法的适用性和海绵层的消能效果，进一步研究了边界层的分离现象。数值结果表明，涡旋的涡量供应主要来自于结构表面的剪切边界层，而这些反旋涡量是由前一时刻的分离涡运动引起的逆压梯度诱导生成的。涡旋的生成、泄放、拉伸、对流、耗散会显著改变水下结构物周围的局部流场，进而影响局部冲淤和受力，工程设计中应考虑涡旋引起的复杂流动效应。

Flow Separation and Vortical Structure Around Submerged Breakwater Subject to Nonlinear Water Waves

Flow separation and vortical dynamics generated by second-order Stokes waves propagating over a submerged rectangular breakwater supported on a rubble mound were investigated using Particle Image Velocimetry technology (PIV) and numerical model based on Reynolds-Averaged-Navier-Stokes equations (RANS). Experimental wave surfaces show nonlinear deformation before and after the structure. The asymmetry and skewness of the weatherside wave profile are -0.21 and 0.04, respectively, which indicates a steep front face and gentle rear face. The asymmetry and skewness of the leeside wave profile are -0.39 and 0.99, respectively, which indicates more significant lack of symmetry relative to vertical and horizontal axes. Phase-averaged velocity and vortex fields calculated from PIV data show that clockwise and counterclockwise vortices are generated periodically on the weather- and lee- side of the structure. However, these vortices are not fully developed. The subsequent flow reversal moves the vortices towards the free surface or the structure to make them dissipated. The weatherside vortex pair is confined within a relatively narrower region of about 0.5 times Keulegan-Carpenter number from the weather side of the structure, which implies smaller wave energy dissipation. Meanwhile, the leeside vortex pair is confined within a relatively wider region of about 1.0 times Keulegan-Carpenter number from the lee side of the structure, which indicates greater dissipation of wave energy. In addition, a small circulation system is found above the upstream shoulder of the rubble mound and its movement is confined within two times the unperturbed wave particle trajectory, which may lead to local scouring. A numerical wave flume was established based on RANS-VOF mode and then checked by experimental results. The applicability of different wave-making methods and the effect of energy dissipation by sponge layer were analyzed. The RANS-VOF (Volume of Fluid) model was then used to further study the flow separation of shear boundary layers. Numerical results show that the supply of vorticity mainly comes from the shear boundary layers at the surface of the structure. The adverse vorticity in the shear boundary layers is induced by the adverse pressure gradient imposed by the movement of the vortices previously shed from the structure. The generation, shedding, stretching, advection and dissipation of vortices is expected to significantly change the local flow around submerged structures and hence cause local scour as well as additional loading. Therefore, the effect of the complex flow induced by vortices should be taken into account in engineering design.