| An experimental investigation of the flow over three microgeometries was conducted in order to study its boundary layer control capabilities. Drag reduction and boundary layer control are two of the most researched areas in fluid mechanics. The necessity of reducing drag over vehicles is imperative to reduce the power needed to move a vehicle, or to save millions of gallons of fuel; this can also contribute to a reduction of the emissions of pollutant gases to the atmosphere. It has been estimated that a reduction in drag of 1% on an airplane can save the airlines around ;Time-resolved digital particle image velocimetry (TR-DPIV) measurements were taken in order to characterize the cavity vortices formed inside the geometries, as well as velocity profile measurements to identify the stability of the boundary layer over the geometries. The cavity vortices introduce a partial slip condition into the flow which affects the stability of the boundary layer. The results indicate that the shark skin can work as a boundary layer control mechanism by delaying or inhibiting separation over the shark's body, thereby reducing pressure drag. The ribs on the front side of the shark skin denticles promoted secondary vorticity that was measured under both laminar and turbulent boundary layer conditions. These secondary vortices appeared to stabilize the flow with respect to the presence of a transverse pressure gradient, while creating a stable, interlocking system of cavity vortices within the geometry. The other two geometries lead to very interesting results. Of the three models tested, the 2-D grooved surface resulted in the most stable boundary layer being formed above the cavities, and the sawtooth geometry resulted in the largest mixing for turbulent boundary layer conditions. |
