When a water jet impinges a pool of water at rest, air bubbles may be entrained and carried away below the pool free surface: this process is called plunging jet entrainment. The study presents new experimental data obtained with a vertical supported jet. Distributions of air concentration and mean air-water velocity, and bubble chord length distributions measured in the developing shear layer are presented. The results indicate that the distributions of void fraction follow closely analytical solution of the diffusion equation. Further, the momentum shear layer and the air bubble diffusion layer do not coincide. Chord length data show a wide range of air bubble sizes and overall the experimental results suggest strong interactions between the entrained air bubbles and the momentum transfer mechanisms.

A t the im pact of a plunging liquid jet with a receiving pool, air bubble s may be entrained if the impact velocity exceeds a critical velocity. New experiments were perform ed in a two-dim ensional plunging jet. Th e¯ow conditions near the inception of air entrainm ent were investigated. Two mechanisms of air entrainment were visualized at low jet velocities: by elongated air cavity and by foam bubble s. The breakage of entrained air bubbles was studied also. The results highlight that the bubble breakage process depends critically upon the initial bubble size. Keywords: plungin g jet; air entrainm ent; inception; bubble breakage; air-water¯ow INTRO DUC TIONW hen a water jet impacts a plunge pool, som e air m ay be carried dow nw ards. Exam ples of mechanical penetration of one phase into another include self aeration in free jets, high speed open channel¯ow s, entrainm ent via surface vortices, drop impact, hydra ulic jum ps and breaking waves (e.g. Bin 1 , Chanson 2 ). Air entrainm ent by plunging jet is an industrially important process where air entrainm ent, during the pouring of one liquid into anothe r, is to be avoided, or entrainm ent is desired but control (e.g. of bubble sizes) is necessary.Past studies (e.g. Ervine and Elsawy 3 , Lara 4 ) show ed that air bubble entrainm ent takes place when the jet im pact velocity V exceeds a characteristic value V e which is primarily a function of¯uid prope rties, jet length and jet core turbulence. For V > V e , air entrainm ent is observed by an individual bubble entrainm ent process for V slightly larger than V e or by a ventilated cavity m echanism at larger jet impact velocities (Cum m ings and Chanson 5,6 ). After being entrained, air bubble s m ay break up if the com bination of¯uid shear stresses and pressures exceed the surface tension forces.Most studies of air entrainm ent inception and individual air bubble entrainm ent were perform ed with circular jets. Sene 7 (1988) studied sloping supported planar jets. In the present paper, the authors present an experimental study of plunging jet entrainm ent with a simple geom etry, speci® -cally a vertical supported plunging jet. The inception¯ow conditions and the individual bubble entrainment process were investigated using high-speed cam era and videocam era, and the results are presented herein. EXPERIMENTA L PROC EDUREThe experiment consists of a fresh water planar jet, 0.27 m wide and 0.012 m thick (Figure 1). The jet plunges into a 1.8 m deep, 0.3 m wide and 3.6 m long channel with glass walls (Figure 1(a)). The jet angle with the horizontal was set at 89°to ensure that the jet remained attached to the support. Although the jet angle affects the inception velocity 1 , a 1-degree deviation from the vertical has very little in¯uence. Discharges were measured with an ori® ce plate meter, and clear-water velocity and velocity¯uctuations were recorded with a 3.3 m m Pitot tube.A PanasonicY F15 CCD W V-F15HS E Cam -corder video cam era was used to visualize air bubble entrainment below the plunge pool surfa...

Air-water bubbly flows are encountered in many engineering applications. One type of air-water shear flows is the developing flow region of a plunging jet. The mechanisms of air entrainment by plunging liquid jets are discussed in the light of new experimental evidence. Then the air bubble diffusion is analyzed analytically in the near-flow field of both circular and two-dimensional plunging jets. The theoretical developments are compared with existing circular plunging jet data and new experiments performed with a two-dimensional vertical supported jet. The study highlights two mechanisms of air entrainment at the plunge point depending upon the jet impact velocity and results suggest that the dispersion of air bubbles within the shear layer is primarily an advective diffusion process.

Prenatal stress exposure increases vulnerability to virtually all forms of psychopathology. Based on this robust evidence base, we propose a “Mental Health, Earlier” paradigm shift for prenatal stress research, which moves from the documentation of stress‐related outcomes to their prevention, with a focus on infant neurodevelopmental indicators of vulnerability to subsequent mental health problems. Achieving this requires an expansive team science approach. As an exemplar, we introduce the Promoting Healthy Brain Project (PHBP), a randomized trial testing the impact of the Wellness‐4‐2 personalized prenatal stress‐reduction intervention on stress‐related alterations in infant neurodevelopmental trajectories in the first year of life. Wellness‐4‐2 utilizes bio‐integrated stress monitoring for just‐in‐time adaptive intervention. We highlight unique challenges and opportunities this novel team science approach presents in synergizing expertise across predictive analytics, bioengineering, health information technology, prevention science, maternal–fetal medicine, neonatology, pediatrics, and neurodevelopmental science. We discuss how innovations across many areas of study facilitate this personalized preventive approach, using developmentally sensitive brain and behavioral methods to investigate whether altering children's adverse gestational exposures, i.e., maternal stress in the womb, can improve their mental health outlooks. In so doing, we seek to propel developmental SEED research towards preventive applications with the potential to reduce the pernicious effect of prenatal stress on neurodevelopment, mental health, and wellbeing.

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