Drag reduction in turbulent channel flow by gas perfusion through porous non-hydrophobic and hydrophobic interfaces: Part I

Abstract

Skin-friction drag in liquid flows can be reduced by several methods including bubble injection into the near-wall region, air layer creation, air cavities, and applied super-hydrophobic coatings. The first three of these methods each has major drawbacks. In the fourth method, the depletion of the trapped air pockets on the super-hydrophobic surfaces (SHSs) in turbulent flows may cause a significant drag increase. To improve FDR in turbulent flows, a novel method is investigated that perfuses air through a porous medium with and without a hydrophobic treatment. A set of experiments has been conducted in a recirculating water tunnel at downstream-distance-based Reynolds numbers to 8.2million. The test model was a flat porous plate and the total wall shear stress was measured by a load cell apparatus. Air was perfused through such interfaces at mass flow rates to 50 L/min. It is noted that downstream persistence is not an issue with this technique as long as perfusion occurs near uniformly along the surface. The results showed that the method with maximum airflow exhibited a total drag reduction of around 10–25% with an untreated surface and about 20–30% with a treated surface.