Assessment of drag reduction at slippery, topographically structured surfaces

Drag reduction at topographically structured surfaces that contain a second immiscible fluid in their corrugations is evaluated. Based on a model for the effective slip length of a grooved surface, a threshold for the structured surface being superior to a flat surface with respect to drag reduction is derived for fluids of arbitrary viscosity filling the grooves and flowing over the surface. The specific magnitude of drag reduction is given exemplarily for pressure-driven pipe flow. Flow transverse to the grooves as well as flow longitudinal to open and closed grooves is considered. For typical surface geometry parameters, a flow rate enhancement by several tens of percent is predicted.