Particle image velocimetry characterization of turbulent channel flow with rib patterned superhydrophobic walls

This paper reports particle image velocimetry (PIV) measurements characterizing turbulent flow in a channel with superhydrophobic surfaces, structured and wetting surfaces, and smooth bottom surfaces. The superhydrophobic and structured surfaces are fabricated with alternating ribs and cavities. Both longitudinal and transverse rib/cavity orientations were considered and the surfaces were made superhydrophobic by application of a Teflon coating. The widths of the ribs and cavities were 8 and 32μm, respectively, and the depth of the cavities was 15μm. PIV measurements were acquired for all surfaces considered over the Reynolds numbers range from 4800 to 10 000. Results from the smooth bottom wall measurements were used as a basis for comparison. The hydraulic diameter of the channel was nominally 8.2mm with an aspect ratio of 8.9. The PIV data captured aggregate velocities over multiple rib/cavity modules, such that a spanwise-averaged (over the width of the laser beam) velocity profile was obtained at the channel centerline. The time-averaged velocity profiles reveal no discernible time-mean slip velocity at the superhydrophobic wall. However, the different surfaces are shown to exhibit a systematic influence on the turbulence intensities, total and turbulent shear stress distributions, turbulence production in the channel, and local friction factors. Superhydrophobic surfaces with the ribs and cavities aligned with the flow are shown to yield an 11% decrease in the friction factor while the same surfaces aligned in the transverse direction are shown to cause a modest increase in the friction factor.