Experimental study of the transitional flow of a sphere located at the free surface

Abstract

The investigation of transitional flow past a sphere at the free surface is a challenging problem due to a complex interaction between the free surface and evolution of the boundary layer and resultant separation on the sphere’s surface. An increased knowledge about the fluid phenomena around bluff bodies would be of benefit to the design of offshore structures and ships’ bulbous bows. In this paper, experiments conducted in a towing tank environment are presented for a 225mm-diameter sphere towed at the free surface across the diameter based Reynolds number range 1.6 x105 < Re < 3.8 x105. Measurements of the total resistance using a bespoke dynamometer are combined with above-water photographs to provide further insight. An abrupt change in flow regime was observed as speed increased to reach Re ~ 3.5 x105. The bow breaking wave grows until transition occurs into a smooth flow over the top of the sphere. This change in flow condition resulted in a 60% decrease in total resistance. This paper considers why this drag crisis phenomenon takes place around Re ~ 2.6 - 3.6 x105, which is similar to single-phase flows. The inertial effect of the free surface deformation as opposed to the influence of the laminar to turbulent boundary layer transition is investigated.

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Identifiers

ORCID for Marion James: orcid.org/0000-0002-6964-4598

ORCID for Dominic Hudson: orcid.org/0000-0002-2012-6255

ORCID for Dominic Taunton: orcid.org/0000-0002-6865-089X

ORCID for Stephen Turnock: orcid.org/0000-0001-6288-0400

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