The flow field around a vertically mounted circular pile exposed to gradually-varied unsteady flow is experimentally investigated. Experiments were conducted in a 30 m-long and 1 m-wide recirculating flume equipped with a variable discharge pump. A circular cylinder with 9 cm diameter was used as the model pile. To understand the influence of accelerating and decelerating flow conditions, three unsteady cases with different unsteadiness degrees were tested as well as a reference steady flow case. The spatial and temporal variations of Reynolds-averaged velocity and turbulence characteristics around the pile, as well as undisturbed flow, were analyzed. Findings show that there are distinct differences between the tested gradually-varied unsteady flow cases and the reference steady flow case. The Reynolds-averaged velocity vs. TKE plots of undisturbed flow indicated a hysteresis effect, such that larger turbulence is generated during the falling stage of the flow compared to the rising stage. This hysteresis was considerably reduced in the pile wake, and even reversed hysteresis was seen at certain cases. The spatial variation of Reynolds-averaged velocity and turbulence in the peak instant of unsteady flow was qualitatively similar to that of steady flow, but quantitatively, turbulence, flow contraction, and velocity deficit in the near-wake region were smaller in the case of unsteady flow. Contrarily, the unsteady flow generated remarkably higher turbulence levels at further downstream in the pile wake. It is concluded that in the case of unsteady flow the pile behaves as if it has a more streamlined shape. The results were also interpreted from structure-bed interactions perspective, explaining the differences between the pile scour induced by steady and unsteady flow conditions.

对暴露于逐渐变化的非定常流的垂直安装的圆形桩周围的流场进行了实验研究。实验在一个 30 m 长和 1 m 宽的循环水槽中进行，该水槽配备了一个可变排放泵。一个直径为 9 cm 的圆柱体用作模型桩。为了了解加速和减速流动条件的影响，测试了三个不同不稳定度的不稳定情况以及一个参考稳定流情况。分析了桩周围雷诺平均速度和湍流特性的空间和时间变化，以及未受干扰的流动。结果表明，经过测试的渐变非定常流情况与参考定常流情况之间存在明显差异。雷诺平均速度与 未受干扰流动的 TKE 图表明存在滞后效应，因此与上升阶段相比，在流动的下降阶段会产生更大的湍流。这种滞后在桩尾流中大大减少，在某些情况下甚至可以看到反向滞后。非定常流峰值时刻雷诺平均速度和湍流的空间变化在定性上与定常流相似，但在定量上，近尾流区域的湍流、流收缩和速度赤字在非定常情况下较小流。相反，不稳定的流动在桩尾流的更下游产生了显着更高的湍流水平。得出的结论是，在不稳定流动的情况下，桩表现得好像它具有更流线型的形状。