Air entrainment by inclined circular plunging water jets with various impingement heights

This paper presents an experimental study on air entrainment by inclined circular plunging water jets issued from long nozzles with various jet impingement heights up to 6.1 m. Particular attention was paid to the jets having a large impingement height, and which disintegrated into small droplets before reaching the water surface. The dominant sizes and velocities of the droplets generated by jet breakup were 1–4 mm and 4–9 m/s. The results show that for plunging water jets having the same hydraulic head, the air entrainment rate increased as the jet impingement height at first grew, then dropped as the jet impingement height kept increasing. The early increase in the air entrainment rate was due to the growth of jet surface roughness, while the later decrease was caused by the water jet disintegration. The momentum of the plunging jet decreased dramatically due to the reduced mass and the significant energy dissipation through the atmosphere caused by jet disintegration. This led to a much smaller penetration depth and air entrainment rate compared to with a continuous jet.