On the Influence of Superhydrophobic Coatings on Turbulent Boundary Layer Separation

This experimental study of turbulent boundary layer separation from a contoured backward facingramp demonstrates that a spray-on superhydrophobic coating modifies the location of turbulent boundary layer separation. A custom water tunnel was designed and constructed to study turbulent boundary layer separation from a backward facing ramp located downstream of a flat plate. Particle image velocimetry was used to measure the velocity field and identify the onset of turbulent boundary layer separation. The flow rate within the water tunnel was adjusted to study the separation of turbulent boundary layers at Reynolds numbers based on the plate length of 500,000 to 6,000,000.Experiments on a smooth baseline surface showed that the turbulent boundary layer remained fullyattached along the ramp at low flow rates. The boundary layer detached as the fluid velocity was increased, and the location of the onset of separation advanced upstream with increasing velocity. The application of a superhydrophobic coating caused the onset of separation to occur at a lower flow rate, as well as the location of separation to advance further upstream at a fixed flow rate. Analysis of the velocities in the turbulent boundary layer upstream of the ramp show that the separation location is modified due to a decrease in the Reynolds shear stress within the boundary layer. This decrease in turbulent mixing, brought about by the presence of a superhydrophobic coating, reduces the transport of momentum to the wall which leads to earlier onset of turbulent boundary layer separation. This identifies a concern that should be addressed as superhydrophobic coatings are investigated as a method for reducing ship resistance within the maritime industry