Comment on “Incipient air entrainment in a translating axisymmetric plunging laminar jet” [Phys. Fluids 14, 781 (2002)]

The writers provided an original contribution to the topic of air bubble entrainment at plunging jets. While there were significant contributions on laminar and turbulent plunging jets,1,2 the writers provided solid flow visualizations high- lighting the influence of a cross-flow. Their contribution is a timely reminder that most research on plunging jet flows has been primarily limited to vertical jets plunging into a quies- cent pool. The discusser argues that the paper's topic of ''plunging laminar jet'' might not be strictly correct. He also adds some comment on incipient conditions. A basic question is: was the plunging jet inflow laminar or turbulent? The writers investigated 42 configurations for which the plunging jet inflow was not completely docu- mented. Table I presents a comparison of inflow conditions for well-documented laminar and turbulent plunging jet flow studies. The jet impact Reynolds number and turbulence lev- els are summarized in columns 2 and 3 ~Table I!. The writ- ers' study was conducted with jet Reynolds numbers and turbulence levels comparable to well-documented turbulent plunging jet studies.3- 6 Their turbulence levels were further two orders of magnitude greater than those observed in a laminar plunging jet study.7 The discusser believes that the writers' study was conducted with turbulent rather than lami- nar plunging jets. For the free-jet, the nozzle edge is a flow singularity and a mixing layer develops next to the jet free- surface downstream ~Figs. 1 and 2!. Several high-speed pho- tographic studies of water jets discharging into air 2,8,9 high- lighted a smooth-jet flow patterns for: r air * Vn 2* Dn /s,200 to 2000.2,10 The writers' experiments were conducted for