大涡模拟及其在海洋湍流数值模拟中的应用

本文概述了大涡模拟的定义及特征,介绍了大涡模拟的几种常用方法以及它在海洋湍流场数值模拟中的应用,并指出了一些尚待改进的问题。     

地形和风速影响下的海气相互作用大涡模拟研究

当水流经过海洋地形时,水流的不稳定性会引起垂向混合并伴随大量湍流过程。针对传统海气耦合模式缺少在湍流尺度上讨论海洋地形与风速对海气相互作用影响的问题,使用并行大涡模拟海气耦合模式(the parallelized large eddy simulation model, PALM)在5 m/s的背景风场下,引入理想立方体地形,对比有无地形的影响;设置地形边长为L,高为3L (其中大气部分高L), L与水深H之比为L/H=1/2;然后保持地形条件不变。设置5、10和15 m/s三种风速,讨论风速对小尺度海气相互作用的影响。研究表明:地形在大气部分减弱顺风向速度,增强侧风向速度,影响0～5L的高度区域,而对垂向作用较小;无地形条件下湍流垂向涡黏系数K_m在-0.3L时,水深达到最大值0.024 m²/s,有地形条件下K_m在-0.8L时,达到最大值为0.16 m²/s,地形的存在使得上层海洋混合加强, K_m最大值增加1个数量级。随风速增大海洋和大气中的净热通量、淡水通量和浮力通量都相应...     

基于大涡模拟的波状前缘水翼空化抑制研究

为了降低空化造成的水动力性能损失,基于仿生学原理,参考座头鲸鳍肢剖面形状,将前缘波浪构型引入到水翼设计中,研究波状前缘水翼的非定常空化特性,并探究前缘参数改变对空化控制的效果和规律。选用NACA63₄-021水翼为基准模型,进行前缘参数化重构,设计出3种不同的波状水翼进行对比研究。采用大涡模拟（LES）方法对空化流场进行精细化数值模拟,针对基准水翼和不同波幅与波长参数下的波状水翼开展了空化周期、升阻力系数、压力脉动以及流向涡结构的对比分析。结果发现,波状水翼在抑制空化和降低压力脉动方面都取得了显著效果。其中,3种不同的波状水翼空化抑制率分别为15.7%、18.6%和27.9%,压力脉动幅值分别降低了55.3%、67.3%和74.6%。分析表明,波浪前缘的引入使得空化的分区效应更加凸显,空化从波谷处初生,增大波幅或减小波长都可以加强对空化的抑制效果,并可以提高升力系数以及显著降低水翼表面的压力脉动。前缘波浪构型还将诱发向下游发展的对转涡结构,不同前缘参数的波状水翼涡结构的演化是相似的,空泡发展与溃灭的整个过程对涡结构的发展也具有显著影响。     

基于大涡模拟耦合模式的小尺度海气相互作用研究

大气和海洋是影响地球气候系统的两个重要因素,它们之间的相互作用是海洋和大气研究的重要课题,海气耦合模式则是研究海气相互作用的重要工具,而海气耦合模式重点考虑的参数是海气通量。针对传统的大尺度海气耦合模式缺少湍流尺度分析的问题,本文使用并行大涡模拟海气耦合模式（The Parallelized Large-Eddy Simulation Model,PALM）,在小尺度上探究风速对海气通量及湍流动能收支（Turbulence Kinetic Energy Budget,TKE Budget）的影响,设置了5、10和15m/s三种地转风速度对大气边界层（Atmospheric Boundary Layer,ABL）和海洋混合层（Oceanic Mixed Layer,OML）进行海气耦合模拟。研究表明：海气通量的分布与风速大小密切相关,风速越大,净热通量和浮力通量相对越大,由于温度上升导致海水蒸发加剧,使得大气的淡水通量增大;海洋湍流动能收支各项在近海面处受风速影响较大,且随着深度加深而逐渐减弱。本研究初步展示了小尺度海气耦合模式在海气通量研究中的应用,对进行小尺度海气相互作用研究具有一定的意义。     

海底管线上波浪力的大涡模拟及三步有限元数值模拟

应用三步有限元法结合大涡模拟方法(LES)离散非定常不可压流动的Navier-Stokes方程,模拟了波浪场中海底管线周围流场及其受力情况,并把数值模拟的结果与物理实验的实测结果进行了比较,两者符合较好。为了检验三维效应的影响,同时进行了二维和三维流场的数值模拟。     

