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

To improve the downstream persistence of bubble drag reduction, while maintaining the air plastrons on hydrophobic surfaces in turbulent flows, a method of gas perfusion through porous surfaces is investigated. Friction drag reduction (FDR) by such a method is measured on a flat-plate test model in fully-developed flows and turbulent-boundary-layer flows. Air is perfused through two porous media with and without hydrophobic treatment. The morphology results of bubbly flow show that gas perfusion occurs near uniformly along the entire surface and a portion of the bubbles remain near the wall at the furthest downstream location. In the fullydeveloped flow, a maximum FDR of 5-10% is obtained in the low injection region on all four surfaces (Refractron Ceramic and Maryland Porous - treated and untreated), and drag increases significantly for the high injection region. For Maryland Porous material, the highest %FDR is measured in the turbulent-boundarylayer flow with maximum airflow. A total drag reduction of typically 10-25% is achieved on the untreated surface and about 20-30% on the hydrophobic surface. This synergistic effect between the surface hydrophobicity and gas perfusion is promoted under higher injection rates and lower flow speeds. Additionally, data collapse of the %FDR is realized using the nominal bubble-layer thickness. (See Part I for additional results using this method.)