Movable bed roughness in unsteady oscillatory flow
Movable bed roughness in unsteady oscillatory flow
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DOI：
10.1029/jc087ic01p00469
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发表时间：
1982-01
影响因子：
--
通讯作者：
W. Grant;O. Madsen
中科院分区：
文献类型：
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作者：
W. Grant;O. Madsen
本文提出了一个非定常振荡流过可动、无粘性床面时的粗糙度预报模型。动床上的粗糙度是边界剪应力的函数，而不是固定的几何尺度，不像固定床上的完全粗糙的湍流边界剪切流那样。该模型将粗糙度分为两个不同的部分。这两个贡献是由于个别河床形态周围的形态阻力和近床面泥沙的输运。床面上的形状阻力被显式地视为边界几何形状和剪应力的函数。波纹被预测为局部表面摩擦力的函数，并使用标准的墙定律参数推导出半经验表达式，该表达式给出了波纹或形状粗糙度作为边界几何的函数。波纹粗糙度与波纹陡度和波纹高度的乘积成正比。形状阻力模型与Bagnold(1946)的固定波纹研究结果进行了比较，结果令人满意。由Carstens等人，S(1969)的实验确定了与平底振荡流中强烈输沙有关的Z0值。该值被发现为7或8个颗粒直径。根据欧文(1964)关于均匀颗粒在空气中跳跃的粗糙度假设，导出了与近底输沙层最大厚度有关的粗糙度的表达式。在边界剪应力相对于泥沙初始运动临界值较大的情况下，所导出的表达式与Smith和McLean(1977)针对振荡流修正的单向流法的结果相似。总粗糙度模型与Carstens等人S(1969)的数据吻合较好。与Smith和McLean(1977)的定常流动结果不同，这里的结果表明，当波纹存在时，它们占边界粗糙度的很大一部分。
A model to predict the roughness in unsteady oscillatory flows over movable, noncohesive beds is presented. The roughness over movable beds is shown to be a function of the boundary shear stress, rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds. The model partitions the roughness into two distinct contributions. These two contributions are due to the form drag around individual bed forms and to the near-bed sediment transport. The form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress. The ripples are predicted as a function of the local skin friction, and a semiempirical expression is derived using standard law-of-the-wall arguments, which gives the ripple or form roughness as a function of the boundary geometry. The ripple roughness is found to be proportional to the product of the ripple steepness and height. Favorable comparison of the form drag model with the results of Bagnold's (1946) fixed ripple study is found. The value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from Carstens et al.'s (1969) experiments. This value is found to be 7 or 8 grain diameters. An expression is derived for the roughness associated with the maximum thickness of a near-bottom sediment-transporting layer consistent with Owen's (1964) roughness hypothesis for saltation of uniform grains in air. At large values of the boundary shear stress relative to the critical value for initial sediment motion, the derived expression is similar to the results of Smith and McLean's (1977) unidirectional flow approach modified for oscillatory flow. The total roughness model is found to compare favorably with Carstens et al.'s (1969) data. In contrast to Smith and McLean's (1977) steady flow findings, the results here show that when ripples are present, they account for a significant portion of the boundary roughness.