雷诺数为3 900时三维圆柱绕流的大涡模拟

自从人们对层流的圆柱绕流现象有了系列研究及清楚的认识后,人们逐渐把目光投向湍流的圆柱绕流,但相关研究主要关注于模拟湍流方法的数值格式和精度问题,而忽略了对高雷诺下圆柱绕流流场本身的认识及规律的总结。基于开源代码Open FOAM的大涡模拟方法以连续方程和Navier-Stokes方程为控制方程,选用Smargorinsky模式为亚格子应力模型,采用有限体积法和一次预测两次修正的PISO算法,对Re=3 900时三维圆柱绕流问题进行了数值模拟研究,并着重分析了其尾流特征和性质。数值计算结果表明:大涡模拟方法可以模拟出细致的流场结构,该雷诺数下的圆柱绕流具有很强的三维及湍流效应,在圆柱后方约一倍直径的范围内存在回流区域,在靠近圆柱壁面的尾流区域的速度剖面呈"U"型,远离壁面的速度剖面呈"V"型。瞬时速度剖面始终围绕着时均速度的周围脉动,且距离圆柱越远瞬时速度场的脉动范围越大。     

基于大涡模拟和局部滤波同化方法的海洋环流模式

结合最小二乘法极值原理,提出了一种基于局部谱展开的滤波同化方法,把测量数据和数值计算过程中出现的高频短波滤掉,并将高度计数据同化到了求解过程中.结果既增加了数值稳定性,又提高了数值模拟的准确性.针对在海洋环流问题中水平的流动性质和垂直的不同的特点,我们还将大涡模拟的思想和直接涡黏的思想分别应用于水平方向和垂直方向,给出的方法是一种适用于海洋环流和浅水环流问题的大涡模拟湍流模式.对热带和北太平洋一年四季非定常季风作用下环流的数值模拟表明,提出的方法非常有效,数值结果与实际相当吻合.     

基于大涡模拟与被动示踪物模型的人工鱼礁数值研究

投放人工鱼礁是解决海洋生态环境问题的重要措施之一。通过使用并行大涡模拟模式（the parallelizeda large-eddy simulation model,PALM）及被动示踪物模型模块,研究了不同流速条件（0.1,0.2,0.4和0.6 m/s）下方型人工鱼礁对流场形态、营养盐的抬升作用、和湍流动能收支的影响。研究表明,鱼礁的存在使得其附近垂向速度增大,产生上升流。受到上升流的抬升作用,鱼礁底部的示踪物迅速进入海洋上层,之后遇到鱼礁后方的背涡流,示踪物的抬升受阻,高度逐渐降低。在上升流区域以及背涡流区域的共同影响下,示踪物抬升区域的最大高度与来流流速无关。不同上升流定义对应的上升流区域的高度与来流流速均不相关;上升流区域的最大速度、平均速度与来流流速都成线性增加的关系;然而随着来流流速的增大,不同上升流定义对应的上升流区域面积的变化趋势却完全不同。投放鱼礁后,鱼礁区域底部的混合增强,区域底部的能量被输运至上层。这说明,鱼礁的存在不仅对营养物质具有抬升作用,还能将能量向上输运。     

基于大涡模拟的三角形地形之层结流体湍动能收支模型

为了探究海底地形对湍流动能收支的影响,本文使用并行大涡模拟模式(The Parallelized Large-Eddy Simulation Model,PALM),以坡陡作为无量纲地形参数(δ),设置了亚临界、临界和超临界三种地形状态(δ=0.5,1和2)进行数值模拟。文章计算并呈现了地形作用下的流体速度和湍流动能收支分布,以及湍流动能平衡方程各参量。通过回归分析和量纲分析重点讨论了地形顶点处耗散和海表处能量的耗散,与地形坡陡的关系,得出其关系均呈指数形式。     

基于大涡模拟的ROV导管推进器梢隙流动数值研究

导管推进器是一种普遍应用于无人遥控潜水器(ROV)等潜器中的特种推进器。在桨叶与导管之间的梢隙中存在非常复杂的流动,本研究基于大涡模拟(LES)对导管推进器的梢隙流动进行了数值模拟分析。通过对时间步长的收敛性研究,建立两套基于不同网格类型的计算模型。将计算与试验结果进行对比,比较两种不同类型网格模拟结果的差异,发现切割体网格能够更好地捕捉到泄涡的细节,并结合梢隙流场的原理分析了泄涡发展的过程,梢隙涡的驱动力是吸力面、压力面之间的压差。此外,随着进速系数增大,梢隙周向的涡管轴向分布范围减小,主泄涡发生位置延后,泄出涡的长度和数量都有所减少。     

