Free-Surface Flow Computations using FreSCo+
Similar to other methods, FreSCo uses a volume-of-fluid (VOF) approach to simulate two-phase flows. The challenge refers to the simulation immiscible fluids, e.g. air and water, which feature a discontinous transition between the two phases. Accordingly, the maintenance of a sharp interface is often difficult to realise.
Although dedicated, downwind-biased interpolation schemes are used in VOF-based two-phase flow simulations, the predicted free surface often tends to become blurred. Particularly when the free surface is oriented perpendicular to the face normals, the predicitve accuracy might be disappointing on coarse mehses. The effect is more or less pronounced, depending on the choice of the interpolation scheme, the orientation of the grid and the application.
Next to a number of downwind-biased interpolation schemes (HRIC, CICSAM, SHIP etc.) FreSCo offers an explicit interface sharpening approach (eis) which can be coupled to any interpolation practice. The latter works as a fully-conservative supplement at negligible computational expenses and is driven by flow physics rather than maths. The approach facillitates interfaces of the utmost sharpness (i.e. transition within one cell) but still resolves liquid splashes in line with the grid resolution. The above given figure comparing experiments and computation for a blunt bare-hull flow at Fn=0.7 and Re=3.4 Mio. illustrates the attainable predictive accuracy for steady wave-field simulations.
Examplary Application - Long Term Behaviour of a Sloshing Tank
An important maritime application pertains to sloshing flows, where the slope of the free-surface significantly varies over space and time on a fixed mesh. Therefore, an sharpening approach is an essential add-on feature. The impact of the sharpening approach is illustrated by the below given sloshing example which refers to an experiment performed at NMRI. The geometry is a simple rectangular at 20% filling tank featuring a width of b = 1.2m, height h = b/2. The translational excitation of the tank refers to a period length T = 1.94s and an amplitude of a=b/20.