VISCOSITY IN SEAKEEPING

Abstract

The paper addresses the issue of actuality in ship hydrodynamics: the estimation of ship’s linear and angular oscillations with respect to the state of equilibrium. The prediction of seakeeping properties raises a question about a relative importance of viscous and free-surface effects (Quérard et al. 2009), yet this question remains of more general importance in fluid mechanics, since it is related to the dynamic characteristics of objects/bodies immersed in a liquid. From a theoretical standpoint, the problem refers to flows with moving boundaries. It can also be considered in terms of fluid-structure interaction (FSI), however, not necessarily linked with the computation of the body deformation and stresses due to the flow. As the Author correctly notices, the computational solution to this problem in its full setup reveals to be extremely costly due to the 3D and unsteady nature of the fluid motion under turbulent flow conditions at nominally high Reynolds numbers (Re~109, as stated by the Author in Tab. 1) in presence of the free surface. For this reason, the full solution, or direct numerical simulation (DNS), of the governing Navier-Stokes (N-S) equations at these Re will remain unfeasible in the foreseeable future; see, e.g., Pozorski (2017) for an estimation of the DNS capability in simple wall-bounded turbulent flows. The situation gets even worse in ship hydrodynamics when a DNS of fluid flow would need to be coupled to the dynamics of the rigid body (of complex geometry, usually).