Large eddy simulation of breaking waves

A numerical model is used to simulate wave breaking, the large scale water motions and turbulence induced by the breaking process. The model consists of a free surface model using the surface markers method combined with a three-dimensional model that solves the flow equations. The turbulence is described by large eddy simulation where the larger turbulent features are simulated by solving the flow equations, and the small scale turbulence that is not resolved by the flow model is represented by a sub-grid model. A simple Smagorinsky sub-grid model has been used for the present simulations. The incoming waves are specified by a flux boundary condition. The waves are approaching in the shore-normal direction and are breaking on a plane, constant slope beach. The first few wave periods are simulated by a two-dimensional model in the vertical plane normal to the beach line. The model describes the steepening and the overturning of the wave. At a given instant, the model domain is extended to three dimensions, and the two-dimensional flow field develops spontaneously three-dimensional flow features with turbulent eddies. After a few wave periods, stationary (periodic) conditions are achieved. The surface is still specified to be uniform in the transverse (alongshore) direction, and it is only the flow field that is three-dimensional.The turbulent structures are investigated under different breaker types, spilling, weak plungers and strong plungers. The model is able to reproduce complicated flow phenomena such as obliquely descending eddies. The turbulent kinetic energy is found by averaging over the transverse direction. In spilling breakers, the turbulence is generated in a series of eddies in the shear layer under the surface roller. After the passage of the roller the turbulence spreads downwards. In the strong plunging breaker, the turbulence originates to a large degree from the topologically generated vorticity. The turbulence generated at the plunge point is almost immediately distributed over the entire water depth by large organised vortices. Away from the bed, the length scale of the turbulence (the characteristic size of the eddies resolved by the model) is similar in the horizontal and the vertical direction. It is found to be of the order one half of the water depth.     

Large eddy simulation of spilling and plunging breakers

A Navier–Stokes solver with a free surface model is used for simulating wave breaking, undertow, and turbulence in breaking waves. The free surface model is based on the Volume of Fluid concept. Turbulence scales larger than the grid scale are simulated directly while turbulence scales smaller than the grid scale are represented by a sub-grid scale model. Two different approaches for the sub-grid scale model have been applied, which are the Smagorinsky model and a model based on a k-equation for the sub-grid scale turbulence. The waves approach the shore in shore-normal direction and break on a plane constant sloping beach. Periodic spilling and plunging breakers are simulated for 20 and 16 wave periods, respectively. The set-up, undertow, and turbulence levels are compared to experimental results. Despite the rather coarse resolution of the computational domain, satisfactory results for the wave height decay and undertow have been obtained. However, the turbulence levels are over-predicted when using the standard values of the model parameters and a complete answer to this problem has not been found. Furthermore, the evolution of vorticity over the wave period has been studied. It shows that at the initial breaking point vorticity is generated around the vertical as well as around the transverse axis. Later vorticity around the longitudinal axis (offshore–onshore direction) is generated, probably through deformation of vorticity around the other axis.     

海面风应力偏离风向的观测与分析

针对海气界面风应力方向与风向不一致的现象,2015年2月4日至3月12日在南海博贺观测平台开展了综合观测,利用涡动相关法计算了海气界面风应力,并在3类大气稳定度条件下分析了风应力矢量偏离风矢量的角度变化,进一步讨论了大气层结稳定时两者角度之差与风速的参数化关系。结果表明:在大气层结稳定条件下,风应力矢量偏向风矢量左侧,且偏离角度随逆波龄和风速增大而减小;当大气层结不稳定时,风应力矢量一般偏向风矢量右侧。海气界面风应力矢量受海表面风、波浪以及大气层结的共同调制。     

Dependence of seafloor boundary layer thickness on the overlying flow direction: a large eddy simulation study

Using large eddy simulation (LES) incorporating the effect of the horizontal component of the earth’s rotation vector, we studied the seafloor turbulent boundary layer to investigate the dependence of the boundary layer thickness on the overlying geostrophic flow orientation. The thickest boundary layer appears for the westward geostrophic flow: it is almost twice that of the eastward flow. The turbulent disturbances in the boundary layer are elongated slightly leftward relative to the geostrophic flow. Linear stability analysis for the Ekman’s spiral flow showed that the growth rate is maximum for the westward geostrophic flow and the unstable roll-like mode appears, which points slightly leftward relative to the geostrophic flow. These properties correspond to the feature near the bottom of the developed turbulent layer.     

