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

James W. Gose
Department of Naval Architecture and Marine Engineering University of Michigan 2600 Draper Dr., Ann Arbor, MI 48109, USA

Kevin B. Golovin
Department of Materials Science and Engineering University of Michigan 2800 Plymouth Rd., Ann Arbor, MI 48109

Julio M. Barros
Department of Mechanical Engineering, United States Naval Academy Annapolis, MD 21402 USA

Michael P. Schultz
Department of Naval Architecture and Ocean Engineering United States Naval Academy Annapolis, Maryland 21402 USA

Anish Tuteja
Department of Materials Science and Engineering University of Michigan 2800 Plymouth Rd., Ann Arbor, MI 48103, USA

Marc Perlin
Department of Naval Architecture and Marine Engineering University of Michigan 2600 Draper Dr., Ann Arbor, MI 48109, USA

Steven L. Ceccio
Department of Mechanical Engineering University of Michigan Ann Arbor, Michigan 48109-2140 U.S.A.

Sinopsis

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 fully-developed 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-1, 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.