Riblets are longitudinal micro-grooved on the wall surface of turbulent boundary layers or channel flows,whose skin-friction drag is lower than that of a smooth surface.In other words,the skin-friction drag of "non-smooth" surface can be made lower than that of a smooth surface by using riblets,which surprises many scientists and engineers since this contradicts with what the Moody chart indicates.Here,the nondimensional size of drag-reducing riblets is of the thickness of the viscous sublayer,suggesting that riblets must be submerged within the viscous flow region to reduce the drag.