Method for analyzing friction reduction effects of bubbles in a ship

摘要

A method is provided for analyzing the effects of bubbles on skin-friction reduction in a ship cruising in a bubbly flow field. A shear stress decrement τ_t, produced by a bubble in a flow field is obtained in a high frequency band region, in which a product of a bubble time-constant T and a turbulence frequency ω_L in a flow field is sufficiently greater than 1, based on a resistive force ΔR_v acting on a bubble derived from the bubble movements in a fluid flow direction along a ship surface and in a direction at right angles to a ship wall surface. Assuming that the shear stress decrement is generated by a decrease in a gas mixing length, an expression for the operative wall constant κ₁ for bubbly flow including a parametric bubble diameter d_b is derived. Based on an expression relating the operative wall constant κ₁ and a local friction factor C_f in bubbly flow, and an expression relating the normal wall constant κ in non-bubbly flow and a local friction factor C_f0 in non-bubbly flow, an analytical solution is obtained for a skin-friction ratio C_f/C_f0.