The near-wall region of an unsteady turbulent pipe flow has been investigated experimentally using hot-film anemometry and two-component particle image velocimetry. The imposed unsteadiness has been pulsating, i.e., when a non-zero mean turbulent flow is perturbed by sinusoidal oscillations, and near-uniformly accelerating in which the mean flow ramped monotonically between two turbulent states. Previous studies of accelerating flows have shown that the time evolution between the two turbulent states occurs in three stages. The first stage is associated with a minimal response of the Reynolds shear stress and the ensemble-averaged mean flow evolves essentially akin to a laminar flow undergoing the same change in flow rate. During the second stage, the turbulence responds rapidly to the new flow conditions set by the acceleration and the laminar-like behavior rapidly disappears. During the final stage, the flow adapts to the conditions set by the final Reynolds number. In here, it is shown that the time-development of the ensemble-averaged wall shear stress and turbulence during the accelerating phase of a pulsating flow bears marked similarity to the first two stages of time-development exhibited by a near-uniformly accelerating flow. The stage-like time-development is observed even for a very low forcing frequency; ω+=ων/u¯τ2=0.00073$\omega ^{+}=\omega \nu /{\overline {u}}_{\tau }^{2}=0.00073$ (or equivalently, ls+=2/ω+=52${l}_{s}^{+}=\sqrt {2/\omega ^{+}}=52$), at an amplitude of pulsation of 0.5. Some previous studies have considered the flow to be quasi-steady at ls+=52${l}_{s}^{+}=52$; however, the forcing amplitude has been smaller in those studies. The importance of the forcing amplitude is reinforced by the time-development of the ensemble-averaged turbulence field. For, the near-wall response of the Reynolds stresses showed a dependence on the amplitude of pulsation. Thus, it appears to exist a need to seek alternative similarity parameters, taking the amplitude of pulsation into account, if the response of different flow quantities in a pulsating flow are to be classified correctly.

中文翻译：

---

关于脉动和加速湍流管流的相似性

已经使用热膜风速测量法和双分量粒子图像测速法对不稳定的湍流管流的近壁区域进行了实验研究。强加的不稳定一直是脉动的，即当非零平均湍流受到正弦振荡的扰动时，并且几乎均匀加速，其中平均流在两个湍流状态之间单调上升。先前对加速流动的研究表明，两种湍流状态之间的时间演化发生在三个阶段。第一阶段与雷诺剪应力的最小响应相关，并且整体平均平均流的演变基本上类似于层流，其流速发生相同的变化。在第二阶段，湍流对加速度设定的新流动条件做出快速响应，层流行为迅速消失。在最后阶段，流动适应由最终雷诺数设定的条件。在这里，表明在脉动流的加速阶段，整体平均壁面剪应力和湍流的时间发展与几乎均匀加速的流动所表现出的前两个时间发展阶段具有明显的相似性。即使在非常低的强迫频率下也能观察到类似阶段的时间发展；ω+=ων/u¯τ2=0.00073$\omega ^{+}=\omega \nu /{\overline {u}}_{\tau }^{2}=0.00073$（或等效地，ls+=2/ ω+=52${l}_{s}^{+}=\sqrt {2/\omega ^{+}}=52$)，脉动幅度为 0.5。以前的一些研究认为流动在 ls+=52${l}_{s}^{+}=52$ 处是准稳态的；然而，这些研究中的强迫幅度较小。集合平均湍流场的时间发展加强了强迫振幅的重要性。因为，雷诺应力的近壁响应显示出对脉动幅度的依赖性。因此，如果要正确分类脉动流中不同流量的响应，则似乎需要寻求替代的相似性参数，同时考虑脉动的幅度。