Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session J29: Drag Reduction I: Superhydrophobic Surfaces |
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Chair: Harish Ganesh, University of Michigan Room: 255 A |
Sunday, November 24, 2024 5:50PM - 6:03PM |
J29.00001: Drag reduction characterization of spray-on superhydrophobic surfaces in a turbulent channel flow Harish Ganesh, Parth Devrajbhai Khokhani, Bozhong Zhuang, Fan-Wei Wang, Anish Tuteja, Steven Louis Ceccio Previous studies have shown that randomized spray-on superhydrophobic surfaces (SHS) achieve drag reduction (DR) at high Reynolds numbers (Re). DR of SHS depends on the Re, with DR reducing with increased Re due to loss of gas pockets. Despite these studies, the role of the roughness profile of surfaces and the underlying chemical composition of the SHS under consideration on the observed DR performance is yet to be fully understood. In this study, drag produced by several SHS made with Fluorinated Silica NanoParticles (FSNP) and Fluorodecyl Polyhedral oligomeric SilSesquioxane (FPOSS) are tested in a turbulent channel flow. These materials are sprayed at various pressures onto a stainless-steel substrate to achieve different roughness profiles. Drag produced by the surfaces is estimated based on pressure drop measurements in a turbulent channel flow for ReH ranging from 10000 to 40000. Measured drag is compared with a hydrodynamically smooth surface to quantify DR. |
Sunday, November 24, 2024 6:03PM - 6:16PM |
J29.00002: Evaluating aerodynamic viscous drag reduction performance of superhydrophobic coatings Jacob Huerta, Connor Wilkinson, Tyler Miller, Mostafa Ojaghloo, Jonathan W Naughton Applying different types of coatings to various surfaces, such as airfoils, has shown a significant improvement in drag reduction. Among these coatings, superhydrophobic (SHP) coatings, while primarily used for anti-icing, have shown evidence of drag reduction as well. However, with the development of different SHP compounds, unique surface characteristics, durability in external environments, and consideration of the large-scale applications such as wind turbines, there is more to be understood about SHP coatings. This study explores the effect of a specialized SHP coating on the surface characteristics and the viscous drag force on a smooth flat plate for a range of Reynolds numbers. A flat plate was coated with SHP coatings using a high-volume, low-pressure (HVLP) spray applicator and were cured using the recommended conditions for 24 to 48 hours. Data were acquired using an in-house developed elastomeric force balance (EFB) and Oil Film Interferometery (OFI). Drag reduction efficiency of the various coatings will be discussed. |
Sunday, November 24, 2024 6:16PM - 6:29PM |
J29.00003: A level set-based model for performance prediction of superhydrophobic surfaces Mehedi Hasan Bappy, Krishnan Mahesh The drag reduction (DR) capabilities of superhydrophobic surfaces (SHS) hinge on their reduced wettability, which is intrinsically linked to the morphology of the air-water interface. Accurate prediction of the topology and stability of this interface is vital. In this study, we employ a variational level set methodology (Alame et al. 2020) in a novel approach to characterize and predict the hydrodynamic performance of SHS. A parallel, highly scalable 3D solver for determining the equilibrium interface topology of realistic SHS is developed; the proposed model encapsulates the properties of the equilibrium interface, its correlation with DR, surface characteristics, and flow dynamics. We have established correlations between the skin friction coefficient for various SHS under different flow scenarios and the properties of their interfaces, as well as the substrate characteristics. This methodology offers a powerful tool for optimizing SHS design for enhanced drag reduction in practical applications. The results are validated against SHS with both canonical structured roughness and realistic roughness over a range of ReH. |
Sunday, November 24, 2024 6:29PM - 6:42PM |
J29.00004: Plastron stability of super-hydrophobic surface with transverse grooves in turbulent flows Shabnam Mohammadshahi, Jordan Breveleri, Daniel O’Coin, Foram S Fanasia, Hangjian Ling We experimentally studied the stability of the gas (or plastron) trapped on super-hydrophobic surface (SHS) consisting of transverse grooves in turbulent flows. We systemically varied the groove width (g), texture height (h), and texture wavelength (λ) in the range of 200 to 800 µm. The experiments were performed in a turbulent channel flow facility, where the mean flow speed varied from 0.5 to 6 m/s and the Reynolds number based on mean flow speed and channel height Rem varied from 2000 to 24000. The status of gas layer on SHS was imaged by a reflected-light microscopy. We found that as increasing Reynolds number, the SHS experienced a sudden wetting transition from Cassie-Baxter state to Wenzel state. A metastable state where the liquid partially filled the grooves was not observed. Moreover, we found that the wetting transition was delayed and occurred at a higher Reynolds number as increasing h, reducing g, and increasing λ. The critical Reynolds number Recr for wetting transition was captured by models based on the force-balance at the gas-liquid interface. Last, we showed that grooves with V-shape maintained a stable plastron in turbulent flows at a higher Reynolds number. |
Sunday, November 24, 2024 6:42PM - 6:55PM |
J29.00005: Drag reduction by plastron in featureless turbulent Taylor-Couette flow: torque and 2D PIV measurements Woobin Song, Haeyeon Lee, Garam Ku, Minjae Kim, Dong Rip Kim, Simon Song In underwater environments, superhydrophobic surfaces can form a plastron between the water and the surface, depending on the surface characteristics. This plastron induces slip at the wall, subsequently reducing friction drag. The drag reduction in laminar flow has been extensively demonstrated based on the geometric characteristics of superhydrophobic surfaces. However, in turbulent flow, it is challenging to experimentally study drag reduction due to the difficulty of maintaining the plastron and precisely measuring friction drag. We developed a Taylor-Couette flow device capable of producing featureless turbulence through exact counter-rotation, enabling precise torque measurement and near-wall velocity field analysis over a wide range of Reynolds numbers using water as the working fluid. The formation and maintenance of the plastron in the turbulent regime are achieved using superhydrophobic surfaces with a hierarchical porous structure and Teflon coating. The presentation will include the drag reduction performance by the plastron for shear Reynolds numbers up to 16,000, comparisons of mean and turbulent quantities with and without the plastron, and validation results of the Taylor-Couette flow measurements. |
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