Correction of the effect of relative wind direction on wind speed derived by advanced microwave scanning radiometer

The sea surface wind speed (SSWS) derived by a microwave radiometer can be contaminated by changes of the brightness temperature owing to the angle between the sensor azimuth and the wind direction (Relative Wind Direction effect: RWD effect). We attempt to apply the method proposed by Konda and Shibata (2004) to the SSWS derived by Advanced Microwave Scanning Radiometer (AMSR) on Advanced Earth Observing Satellite II (ADEOS-II), in order to correct for the RWD effect. The improvement of accuracy of the SSWS estimation amounts to roughly 60% of the error caused by the RWD effect. Comparison with in situ observation at the Tropical Atmosphere Ocean (TAO) array shows that the root mean square error of the corrected SSWS is 1.1 ms⁻¹. It is found that the impact of the RWD effect on the estimation of the latent heat flux can amount to about 30 Wm⁻² on average. We applied the method to the SSWS derived by AMSR for Earth Observing System (AMSR-E) and obtained a similar result.     

Large eddy simulation of stratified mixing in a three-dimensional lock-exchange system

Accurate and computationally-efficient modeling of stratified mixing processes are of paramount importance in both coastal and large-scale ocean circulation. In this study, our main objective is to investigate the feasibility and accuracy of large eddy simulation (LES) as a possible tool to study small-scale oceanic processes. To this end, LES is evaluated in a 3D lock-exchange problem, which contains shear-driven mixing, internal waves, interactions with boundaries and convective motions, while having a simple domain, initial and boundary conditions, and forcing.Two general classes of LES models are tested, namely eddy viscosity (EV) models based on constant–coefficient and dynamic Smagorinsky models, and an approximate deconvolution (AD) model. By noting that the dynamic Smagorinsky and AD models have different strengths in that the former is good in providing appropriate dissipation while the latter in preserving the detail of coherent structures on coarse resolution meshes, a hybrid approach combining EV and AD models is also evaluated. A direct numerical simulation (DNS) is performed as the benchmark solution, and all LES models are tested on three coarse meshes. The main measure of mixing is taken as the temporal evolution of background potential energy.It is found that constant-coefficient Smagorinsky models can only provide a marginal improvement over under-resolved simulations, while both dynamic Smagorinsky and AD models lead to significant improvements in mixing accuracy. The primary accomplishment of this study is that it is shown that the hybrid approach attains the best agreement with the mixing curve from DNS, while being computationally approximately a thousand times faster.     

The dependence of precipitation on wind direction over the Kuroshio front in the winter East China Sea

This study investigates atmospheric responses to the directions of surface wind over the Kuroshio front in the East China Sea, using wintertime satellite-derived data sets. Composite maps of sea surface temperature, wind speed, precipitation, turbulent heat flux, surface wind divergence, and the curl of wind vectors above the atmospheric boundary layer are depicted based on the classification of intense northeasterly (along the front) and northwesterly (across the front) winds over the East China Sea. When northeasterly winds prevail, considerable precipitation occurs on the offshore side of the Kuroshio front, in contrast to periods when northwesterly winds prevail. First, the northeasterly winds strengthen above the front because of the downward transfer of momentum from the fast-moving air at higher levels and/or an adjustment of sea level pressure over the oceanic front, although the process by which the influence of the Kuroshio penetrates beyond the marine atmospheric boundary layer remains unclear. Second, a cyclonic vortex forms above the marine atmospheric boundary layer (at 850-hPa height) on the offshore side of the front, and thereafter, surface wind convergence via Ekman suction (hence, enhanced precipitation) occurs over the East China Sea shelf breaks. The northeasterly winds blow over the East China Sea when the Aleutian Low retreats to the east and when high sea level pressure covers the northern Sea of Japan.     

Methods of obtaining offshore wind direction and sea-state data from X-band aircraft SAR imagery of coastal waters

X-band synthetic aperture radar (SAR) imagery of the Goto Islands of Japan was digitally analyzed to extract air-sea interaction parameters and to assess the potential of texture measures in analysis of SAR ocean imagery. Wind direction is extracted from wind rows, wind streaks, and random turbulence patterns observed in the SAR imagery. Sea-state parameters are either extracted directly from the imagery or estimated using the extracted information in previously established empirical formulas. A convenient method of digitally presenting imagery, local power spectra, and the extracted/estimated parameters is presented. Texture analysis based on gray-level co-occurrence (GLC) matrices is applied to SAR ocean imagery. The inertia measure is shown to extract similar information to the power spectrum. The cluster-shade measure is shown to be sensitive to image phase.