Effect of Finite Spatial Resolution on the Turbulent Energy Spectrum Measured in the Coastal Ocean Bottom Boundary Layer
Effect of Finite Spatial Resolution on the Turbulent Energy Spectrum Measured in the Coastal Ocean Bottom Boundary Layer
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DOI：
10.1175/2009jtecho647.1
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发表时间：
2009-12-01
影响因子：
2.2
通讯作者：
Osborn, Thomas R.
中科院分区：
文献类型：
--
作者：
Hackett, Erin E.;Luznik, Luksa;Osborn, Thomas R.
利用在大西洋大陆架底部边界层进行的原位粒子图像测速（PIV）测量，对有限空间分辨率对测量能谱的影响进行了参数化研究。对数据应用不同尺度的二维盒子空间滤波器，并利用这些滤波后的分布计算频率域和波数域的一维能谱。研究发现，在小于滤波器尺度10倍的所有长度尺度上，能级衰减幅度都在15%以上。在频谱方向以及垂直于它的方向上进行滤波有助于衰减的程度，后者通过对所有波数的隐式积分。在比滤波器更大的尺度上，高斯滤波器、非线性巴特沃斯滤波器和中值滤波器比盒滤波器衰减更小的能量。当使用泰勒假设转换频谱时，波能在波数空间中出现在不同于其真实物理尺度的位置，这比滤波器尺寸大得多。因此，波能没有衰减，并在这个光谱范围内主导了湍流。由于波能和湍流对滤波的响应不同，在波主导区和湍流主导区之间的过渡发生了修正的光谱斜率，导致光谱斜率异常陡峭。最后，去除速度信号的压力相干部分不足以揭示波峰光谱范围内的湍流。在这个范围内的剩余能量仍然被更大的尺度所支配。
The effect of finite spatial resolution on the measured energy spectrum is examined via a parametric study using in situ particle image velocimetry (PIV) measurements performed in the bottom boundary layer on the Atlantic continental shelf. Two-dimensional (2D) box spatial filters of various scales are applied to the data, and these filtered distributions are used to compute 1D energy spectra in both frequency and wavenumber domains. It is found that energy levels are attenuated by more than 15% at all length scales that are smaller than 10 times the scale of the filter. Filtering both in the direction of the spectrum as well as perpendicular to it contributes to the extent of attenuation, the latter via implicit integration over all wavenumbers. At scales larger than that of the filter, Gaussian, nonlinear Butterworth, and median filters attenuate less energy than the box filter. When frequency spectra are converted using Taylor's hypothesis, wave energy appears in wavenumber space at a location different than its true physical scale, which is much larger than the filter sizes. Consequently, wave energy is not attenuated and dominates over the turbulence through this spectral range. Because wave energy and turbulence respond differently to the filtering, modified spectral slopes at the transition between wave-and turbulence-dominated regions occur, resulting in inordinately steep spectral slopes. Finally, removal of the pressure-coherent part of the velocity signal is not sufficient to reveal the turbulence within the wave peak spectral range. Remaining energy in this range is still dominated by much larger scales.