Leidenfrost spheres, projectiles, and speed model boats: assessing the superhydrophobic surfaces drag reduction

Superhydrophobic surfaces are anticipated to lower the drag on bluff bodies moving through water by creating a thin air layer around the object, effectively providing a free-slip boundary condition. A proven method to assess the maximum possible drag reduction by such air layers involves the use of Leidenfrost vapor layers on heated metal bodies. Previously, this method was applied to calculate drag reduction on a free-falling heated sphere, primarily affected by form drag. We now apply this method to assess the impact of thin gas layers on the hydrodynamic drag of free-falling streamlined projectiles and towed model boats, where skin friction drag is predominant, and form drag is less significant [1]. Our comparisons of streamlined bodies, with and without these air layers, show only minor drag reductions at the sub-critical Reynolds numbers tested. This observation also applies to towed model boats. Both superhydrophobic surfaces and Leidenfrost vapor films yield similar results, indicating that skin friction drag is less affected by thin gas layers compared to form drag within the range of sub-critical Reynolds numbers studied [2]. These findings have considerable implications for the practical application of gas layers on superhydrophobic surfaces for energy conservation.