Unsteady stress partitioning and momentum transfer in the wave bottom boundary layer over movable rippled beds
Unsteady stress partitioning and momentum transfer in the wave bottom boundary layer over movable rippled beds
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活动波纹床上波底边界层的非稳态应力分配和动量传递
DOI：
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发表时间：
2014
期刊：
影响因子：
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通讯作者：
D. L. Foster
中科院分区：
文献类型：
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作者：
S. Rodriguez‐Abudo;D. L. Foster
利用粒子图像测速（PIV）系统，对波浪非对称作用下波纹状沉积物床面的近床速度场进行了观测。为了研究床面形状引起的动力学在总动量传递中的作用，通过全动量方程对二维时变速度场实施了双平均技术。这种方法允许直接确定河床形状引起的应力，即，由于床形的存在而产生的应力，在平床条件下为零。这一分析表明，床形引起的应力是密切相关的相干运动的存在，并可能从湍流应力分区。由推移质输运模型提供的应力推断表明，从双平均技术获得的总动量传递能够再现床形动员。从当地的速度剖面得到的总动量传递和应力估计之间的比较显示出显着的波动，并建议一个阵列的传感器是必要的，以重现床形演变。通过分解构成近床动量平衡的不同项获得的动量不平衡（即，加速度不足、应力梯度和河床形状引起的表面摩擦）提供了与流动分离一致的河床形状引起的压力的估计。该分析揭示了三个区域的流动：自由流，所有条款都相对平衡;近床，动量不平衡是显着的流动减弱;和涟漪波峰，其中床的形式引起的压力是领先的顺序机制。
Observations of the nearbed velocity field over a rippled sediment bed under asymmetric wave forcing conditions were collected using a submersible particle image velocimetry (PIV) system. To examine the role of bed form-induced dynamics in the total momentum transfer, a double-averaging technique was implemented on the two-dimensional time-dependent velocity field by means of the full momentum equation. This approach allows for direct determination of the bed form-induced stresses, i.e., stresses that arise due to the presence of bed forms, which are zero in flat bed conditions. This analysis suggests that bed form-induced stresses are closely related to the presence of coherent motions and may be partitioned from the turbulent stresses. Inferences of stress provided by a bed load transport model suggest that total momentum transfer obtained from the double-averaging technique is capable of reproducing bed form mobilization. Comparisons between the total momentum transfer and stress estimates obtained from local velocity profiles show significant variability across the ripple and suggest that an array of sensors is necessary to reproduce bed form evolution. The imbalance of momentum obtained by resolving the different terms constituting the near-bed momentum balance (i.e., acceleration deficit, stress gradient, and bed form-induced skin friction) provides an estimate of the bed form-induced pressure that is consistent with flow separation. This analysis reveals three regions in the flow: the free-stream, where all terms are relatively balanced; the near-bed, where momentum imbalance is significant during flow weakening; and below ripple crests, where bed form-induced pressure is the leading order mechanism.