Propagation of non-linear transient pressures through fresh and sea water

The propagation of pressures generated by wave or jet impact into water filled cracks and fissures can create internal bursting pressures and resulting damages in structures such as blockwork breakwaters or coastal rock cliffs. Experimental investigations of pressure pulse propagation showed that the pressure pulses travelled as elastic waves in a water-air mixture inside of the cracks. Most previous experimental investigations were conducted at small scale, with near linear air compressibility. In reality, pressures will be well above atmospheric pressure so that non-linear effects can be expected. A series of drop tests was conducted to investigate the characteristics of pressure pulse propagation in the non-linear range of air compressibility in fresh and sea water. It was found that with pressure magnitudes exceeding approximately 100 kPa, the pulses become distinctly non-linear, with steep and short peaks and long, shallow troughs. The speed of propagation in fresh water ranged from 300 to 400 m/s, indicating air contents of 0.11 to 0.07% air. In sea water, the speed of propagation reached 290 m/s, indicating a slightly larger resident air content of 0.12%. Cavitation occurred and was accompanied in fresh water by short pressure bursts, which could not be observed in sea water.