High-speed Jet Formation after Solid Object Impact

TL;DR

This study investigates the rapid jet formation caused by a solid object impacting water, revealing the flow dynamics and providing a quantitative model that aligns with experimental and numerical observations.

Contribution

It uncovers the flow mechanism feeding the jets and extends an analytical model of cavity collapse to accurately predict jet formation.

Findings

Jetting is driven by local flow around the jet base, not the stagnation point flow.

The analytical model matches experimental and numerical data well.

High-speed imaging and simulations confirm the jet formation mechanism.

Abstract

A circular disc impacting on a water surface creates a remarkably vigorous jet. Upon impact an axisymmetric air cavity forms and eventually pinches off in a single point halfway down the cavity. Immediately after closure two fast sharp-pointed jets are observed shooting up- and downwards from the closure location, which by then has turned into a stagnation point surrounded by a locally hyperbolic flow pattern. This flow, however, is {\it not} the mechanism feeding the two jets. Using high-speed imaging and numerical simulations we show that jetting is fed by the local flow around the base of the jet, which is forced by the colliding cavity walls. Based on this insight, we then show how the analytical description of a collapsing void (using a line of sinks along the axis of symmetry) can be continued beyond the time of pinch-off to obtain a quantitative model for jet formation which is in good agreement with the numerical and experimental data.