A thin sheet of water flowing over surfaces with alternating hydrophilic (water-attracting) and hydrophobic (water-repelling) bands introduces "scales" in the flow, leading to the formation of a pinecone-like structure.
Active Flow Control (AFC)
Sweeping jet actuators, which emit a continuous jet that swings from one side of the outlet nozzle to the other, are a promising type of active flow control devices for aircraft tails and rudders.
The size of aircraft vertical tails are usually based on the needs of emergency situations, such as engine failure at low airspeeds. As a result, they are oversized for normal operating conditions. Sweeping jet actuators can be employed to delay flow separation over the tails and increase side force generation, thus allowing aircraft to be built with smaller tails that still perform as necessary under emergency conditions.
Our work with hydrophobic (water repelling) and hydrophilic (water attracting) surfaces extends to liquid bridges. Liquid bridges occur when water rivulets flowing over horizontal or partially inclined hydrophobic plates are lifted off the surface. The rivulets arch through the air as gravity then pulls them back down to the surface.
Imaging Techniques for AFC
The 3-dimensional flow over an aircraft's vertical tail and rudder is obtained using defocusing digital particle image velocimetry (DDPIV).