Large Drag Reduction over Superhydrophobic Riblets

摘要

Riblets and superhydrophobic surfaces are two demonstrated passive dragreduction techniques. We describe a method to fabricate surfaces that combineboth of these techniques in order to increase drag reduction properties.Samples have been tested with a cone-and-plate rheometer system, and havedemonstrated significant drag reduction even in the transitional-turbulentregime. Direct Numerical Simulations have been performed in order to estimatethe equivalent slip length at higher rotational speed. The sample with100~$\mu$m deep grooves has been performing very well, showing drag reductionvarying from 15 to 20 $\%$ over the whole range of flow conditions tested, andits slip length was estimated to be over 100 $\mu$m.