EXPERIMENTAL STUDY OF TURBULENT OSCILLATORY BOUNDARY LAYERS IN A NEW OSCILLATORY WATER TUNNEL
EXPERIMENTAL STUDY OF TURBULENT OSCILLATORY BOUNDARY LAYERS IN A NEW OSCILLATORY WATER TUNNEL
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DOI：
10.1016/j.coastaleng.2014.03.007
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发表时间：
2012-10
影响因子：
4.4
通讯作者：
Jing Yuan;O. Madsen;E. Chan
中科院分区：
文献类型：
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作者：
Jing Yuan;O. Madsen;E. Chan
采用振荡水洞进行流动生成，采用粒子图像测速系统进行速度测量，进行了高质量的实验研究，包括大量与全尺寸海岸边界层流动相对应的试验。在三种固定的底部粗糙度配置（即光滑、“砂纸”和陶瓷大理石底部）上对正弦波、斯托克斯波和前倾波进行了测试。实验结果表明，对数剖面可以较准确地反映极近底部区域的边界层流动，因此对数剖面拟合分析可以较准确地确定理论底部位置和底部粗糙度。正弦波和非线性波的一次谐波速度，以及非线性波的二次谐波速度，都表现出相似的垂直变化模式。两个无量纲特征边界层厚度，即1%速度赤字标高和最大振幅标高，与粗糙底试验的相对粗糙度呈幂律关系。观察到嵌入在非线性波中的弱边界层流和嵌入在正弦波中的小但有意义的三次谐波速度。它们只能用随时间变化的湍流涡流粘度的作用来解释。测量的周期平均垂直速度表明，在试验通道中存在第二类普朗特尔二次流。在三种通过速度测量推断底部剪应力的方法中，雷诺应力法由于缺少湍流涡流而低估了底部剪应力，动量积分法由于二次流的影响也明显低估了粗糙底部试验的底部剪应力，因此只有测井剖面拟合方法才能得到正确的估计。对得到的底部剪应力进行了分析，给出了最大剪应力和前三次谐波，并对已有的理论模型进行了验证。
A high-quality experimental study including a large number of tests which correspond to full-scale coastal boundary layer flows is conducted using an oscillating water tunnel for flow generations and a Particle Image Velocimetry system for velocity measurements. Tests are performed for sinusoidal, Stokes and forward-leaning waves over three fixed bottom roughness configurations, i.e. smooth, “sandpaper” and ceramic-marble bottoms. The experimental results suggest that the logarithmic profile can accurately represent the boundary layer flows in the very near-bottom region, so the log-profile fitting analysis can give highly accurate determinations of the theoretical bottom location and the bottom roughness. The first-harmonic velocities of both sinusoidal and nonlinear waves, as well as the second-harmonic velocities of nonlinear waves, exhibit similar patterns of vertical variation. Two dimensionless characteristic boundary layer thicknesses, the elevation of 1% velocity deficit and the elevation of maximum amplitude, are found to have power-law dependencies on the relative roughness for rough bottom tests. A weak boundary layer streaming embedded in nonlinear waves and a small but meaningful third-harmonic velocity embedded in sinusoidal waves are observed. They can be only explained by the effect of a time-varying turbulent eddy viscosity. The measured period-averaged vertical velocities suggest the presence of Prandtl's secondary flows of the second kind in the test channel. Among the three methods to infer bottom shear stress from velocity measurements, the Reynolds stress method underestimates shear stress due to missed turbulent eddies, and the momentum integral method also significantly underestimates bottom shear stress for rough bottom tests due to secondary flows, so only the log-profile fitting method is considered to yield the correct estimate. The obtained bottom shear stresses are analyzed to give the maximum and the first three harmonics, and the results are used to validate some existing theoretical models.