AbstractsPhysics

The effect of introducing an air film around a submerged water jet is examined in laboratory experiments. The objective is to reduce the friction forces between the water jet and the ambient fluid, making a jet potentially more effective at a larger distance. The test are conducted at a physical scale comparable to dredging practise, contrary to earlier experiments reported in the literature, where small scale jets are tested. During the tests the development of the jet was measured in axial and radial direction. The jet pressure and air discharge are varied, leading to a number of combinations of jet pressures and air discharges. The results showed that also at this scale the development of the water jet is influenced by the air film, although not as much as in the earlier experiments. Governing parameters have been established. Examples are the volume, mass and momentum ratios between the air flow and water flow. The ratios of the air flow and water flow are not in the same order if the comparison is made between the large and small scale water jets. Relatively less air was added in the test of a water jet at the scale used in dredging practise. Adding air around a water jet has a positive effect on the development of a water jet, if enough air is added. With increasing size of the nozzle diameter, more air has to be injected to create an effective air film. If enough air is added to create an effective air film, the effect is observed at very large axial distances. A description of this type of jets is given based on the description of Yahiro and Yoshida (1974) of water jets surrounded by an air film and the description of Rajaratnam (1976) of submerged water jets. The total amount of energy of the water jet and air film can also be used to create a water jet without an air film. A water jet without an air film with the same total amount of energy does not result in the same velocity increase as observed if an air film is created.