Experiments in Fluids ( IF 2.3 ) Pub Date : 2021-03-31 , DOI: 10.1007/s00348-020-03132-0 Amitosh Dash 1 , Willian Hogendoorn 1 , Christian Poelma 1
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Abstract
We discuss empirical techniques to extract quantitative particle volume fraction profiles in particle-laden flows using an ultrasound transducer. A key step involves probing several uniform suspensions with varying bulk volume fractions from which two key volume fraction dependent calibration parameters are identified: the peak backscatter amplitude (acoustic energy backscattered by the initial layer of the suspension) and the amplitude attenuation rate (rate at which the acoustic energy decays with depth owing to scattering losses). These properties can then be used to reconstruct spatially varying particle volume fraction profiles. Such an empirical approach allows circumventing detailed theoretical models which characterize the interaction between ultrasound and suspensions, which are not universally applicable. We assess the reconstruction techniques via synthetic volume fraction profiles and a known particle-laden suspension immobilized in a gel. While qualitative trends can be easily picked up, the following factors compromise the quantitative accuracy: (1) initial reconstruction errors made in the near-wall regions can propagate and grow along the reconstruction direction, (2) multiple scattering can create artefacts which may affect the reconstruction, and (3) the accuracy of the reconstruction is very sensitive to the goodness of the calibration. Despite these issues, application of the technique to particle-laden pipe flows shows the presence of a core with reduced particle volume fractions in laminar flows, whose prominence reduces as the flow becomes turbulent. This observation is associated with inertia-induced radial migration of particles away from the pipe axis and is observed in flows with bulk volume fractions as high as 0.08. Even transitional flows with low levels of intermittency are not devoid of this depleted core. In conclusion, ultrasonic particle volume fraction profiling can play a key complementary role to ultrasound-based velocimetry in studying the internal features of particle-laden flows.
Graphic abstract
中文翻译：
超声粒子体积分数分析：经验方法的评估
摘要
我们讨论了使用超声换能器在充满颗粒的流动中提取定量颗粒体积分数剖面的经验技术。一个关键步骤涉及探测具有不同体积体积分数的几个均匀悬浮液，从中确定两个关键体积分数相关校准参数：峰值反向散射幅度（由悬浮液的初始层反向散射的声能）和幅度衰减率（在由于散射损失，声能随深度衰减）。然后可以使用这些属性来重建空间变化的粒子体积分数分布。这种经验方法允许规避描述超声和悬浮液之间相互作用的详细理论模型，这些模型并不普遍适用。我们通过合成体积分数剖面和固定在凝胶中的已知载有颗粒的悬浮液来评估重建技术。虽然可以很容易地发现定性趋势，但以下因素会影响定量精度：（1）在近壁区域产生的初始重建误差可以沿着重建方向传播和增长，（2）多次散射会产生可能影响的伪影重建，以及 (3) 重建的准确性对校准的优劣非常敏感。尽管存在这些问题，但将该技术应用于载有颗粒的管道流动表明，在层流中存在颗粒体积分数减少的核心，其突出度随着流动变得湍流而降低。这种观察与惯性引起的颗粒远离管轴的径向迁移有关，并且在体积分数高达 0.08 的流动中观察到。即使是间歇性较低的过渡流也不是没有这个耗尽的核心。总之，超声粒子体积分数分析可以在研究含粒子流的内部特征方面对基于超声的测速法起到关键的补充作用。
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