Laser Doppler Velocimetry Measurements of A Turbulent Boundary Layer Flow over Sprayed Superhydrophobic Surfaces

Abstract

Measurements of near-zero pressure gradient turbulent boundary layer (TBL) flow over several superhydrophobic surfaces (SHSs) are presented and compared to those for a hydraulically smooth baseline. The surfaces were developed at the University of Michigan as part of an ongoing research thrust to investigate the feasibility of SHSs for skin-friction drag reduction in turbulent flow. The SHSs were previously evaluated in fullydeveloped turbulent channel flow and have been shown to provide meaningful drag reduction. The TBL experiments were conducted at the U.S. Naval Academy in a water tunnel with a test section 2.0 m (L) × 0.2 m (W) × 0.1 m (H). The free-stream speed was set to 1.25 ms, nominally, which corresponded to a friction Reynolds number, Reτ, of 1,600. The TBL was tripped at the test section inlet with a 0.8 mm diameter wire. The upper and side walls provided optical access, while the lower wall was either the smooth baseline or a spray coated SHS. The velocity measurements were obtained with a two-component Laser Doppler Velocimeter (LDV) and custom-designed beam displacer operated in coincidence mode. The LDV probe volume diameter was 45 μm (approx. two wallunits). The measurements were recorded 1.5 m downstream of the trip. When the measured quantities were normalized using inner variables, the results indicated a significant reduction in the near wall viscous and total stresses. Increased stresses were also measured in the overlap layer when compared to the smooth wall. Nevertheless, consideration of the total stress and a log layer with a wake analysis shows drag reduction of -11 to 36% for the SHS analyzed. INTRODUCTION Nature has provided an exhaustive source of evolutionary functional materials to be mimicked for everyday applications (Jung & Bhushan, 2010). One notable case relevant to the marine environment is the lotus leaf which is known for its self-cleaning properties and resistance to wetting (Neinhuis & Barthlott, 1997).