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Abstract
Superhydrophobic surfaces have been shown reduce drag in laminar flows; however, in turbulent flows, the literature is divided with drag reductions between 0% and 70% being achieved. With frictional drag accounting for over half of the resistance of most ships, a method of decreasing drag would result in both significant fuel savings, and a reduction in carbon dioxide emissions. In order to ascertain whether these surfaces can provide a reduction in drag in turbulent flows, experimental and detailed computational fluid dynamics studies have been undertaken. In addition to determining whether turbulent flow drag reduction is achievable, this work investigated both the mechanics and conditions under which the drag reduction occurs, and quantified the interaction between the hydrophobic surface and the turbulent multiphase flow.
The experimental program aimed to determine if drag reduction in high Reynolds number flows were achievable. As part of the research, a test rig was designed and constructed that allowed measurement of skin friction drag whilst minimising the effects of pressure drag. A hydrophobic surface was compared to a smooth plate across a range of turbulent flow Reynolds numbers with no noticeable drag reductions shown. Further investigations into the reasons for the lack of drag reduction were then achieved using computational fluid dynamics.
The lattice Boltzmann method was used to accurately simulate the interactions between air and water at a scale where surface tension dominates. A code featuring methods which include fractional propagation, a novel technique of mesh refinement, a multiphase model and pseudo direct numerical simulation of turbulence has been devised and implemented. Validation across a range of benchmark tests was performed and the code proven to produce accurate results. An optimisation process was also undertaken to maximise efficiency.
This code was then used to simulate laminar, transitional and turbulent flows through a smooth walled channel, and over a series of roughened and hydrophobic surfaces.
The results of this research have confirmed that drag reductions in laminar and transitional flows are achievable; however, at Reynolds numbers greater than Re_tau = 390, minimal benefit was found because the air layer against the surface was removed. The drag reduction effect has been shown to be dependent on the location of the free-surface, and the way in which it insulates the ridges and posts on the hydrophobic surface from the water. The geometry of the surface has also been shown to have an effect on both the overall drag and the ability of the surface to maintain the insulating air layer.