The effects of porous material on microbubble skin friction reduction

Previous results have shown that the injection of a high concentration of microbubbles into a turbulent boundary layer can produce sizeable skin friction reductions. The present paper investigates the role of the porous material in this phenomenon. A variety of different porous surfaces have been tested: a series of sintered metal plates with widely different pore sizes; a specially constructed porous material, composed of plastic strips; and two different porous section lengths. The effects of gravity were determined by changing the orientation of the porous surface with respect to the boundary layer. A notable conclusion is that the skin friction reduction is not critically related to the characteristics of the porous material.