Entrainment effects in periodic forcing of the flow over a backward-facing step
Entrainment effects in periodic forcing of the flow over a backward-facing step
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DOI：
10.1103/physrevfluids.2.074605
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发表时间：
2017-02
期刊：
影响因子：
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通讯作者：
T. Berk;T. Medjnoun;B. Ganapathisubramani
中科院分区：
文献类型：
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作者：
T. Berk;T. Medjnoun;B. Ganapathisubramani
实验研究了Strouhal数对后向台阶（高度，H）上流动周期性强迫的影响。在雷诺数ReH = HU∞/ν = 41000时，在Strouhal数为0.21 < StH < 1.98（StH = f H/U∞）的台阶边缘处通过合成射流施加强迫。在文献中，常将Strouhal数对再附着长度的影响分为低频和高频激励，指的是非强迫流中存在的特定频率。在本研究中，斯特劳哈尔数的变化解释的基础上的一个连续的变化，而不是特定的频率夹带到回流区的动量。结果表明，再附着长度随动量卷吸的增加而线性减小。垂直动量通量驱动的强迫和本地垂直动量通量所产生的涡被证明是定性相似的环流考虑的所有情况下。总循环（与夹带的动量和对再附着长度的影响）示出减少与Strouhal数，而这是不预测的模型的基础上，特定的低和高频率。通过跟踪锁相数据中的涡旋，推导出环流（衰减）的经验模型。该模型被用来破译相关的标度参数，解释环流，夹带的动量和再附着长度的变化。斯特劳哈尔数的三个政权被确定。一个低Strouhal数制度观察到的旋涡形成在后期阶段相对于回流区，导致有效性降低。对于高Strouhal数，涡被重新摄入致动器中，或者被挤得如此之近以至于它们彼此抵消，这两者都降低了强迫的有效性。在中间状态下，会形成涡旋串，随着斯特罗哈尔数的增加，环流的衰减也会增加。这种衰减的比例完全解释了所观察到的再附着长度的变化。在这项研究中所作的动量卷吸的观察，预计也适用于其他钝体流的周期性强迫。
The effect of the Strouhal number on periodic forcing of the flow over a backward-facing step (height, H) is investigated experimentally. Forcing is applied by a synthetic jet at the edge of the step at Strouhal numbers ranging from 0.21 < StH < 1.98 (StH = f H/U∞) at a Reynolds number of ReH = HU∞/ν = 41000. In the literature, the effect of Strouhal number on the reattachment length is often divided into low- and high frequency actuation, referring to specific frequencies present in the unforced flow. In the present study, variations with Strouhal number are explained based on a continuous variation of entrainment of momentum into the recirculation region rather than on specific frequencies. The reattachment length is shown to decrease linearly with entrainment of momentum. Vertical momentum flux is driven by vortices generated by the forcing and locally vertical momentum flux is shown to be qualitatively similar to circulation for all cases considered. Total circulation (and therewith entrainment of momentum and the effect on the reattachment length) is shown to decrease with Strouhal number whereas this is not predicted by models based on specific low- and high frequencies. An empirical model for the (decay of) circulation is derived by tracking vortices in phase-locked data. This model is used to decipher relevant scaling parameters that explain the variations in circulation, entrainment of momentum and reattachment length. Three regimes of Strouhal number are identified. A low-Strouhal-number regime is observed for which vortices are formed at a late stage relative to the recirculation region, causing a decrease in effectiveness. For high Strouhal numbers vortices are being re-ingested into the actuator or are packed so close together that they cancel each other, both decreasing the effectiveness of forcing. In the intermediate regime a vortex train is formed of which the decay of circulation increases for increasing Strouhal number. The scaling of this decay fully explains the observed variation in reattachment length. The observations on entrainment of momentum made in this study are expected to also hold for periodic forcing of other bluff-body flows.