The initial development of a jet caused by fluid, body and free surface interaction. Part 5. Parasitic capillary waves on an initially horizontal surface

In Part 3 of this series of papers (Needham et al., Q. J. Mech. Appl. Maths, vol. 61, 2008, pp. 581-614), we studied the free surface flow generated in a horizontal layer of inviscid fluid when a flat, rigid plate, inclined at an external angle α to the horizontal, is driven into the fluid with a constant, horizontal acceleration. We found that the most interesting behaviour occurs when α >π/2 (the plate leaning into the fluid). Whenπ/2<α<αc with αc ≈102:6, we were able to find the small-time asymptotic solution structure, and solve the leading-order problem numerically. When α=αc, we found numerical evidence that a 120 corner exists on the free surface, at leading order as t→0. For α>αc, we could find no numerical solution of the leading-order problem as t→0, and hypothesised that the solution does not exist for any t > 0 for these values of α. At the present time, there is no rigorous proof of this hypothesis. In this paper, we demonstrate that the likely non-existence of a solution for any t > 0 when α αc can be reconciled with the physics of the system by including the effect of surface tension in the model. Specifically, we find that for α αc, the solution exists for 0≤t,<t_c, with t_c Bo^-1/^(3y-1)τc and tc O(1) as Bo^-10, where Bo is the Bond number (Bo^-1 is the square of the ratio of the capillary length to the fluid depth) and γ1=.1/(1-π/4α). The solution does not exist for t>tc due to a topological transition driven by a nonlinear capillary wave, i.e. the free surface pinches off (self-intersects) when t D tc. We are also able to compare this asymptotic solution with experimental results which show that, in an experimental case where the contact angle remains approximately constant (the modelling assumption that we make in this paper), the asymptotic solution is in good agreement. In general, the inclusion of surface tension leads to the generation of capillary waves ahead of the wavecrest, which decay as t increases for t ≥t_c but dominate the flow and lead to pinch-off for t=tc. These capillary waves are an unsteady analogue of the parasitic capillary waves that can be generated ahead of steadily propagating, periodic capillary-gravity waves, e.g. Lin & Rockwell (J. Fluid Mech., vol. 302, 1995, pp. 29-44).