Sinusoidal free surface sloshing at the centre of a square tank: (a) analytical (solid line) and GN-predicted (cross symbols) crest-induced free surface time histories for a = 0·005 m; (b) GN-predicted crest-induced free surface time history for a = 0·015 m; (c) GN-predicted trough-induced free surface time history for a = 0·015 m; and (d) GN-predicted even harmonics time history for a = 0·015 m