The effects of salinity on bubbly drag reduction in turbulent Taylor–Couette flow

  • Blaauw, Luuk (University of Twente)
  • Lohse, Detlef (University of Twente)
  • Huisman, Sander (University of Twente)

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Bubbles in turbulent Taylor–Couette flow can greatly reduce the drag on the inner cylinder. An air volume fraction of only 4% can lead to a drag reduction of 40% van Gils et al. [2013]. This phenomena has very promising applications in the naval industry, where by injection of bubbles underneath the ship hull the drag, and with that the fuel consumption, can be reduced. The effects of water contamination on drag reduction has been given limited attention, although small amount of contaminants can drastically change the system’s behaviour Verschoof et al. [2016]. Given the abundance of ions (salts) in the ocean, we want investigate the effects of these salts on bubbly drag reduction. Using the Twente Turbulent Taylor–Couette setup van Gils et al. [2011] we investigate bubbly drag reduction in a highly turbulent flow (Rei ≈ 2 × 106). We investigate salinities from fresh water up to salt concentrations comparable to seawater. Using a variety of salts (NaCl, MgCl2, Na2SO4, NaCH3COO, and substitute ocean salt) we identify the effects of different salts on the drag reduction. High speed imaging is used to capture the bubble sizes and calculate the bubble Weber number, characterizing the bubble deformability. We connect the bubble deformability to the observed drag reduction. Salts present in the working fluid inhibit the coalescence of bubbles, leading to a smaller bubble sizes in the flow. These smaller bubbles are found to be less effective to reduce the drag, and we see diminishing drag reduction for increasing salinity.