title: Experimental investigation and theoretical analysis of passive drag reduction over surfaces of various degrees of wettability
Creators: name: Alsharief, Ahmed Faraj Alarbi
Publisher: name: Memorial University of Newfoundland
publisherIdentifier: https://ror.org/04haebc03
publisherIdentifierScheme: ROR
Year: 2024

description: In recent years, researchers have focused on studying skin drag reduction through surface modification, with nanocoating technology playing a significant role. Various techniques, such as chemical etching, solution immersion, laser electrodeposition, and coating, have been employed to modify surfaces. Critical parameters in the manufacturing process include simplicity, speed, cost-effectiveness, and versatility. Commercially available superhydrophobic/oleophobic coatings, introduced in April 2016, operate by reducing surface energy to minimize the contact area between liquid and solid surfaces. The impact of superhydrophobic surfaces on flow is characterized by slip length and wetting degree parameters. However, many of these parameters related to drag reduction remain poorly understood, and relevant data is scarce in the literature. An experimental and theoretical study of Taylor-Couette (TC) flows and open channel flows were performed to investigate the passive viscous drag reduction using various randomly fabricated SHSs. Three different commercial SH coatings were used to fabricate the tested surface. In the TC flow study, the experiments were carried out in a small-scale facility using the MCR-301 Rheometer, along with concentric disposal cups (CDCs) as TC cells purchased from Anton Paar Instruments. The tested liquids were water, 5 cSt silicone oil and 10 cSt silicone oil. An open channel was modified to use a replaceable test surface with constant water depth in the second phase. The test surface will be painted with three commercial superhydrophobic coatings. A laser Doppler velocimetry (LDV) system was used to measure the velocity over the modified test surface at seven locations to investigate the shear stress when the test surface is in both coated and uncoated states. In the first part of the Taylor-Couette (TC) study, slippage was demonstrated over three fabricated SHSs in laminar and low turbulent flow. The investigation explored how slip length increased with rising Reynolds (Re) numbers over the tested SHSs, employing a viscous model to study the generated plastron thickness. The mean skin friction coefficient (Cf) was fitted to a modified semiempirical logarithmic law in the Prandtl–von Kármán coordinate. An effective slip length was estimated within the 35-41 μm range, resulting in a drag reduction (DR) of RMS value between 7-11% for the surfaces. This highlights the proportional relationship between b⁺, δ⁺, and the Re number. Statistical analysis, including regression modelling, was applied to experimental data, yielding an R² of 0.87 and strong agreement with the experimental results. The regression model indicated that b+ and Re numbers exerted a greater influence on δ⁺ than the wetting degree, emphasizing minimal differences in the wetting degree among the three tested surfaces. The second part of the Taylor-Couette (TC) cell study investigates the impact of surface wettability on drag reduction using three hydrophobic coatings—FlouroPel Coating (FPC-800M), Superhydrophobic Binary Coating (SHBC), and Ultra-Ever Dry (UED)—applied to curved aluminum surfaces. The study employs three liquids with different viscosities to characterize wettability and flow features. Static and dynamic contact angles on the surfaces were measured, and a rheometer-equipped Taylor-Couette flow cell was used to evaluate drag reduction. Static contact angle measurements revealed superhydrophobic behaviour for water (maximum static contact angle (SCA) of 158°) and oleophilic behaviour for the 10 cSt silicone oil (SCA of 13°). Water rheometer measurements demonstrated a maximum drag reduction of 18% for UED-coated surfaces. Interestingly, oleophilic surfaces exhibited a maximum drag reduction of 6% and 7% in the silicone oils with low static contact angles. The observed drag reduction is attributed to an increase in plastron thickness, influenced by an elevation in Reynolds number and dynamic pressure, coupled with a decrease in static pressure normal to the superhydrophobic wall. The open channel study explores the effectiveness and sustainability of commercial superhydrophobic coatings in reducing skin friction drag. Three distinct SHSs applied through a spray coating technique on an acrylic flat plate are investigated for their drag reduction (DR) properties. The SCA of water on these surfaces measure 145°, 147°, and 155°. Turbulent flow measurements are conducted using a two-dimensional Laser Doppler Velocimetry (LDV) system in an open channel flow facility, with experiments performed at an approaching Reynolds number of 34200. A unique aspect of this study is the characterization of drag reduction along the entire span of the fabricated surface in the streamwise flow direction. Velocity measurements are taken in a spanwise direction at each selected plane, and a theoretical prediction model for slip velocity and slip length is evaluated. The results show that the slip length equals the coating thickness as the plastron depletes. The SHSs enhance turbulence intensity and streamwise normal shear stress, particularly near the wall. As one moves away from the wall, the impact of turbulence diminishes, indicating the existence of an interface region near the wall due to the SHSs. Overall, the study demonstrates average drag reductions of 11%, 7%, and 18% for the tested SHSs. Importantly, it provides compelling evidence for the consistent reduction of viscous drag across the entire span of the plate, from the leading edge to the trailing edge.