Abstract
Water patch topology and momentum loss resulting from a jet impacting on the underside of a flat plate with varied hydrophobicity were studied. The jet's Reynolds and Froude numbers ranged from 3700 to 31 000, and from 1 to 23, respectively. Hence effects of gravity were expected to be non-negligible, and data suggest that this is the case for hydrophobic surfaces. The interplay of gravity, surface tension, inertia and viscosity resulted in two distinct behaviours. On hydrophilic surfaces, water spread uniformly. Friction reduced momentum, and led to accumulation at the edges of the patch until gravity overcame surface tension and produced droplets. On hydrophobic surfaces, two rims formed, enclosing a thin laminar film. The patch shape was mostly determined by the balance of kinetic and surface energy. Dewetting occurred in most cases when the two rims merged, but for a narrow parameter, range water detached soon after impact and formed a type of skewed water bell. The transition in detachment topology was predicted reasonably by a simple model considering whether an attached or detached rim minimizes energy. Due to promotion of dewetting by gravity, the water patch area decreased compared to that reported in previous studies, which considered jet impingement on vertical surfaces and tops of horizontal surfaces. Owing to the application that motivated this study, the streamwise force on the plate was also measured. On hydrophobic surfaces the reduction in force correlated with the reduction in water patch area. Water patch area and momentum loss were both found to scale best with the contact-angle-modified Weber number.
Highlights
Water impacting on the underside of surfaces has relevance to a range of applications, from surface cleaning and cooling, to air layer drag reduction
We note that the parameter ranges studied were chosen to match, to the extent possible, those expected to be relevant for air layer drag reduction over superhydrophobic surfaces (Callahan-Dudley et al 2020; Peifer et al 2020)
The flow topology and horizontal force imparted by a water jet impinging on the underside of a flat plate were studied over a range of parameters
Summary
Water impacting on the underside of surfaces has relevance to a range of applications, from surface cleaning and cooling, to air layer drag reduction In an attempt to simplify the full problem and to gain better understanding of the effects of the key parameters, it was decided to study isolated wetting events in a reproducible and controlled manner In this simplified case, the slug of water that contacts the surface through the air layer is replaced by an upward inclined jet, and the ship’s hull is represented by a flat plate with varied hydrophobic and roughness properties. Kibar et al (2010) showed how surface properties can affect the topology resulting from an oblique jet impinging on a vertical surface This was studied further numerically by Kibar (2015), who presented an energy argument on the water patch topology that builds on the experimental work of Kaps et al (2014).
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