Impact of riblets on the drag of finite-sized bodies with non-zero pressure gradients

Riblets are an effective method of passive drag reduction with between 8-10% reductions observed in laboratory and less than 3% reported in industrial applications. The majority of implementation in both spaces has focused on the case of asymptotic boundary layers, well suited for larger vehicles such as ships and planes. Less exploration is available on the impact of riblets on the flow field around finite-sized, smaller bodies where the development of the boundary layers deviates from the zero pressure gradient conditions and the dependence on the streamwise direction becomes more pronounced. In this talk, we will consider the case of riblet-covered, finite-sized bodies in flow and discuss how the limited size of the body and riblets interact with each other and affect the development of the boundary layer and the pressure distribution around the sample. We use riblet covered foils in a water tunnel and employ particle image velocimetry experiments to capture the velocity distribution and subsequently the pressure field around the bodies and connect the measurements to the drag experienced by the sample and the differences with that of a smooth reference. We will use the experimental evidence to explore the impact of the pressure distribution on the performance of the riblets in altering the frictional and pressure components of the drag and how this can translate into practical implementation of riblets for vehicles with smaller or bulkier silhouettes.