Abstract Under the right conditions, viscoelastic fluids can exhibit elastic flow instabilities in the absence of inertia. If the fluid is in contact with a flexible or flexibly-mounted structure, these elastic flow instabilities lead to time-dependent forces on the structure, which can cause the structure to oscillate. This constitutes a new class of Viscoelastic Fluid–Structure Interactions (VFSI). Up until now, the VFSI reported in the literature demonstrated a one-way coupling between the fluid and the structure where little feedback from the structure to the flow was observed. Here, we report for the very first time, the presence of a lock-in behavior in VFSI. We have designed and conducted a set of experiments in which the frequency of the elastic flow instabilities and the natural frequency of the flexible structure become equal as the flow velocity is increased, and therefore lock-in is observed. We present amplitude and frequency responses of two sets of flexibly-mounted cylinders over a range of Weissenberg numbers and reduced velocities to show the lock-in range. The time-varying flow fields inside and outside the lock-in range were studied through streak-line imaging and particle image velocimetry. The viscoelastic lock-in range was observed for a Weissenberg number range of 5 ≤ W i ≤ 15 for one of the cylinders tested and for a range of 20 ≤ W i ≤ 30 for the other. For both cylinders, the lock-in started at a reduced velocity of V r ≈ 1 . 5 . In Cylinder 1, the lock-in ended at V r ≈ 4 , while for Cylinder 2 the end of the lock-in was not observed, because the data points did not cover a wide enough range. Similar to the lock-in observed in the typical VIV response in a Newtonian fluid, the frequency of the elastic flow instability was found to increase with increasing flow velocity before plateauing at the natural frequency of the cylinder in the lock-in range. Unlike the VIV response of a Newtonian fluid, where the lock-in range corresponds to the maximum observed amplitude of oscillations, in VFSI the amplitude of oscillations reached a plateau while in lock-in, but increased with Weissenberg number and reduced velocity before and after the lock-in range.

中文翻译：

---

观察粘弹性流固耦合的锁定

摘要 在适当的条件下，粘弹性流体可以在没有惯性的情况下表现出弹性流动不稳定性。如果流体与柔性或柔性安装的结构接触，这些弹性流动不稳定性会导致结构上的时间相关力，这会导致结构振荡。这构成了一类新的粘弹性流体-结构相互作用 (VFSI)。到目前为止，文献中报道的 VFSI 证明了流体和结构之间的单向耦合，几乎没有观察到从结构到流动的反馈。在这里，我们首次报告了 VFSI 中存在锁定行为。我们设计并进行了一组实验，其中随着流速的增加，弹性流动不稳定性的频率和柔性结构的固有频率变得相等，因此观察到锁定。我们在魏森伯格数和降低的速度范围内展示了两组灵活安装的气缸的幅度和频率响应，以显示锁定范围。通过条纹线成像和粒子图像测速法研究了锁定范围内外的时变流场。粘弹性锁定范围在 5 ≤ Wi ≤ 15 的 Weissenberg 数范围内观察到，其中一个测试圆柱体和 20 ≤ W i ≤ 30 的另一个圆柱体。对于两个气缸，锁定开始于 V r ≈ 1 的降低速度。5 . 在圆柱体 1 中，锁定在 V r ≈ 4 处结束，而对于圆柱体 2，没有观察到锁定的结束，因为数据点没有覆盖足够宽的范围。类似于在牛顿流体中典型的 VIV 响应中观察到的锁定，发现弹性流动不稳定性的频率随着流速的增加而增加，然后在锁定范围内的圆柱体固有频率处达到稳定状态。与牛顿流体的 VIV 响应不同，其中锁定范围对应于观察到的最大振荡幅度，在 VFSI 中，在锁定时振荡幅度达到平台，但随着魏森伯格数和前后速度降低而增加锁定范围。类似于在牛顿流体中典型的 VIV 响应中观察到的锁定，发现弹性流动不稳定性的频率随着流速的增加而增加，然后在锁定范围内的圆柱体的固有频率处达到稳定状态。与牛顿流体的 VIV 响应不同，其中锁定范围对应于观察到的最大振荡幅度，在 VFSI 中，在锁定时振荡幅度达到平台，但随着魏森伯格数和前后速度降低而增加锁定范围。类似于在牛顿流体中典型的 VIV 响应中观察到的锁定，发现弹性流动不稳定性的频率随着流速的增加而增加，然后在锁定范围内的圆柱体的固有频率处达到稳定状态。与牛顿流体的 VIV 响应不同，其中锁定范围对应于观察到的最大振荡幅度，在 VFSI 中，在锁定时振荡幅度达到平台，但随着魏森伯格数和前后速度降低而增加锁定范围。