Effect of Surface Fluid Entrapment in a Laminar Flat Plate Boundary Layer*

For streamlined objects in laminar flow, the drag is dominated by friction drag. To counteract this, the boundary condition of the object can be changed to introduce an apparent slip at the surface. This work investigates a novel approach to providing slip via fluid entrapment in a perforated domain embedded in the surface. The fluid entrapment system uses a cavity underneath the perforated domain to allow entrapment of fluid of the same type, or entrapment of gas (in a liquid flow) via gas supply to the cavity. Either case provides a fluid at the surface perforations and relaxes the no-slip condition. As a model case, fluid entrapment was used for flow over a flat plate with the perforated domain at various downstream locations. Flow over the flat plate was provided by a miniature water tunnel. Velocity profiles were measured using Particle Image Velocimetry (PIV) for a Reynolds number (Re) range of 4,000-6,000. Boundary layer profiles with air entrapment were analyzed and compared with the results for water entrapment and for a solid (control) flat plate. The results show slip flow at the surface for the air entrapment and water entrapment cases, with the air entrapment case producing larger slip flow, in terms of slip velocity and slip length, than the water entrapment case.