Numerical simulation of air retaining surfaces - possibilities and challenges

Christoph Wilms, Judith Geils, Antonia B. Kesel, Albert J. Baars

Biomimetic-Innovations-Centre City University of Applied Sciences Bremen Neustadtswall 30, 28199 Bremen, Germany e-mail: [email protected], web page: http://www.bionik-bremen.de

ABSTRACT

In the last few years more and more research in the field of air retaining surfaces has been carried out. Due to the lower viscosity of air in comparison to water these surfaces can lead to a reduction in frictional resistance of up to 30% if moved in water [1]. Applied on ship hulls a reduction in energy and emissions follows. An advice of suitable surface properties deliver different plants and animals: The water moss Salvinia molesta and the backswimmer Notonecta glauca use a superhydrophobic surface of hairlike structures and in case of S. molesta with a hydrophilic tip, which stabilizes the water-air interface. Parallel to experiments different research groups have carried out numerical simulations to understand the influence of relevant parameters on air retainment and drag. First numerical investigations used a homogenized and predefined slip length in direct numerical simulations [2]. Currently, investigations apply a slip boundary condition at the estimated interface areas and a no slip condition at the surfaces of the structure exposed to water [3]. They suppose a stable Cassie-Baxter state with a flat interface, which indicates a infinite surface tension. Rastegari and Akhavan [4] chose a rigid curved surface with slip condition for the interface to investigate different protrusion angles. Seo et al. 2018 [1] used a direct coupling between the flow and the deformation of the interface. As they mentioned all these investigations do not allow to study the process of bubble draining. Hence, this contribution uses the volume of fluid method to simulate a two phase system, which allows to understand the process of bubble draining. In addition, the challenges as well as the advantages and disadvantages of the methods mentioned above are presented. These investigations are part of the EU project AIRCOAT (Air Induced friction Reducing ship Coating), which targets the large scale application on ship hulls.

REFERENCES

[1] Seo, J., Garcia-Mayoral, R. and Mani, A. (2018): Turbulent flows over superhydrophobic surfaces: flow-induced capillary waves, and robustness of airwater interfaces, J. Fluid Mech. 835, 4585. [2] Min, T and Kim, J. (2004): Effects of hydrophobic surface on skin-friction drag, Phys. Fluids 16, 7. [3] Park, H., Park, H. and Kim, J. (2013): A numerical study of the effects of superhydrophobic surface on skin-friction drag in turbulent channel flow, Phys. Fluids 25, 110815. [4] Rastegari, A. and Akhavan, R. (2018): The common mechanism of turbulent skin-friction drag reduction with superhydrophobic longitudinal microgrooves and riblets, J. Fluid Mech. 838, 68- 104. The AIRCOAT project has received funding from the European Unions Horizon 2020 research and innovation programme under grant agreement N◦ 764553