Measuring turbulent large-eddy structures with an ADCP. 1. Vertical velocity variance
Measuring turbulent large-eddy structures with an ADCP. 1. Vertical velocity variance
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使用 ADCP 测量湍流大涡流结构。
DOI：
10.1357/002224008785837149
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发表时间：
2008
影响因子：
0.5
通讯作者：
C. Grosch
中科院分区：
文献类型：
--
作者：
A. Gargett;A. Tejada;C. Grosch
两种不同的湍流，Langmuir超级单元和不稳定对流，用VADCP进行了采样，VADCP是一种附加垂直(V)束的声学多普勒电流分析器（ADCP）。利用垂直波束提供的垂直速度变化剖面的直接测量值，计算了从标准ADCP的4个波束导出垂直速度的算法的观测响应函数。推导了单对对斜光束垂直速度估计的理论响应函数，说明了大尺度准相干流结构的重要性，以及不同角度斜光束垂直方向的影响。Langmuir超级单体和不稳定对流的不同大涡特性产生不同的理论响应，但在这两种情况下，理论响应与观测结果定性一致。对于Langmuir超级单体，通过使用VADCP的几何形状对大涡模拟（LES）中可用的三维流场进行采样而生成的数值响应函数具有额外的一致性。三种方法的结果都表明，由斜波束速度推断出的垂直速度有很大的误差。误差可能过高或过低，这取决于（通常未知的）湍流场大涡流的特征，如垂直/水平各向异性、相位相干性和相对于仪器的水平各向异性湍流结构的方向。给定一个标准的ADCP，垂直速度方差的“最佳”估计不是通常的4波束估计，而是两个对估计中较大的那个。
Two different turbulent flows, Langmuir supercells and unstable convection, have been sampled with a VADCP, an acoustic Doppler current profiler (ADCP) with an additional vertical (V) beam. Direct measurements of the profile of vertical velocity variance provided by the vertical beam are used to calculate observational response functions for algorithms used to derive vertical velocity from the 4 beams of a standard ADCP. A theoretical response function derived for the vertical velocity estimate from a single pair of opposed slant beams illustrates the importance of large-scale quasi-coherent flow structures, as well as effects of different angles of slant beams from vertical. Different large-eddy characteristics for Langmuir supercells and unstable convection yield different theoretical response: however in both cases, the theoretical response agrees qualitatively with that derived from observations. For Langmuir supercells, there is additional agreement with numerical response functions generated by using the geometry of a VADCP to sample three-dimensional flow fields available from large eddy simulations (LES). The results from all three approaches show that there can be significant error in vertical velocity inferred from slant beam velocities. The error may be either overor under-estimation, depending upon (usually unknown) features of the large eddies of the turbulent field, such as vertical/horizontal anisotropy, phase coherence, and orientation of horizontally anisotropic turbulent structures relative to the instrument. Given only a standard ADCP, the “best” estimate of vertical velocity variance is not the usual 4-beam estimate, but the larger of the two pair estimates.