Salinity reduces bubbly drag reduction

We investigate bubbly drag reduction in the context of air lubrication for maritime applications. To do so, we utilize our Twente Turbulent Taylor–Couette facility to study the effect of naturally-present salts in the ocean on the efficacy of bubbly drag reduction for Re = O(10^6). Whereas the drag reduction is up to 40% for 4% of air in fresh water, we find that the most common salts in the ocean dramatically mitigate the drag reduction. Using high-speed imaging, we find that the bubble size is reduced because bubble coalescence is inhibited due to the presence of the salt. The smaller bubbles have different Weber and Stokes numbers, becoming less deformable and more mobile, respectively, i.e., more tracer-like, reducing their effect on the flow.