Data for figure 2 from Sodium chloride inhibits effective bubbly drag reduction in turbulent bubbly Taylor–Couette flows

Using the Taylor–Couette geometry we experimentally investigate the effect of salt on drag reduction caused by bubbles present in the flow. We combine torque measurements with optical high-speed imaging to relate the bubble size to the drag experienced by the flow. Previous findings have shown that a small percentage of air (4%) can lead to dramatic drag reduction (40%). In contrast to previous laboratory experiments, which mainly used fresh water, we will vary the salinity from fresh water to the average salinity of ocean water. We find that the drag reduction is increasingly more inhibited for increasing salt concentration; going from 40% for fresh water to just 15% for seawater. Salts present in the working fluid inhibit coalescence events, resulting in smaller bubbles in the flow and with that, decreasing the drag reduction. Above a critical salinity, increasing the salinity has no further effect on the bubble sizes in the flow and thus the drag experienced by the flow. Our new findings demonstrate the importance of NaCl on the bubbly drag reduction mechanism, and will further challenge the naval architects to implement promising air lubrication systems on marine vessels.This article is part of the theme issue ‘Taylor–Couette and related flows on the centennial of Taylor’s seminal <i>Philosophical Transactions</i> paper (Part 1)’.

本研究采用泰勒-库埃特（Taylor–Couette）流场几何构型，通过实验探究盐度对流动中气泡引发的减阻效应的影响。我们结合扭矩测量与光学高速成像技术，构建气泡尺寸与流动阻力之间的定量关联。已有研究证实，仅需体积占比4%的少量空气，即可实现高达40%的显著减阻效果。与此前多数采用淡水的实验室实验不同，本实验将盐度调控范围设置为从淡水至海水平均盐度。实验结果表明，盐浓度升高会对减阻效应产生愈发显著的抑制作用：减阻率从淡水条件下的40%降至海水条件下仅15%。工作流体中的盐分会抑制气泡聚并过程，使得流动内气泡尺寸更小，进而削弱减阻效果。当盐度超过临界阈值后，进一步提升盐度对流动内的气泡尺寸及流动阻力均无额外影响。本研究的新发现明确了氯化钠（NaCl）对气泡式减阻机制的关键作用，同时也将为船舶设计师在海事船舶上部署前景可观的空气润滑系统带来新的设计考量。本文隶属于‘纪念泰勒开创性《哲学汇刊》论文发表百年——泰勒-库埃特流及相关流动（第一部分）’专题刊。