The wake of a three-dimensional underwater obstacle: Effect of bottom boundary conditions
The wake of a three-dimensional underwater obstacle: Effect of bottom boundary conditions
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三维水下障碍物的尾流：底部边界条件的影响
DOI：
10.1016/j.ocemod.2020.101611
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发表时间：
2020
期刊：
影响因子：
3.2
通讯作者：
Sarkar, Sutanu
中科院分区：
文献类型：
--
作者：
Puthan, Pranav;Jalali, Masoud;Ortiz-Tarin, Jose Luis;Chongsiripinyo, Karu;Pawlak, Geno;Sarkar, Sutanu
摘要海底三维障碍物是海流作用下产生旋涡、内波和湍流的常见场所。湍流解析模拟进行分层流过去的锥形山，一个典型的例子，三维障碍。出于使用滑移边界条件（BC）和阻力法（有效地部分滑移）BC在文献中的地球物理尾迹，我们研究的敏感性的障碍物表面和平底的流量到BC。四种BC类型的非旋转尾流的稳定电流冲击在一个锥形障碍物，与两种情况下，即NOSL（无滑移BC用于所有固体边界）和SL（滑移BC用于所有固体边界）之间进行了详细的比较。其他两种情况如下：混合，在平底处有滑移，在障碍物边界处无滑移; DL情况，在所有固体边界上采用二次阻力定律BC。无滑移BC允许形成边界层，该边界层分离涡量并将涡量释放到尾流中。当边界条件发生变化时，背风涡和尾流的结构发生了明显的变化。例如，在无滑移情况下，底壁摩擦抑制了流动分离的不稳定性，导致稳定的附着背风涡。相反，当底壁有滑移BC（SL和Hybrid情况）或有部分滑移（DL情况）时，非定常分离导致近尾流中的涡街和湍流增强。在滑移或部分滑移BC的情况下，回流区较短，尾流恢复明显较快。在障碍物的背风面，NOSL情况下的湍流动能（TKE）集中在回流尾流和自由流之间的剪切层中，而在其他三种情况下，TKE是底部增强的。DL是高Re尾流的适当BC，其中边界层无法解析。在这项研究中，还检查了背风涡度的来源，为每一个选择的BC。倾斜的侧面导致障碍物处密度的水平梯度，这通过斜压扭矩产生涡度。与BC的类型无关，斜压扭矩占主导地位。漩涡的伸展和倾斜也很大。模拟了一个附加的非分层自由滑移情况（SL-UN），发现尾流很薄，没有大的尾涡。因此，分层是必要的形成相干背风涡的类型中看到的地球物理尾流。
Abstract Three-dimensional (3D) obstacles on the bottom are common sites for the generation of vortices, internal waves and turbulence by ocean currents. Turbulence-resolving simulations are conducted for stratified flow past a conical hill, a canonical example of 3D obstacles. Motivated by the use of slip boundary condition (BC) and drag-law (effectively partial slip) BC in the literature on geophysical wakes, we examine the sensitivity of the flow to BCs on the obstacle surface and the flat bottom. Four BC types are examined for a non-rotating wake created by a steady current impinging on a conical obstacle, with a detailed comparison being performed between two cases, namely NOSL (no-slip BC used at all solid boundaries) and SL (slip BC used at all solid boundaries). The other two cases are as follows: Hybrid, undertaken with slip at the flat bottom and no-slip at the obstacle boundaries, and case DL wherein a quadratic drag-law BC is adopted on all solid boundaries. The no-slip BC allows the formation of a boundary layer which separates and sheds vorticity into the wake. Significant changes occur in the structure of the lee vortices and wake when the BC is changed. For instance, bottom wall friction in the no-slip case suppresses unsteadiness of flow separation leading to a steady attached lee vortex. In contrast, when the bottom wall has a slip BC (the SL and Hybrid cases) or has partial slip (DL case), unsteady separation leads to a vortex street in the near wake and the enhancement of turbulence. The recirculation region is shorter and the wake recovery is substantially faster in the case of slip or partial-slip BC. In the lee of the obstacle, turbulent kinetic energy (TKE) for case NOSL is concentrated in a shear layer between the recirculating wake and the free stream, while TKE is bottom-intensified in the other three cases. DL is the appropriate BC for high-R e wakes where the boundary layer cannot be resolved. The sources of lee vorticity are also examined in this study for each choice of BC. The sloping sides lead to horizontal gradients of density at the obstacle, which create vorticity through baroclinic torque. Independent of the type of BC, the baroclinic torque dominates. Vortex stretch and tilt are also substantial. An additional unstratified free-slip case (SL-UN) is simulated and the wake is found to be thin without large wake vortices. Thus, stratification is necessary for the formation of coherent lee vortices of the type seen in geophysical wakes.
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发表时间：
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期刊：
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