Pressure transients and energy dissipation in liquid-liquid impacts

Abstract

In or near some engineering structures, such as sea walls, breakwater caissons or stilling basins, a moving water mass can strike another water mass at rest close to the structure. Very little is known about the pressures generated by such water-water impacts, although it has recently been realized that such impacts may affect the stability or integrity of these structures. A series of model tests was conducted at Queen's University Belfast in order to establish whether or not fluid-fluid impact can generate high transient pressure pulses, and to investigate the characteristics of such pressures. It was found that fluid-fluid impacts can generate high impulsive pressures, and that these pressures propagate away from the impact zone. Larger water masses appeared to lose their compact shape while falling, and generated smaller (but longer lasting) impact pressures than the smaller masses. The experiments also showed that the water, even when at rest, retains a small amount of air in the form of microbubbles which reduces the speed of propagation of the compression wave dramatically. The energy contained in the pressure pulses was found to be small when compared with the total energy contained in the impacting masses.

Keywords:

• Fluid-fluid impact
• fluid-solid impact
• pressure propagation