A composite procedure for ship viscous flow with free surface
Author Windt J. and Raven H.C.
Title A composite procedure for ship viscous flow with free surface
Conference/Journal 3rd Numerical Towing Tank Symposium (NuTTS)
Month June
Year 2000
Pages 114-117

Abstract
One of the most active fields of ship hydrodynamics research today is the development of methods for computing the steady viscous flow with free surface around a ship hull. Compared to the classical separate consideration of wave making and viscous flow, such methods aim at making a step forward by incorporating free-surface effects on the viscous flow, and viscous effects on the wave pattern. Methods proposed generally solve the RANS equations subject to dynamic and kinematic free-surface boundary conditions (FSBC's).
While appreciable progress has been made in recent years, the problem is not really solved yet.Mostly a good prediction of the wave profile along the hull is obtained, but the wave pattern away from the hull usually is rather poor due to substantial numerical damping, unless dense grids with several millions of cells are used. Moreover, virtually all methods consider a time-dependent problem and integrate in time until a steady result is obtained; but the approach to steady state may be quite slow due to transient waves, reflection s at open boundaries, and small permissible time steps. The resulting large calculation times and reduced robustness complicate the effective use in practice and make it hard to achieve grid-independent solutions, in particular for lower Froude numbers. Consequently, these methods are only rarely used routinely in ship design. Most RANS computations in practical ship design therefore still use a double-body approximation.
However, a step forward is possible without any significant computation time penalty. As many validations have demonstrated, inviscid methods can predict most of the wave pattern very efficiently and accurately. An obvious procedure therefore is to first run a panel code to determine the wave pattern, and next to compute the viscous flow, imposing free-slip boundary conditions at the wave surface. In this non-interactive formulation, most free-surface effects on the viscous flow are taken into account, but not the viscous effects on the wave pattern, which are known to be localised near the stern.
Surprisingly, this possibility seems to have been largely disregarded so far, except for the similar work at NTUA [6] in which viscous flow was computed under a measured wave surface.

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