A CFD Study On Turbulence Drag Reduction With Superhydrophobic Riblets

There is a common phenomena of energy consumption caused by fluid drag in industry,and the efficiency of energy can be improved with the help of drag reduction.Inspired by the fast-swim sharks in the sea,a lots of experiments about drag reduction with riblets have been conducted,showing that riblets can reduce fluid drag considerably compared with plate.In this paper,a review on drag reduction with riblets is discussed,and the key points and flaws in these studies are also discussed.It should be noticed that the studies of drag reduction with riblets mainly focus on the same mechanisms,while ignoring the different mechanisms between various riblets.So a numerical study is carried out to study the drag reduction,optimize the parameters and explore the difference in mechanisms of drag reduction with riblets.And inspired by the lotus effects,superhydrophobicity is introduced and combined with riblets to further the turbulence drag reduction,and a numerical method is applied to study drag reduction and mechanisms of superhydrophobic riblets.Based on the previous studies,blade and sawtooth riblets are selected.Reynolds Averaged Navier-Stokes Equations(RANS)is applied to study the drag reduction and optimal design with various parameters(dimensionless spacing S~+and dimensionless height H~+),which can be meaningful for exploring mechanisms of drag reduction.The numerical results show that blade riblets gain more drag reduction compared with sawtooth riblets under the same parameters,and drag reduction is enhanced with the increase of Reynolds number.A maximum drag reduction of 9.5%and 7.3%is obtained with blade and sawtooth riblets partly for Re=2900,while 13.7%and 10.1%at higher Re 15000.Mechanisms are explored based on the optimal riblets using Large Eddy Simulation(LES),and velocity filed,Reynolds stress and vortex distribution are considered.Results show that there are different mechanisms between blade and sawtooth riblets:velocity boundary layer is controlled by sawtooth rather than blade,which reduces the fluid drag.While blade riblets focus on controlling the turbulent fluctuation to reduce drag.It should be also noticed that vortices are lifted off the wall and reduce the drag reduction.Coherent structures,especially vortex structures can be controlled by riblets,and energy transmission between vortex structures is weaken,hindering the turbulence process.Finally,superhydrophobic riblets is introduced to further the turbulence drag reduction,and superhydrophobic riblets is studied based on the optimal riblets.Slip wall model is used to study the superhydrophobicity because of the difficulties of forming the micro-nano structures on riblets.Then riblets and slip wall are combined to study superhydrophobic riblets.It shows that drag reduction is enhanced with descent of f(ratio of no-slip boundary),and maximum drag reduction of 63%and 58%are obtained with use of superhydrophobic sawtooth and blade riblets.LES is used to detect the mechanisms,which shows the impaired wall stress,lower velocity fluctuation,weaken vorticity and ruptured vortex structures all contribute to the further drag reduction with the use of superhydrophobic riblets.