Energy dissipation through entrained air compression in plunging jets

The energy dissipation in plunge pools is often related to air entrainment in the plunging jet. Recent experimental work showed that the void fractions below the impinging jet are significantly smaller than outside the jet impact. This was attributed to the compression of the entrained air bubbles. In this paper, the thermodynamics of air bubble compression is introduced as a novel energy dissipation mechanism in civil engineering hydraulics. For the analysis of reported experimental data, isothermal conditions were applied as a lower bound approximation. It was found that the energy dissipation rates range from 2.7-9.7% for a jet velocity of 10 m s(-2) and void fractions of 0.11 and 0.31, to 15.9-36.0% for a jet velocity of 24.5 m s(-1) and void fractions of 0.35-0.55. The results may explain the difference between energy dissipation rates in plunge pools at model and full scale.