Abstract:
It is well known that proper surface patterning on a flat plate can decrease drag. Whether it be the dimples on a golf ball or riblets on a flat plate, there are many proven ways to alter a smooth surface to decrease drag. Continuing the previous research related to biomimetic models of shark skin, this research focuses on finding similar, but simpler, geometries that could lead to a decrease in drag. This experiment studies an array of embedded hexagonal cavities on a flat plate in a water tunnel facility. The flow over the modified flat plate generates vortices in the cavities which cause a partial slip condition that can alter boundary layer flow. Fluorescent dye in the cavities was illuminated with a UV lamp to capture images of the flow inside the cavities. Digital particle image velocimetry, which uses a laser to illuminate particles in the flow and captures images with a high speed camera, was used to measure the velocity flow field over the cavities. The flow fields were analyzed for the purpose of studying velocity profiles above the cavities, measuring partial slip velocities, calculating momentum thicknesses and shape factors, as well as determining the difference in boundary layer characteristics of the hexagonal cavities at two different orientations. Finally, the Reynolds stresses over the hexagonal cavities were compared to those over the flat plate under turbulent conditions to attempt to discern the effect of orientation on turbulence augmentation. The results show that the hexagonal cavities cause the boundary layer to transition to turbulence sooner than the flow over a flat plate. Also, in a turbulent flow, the cavities bring higher momentum fluid closer to the wall compared to the flat plate case, which is a proven requirement for the prevention of boundary layer separation. The results also suggested a decrease in the cavity circulation at the downstream portion of the model; confirming this trend should be a focus of future work. If this result is indeed confirmed, only applying this pattern to local regions of incipient separation may prove to be most effective.
