Air–Water Properties in Rectangular Free-Falling Jets

Abstract

This study analyzes the air–water flow properties in overflow nappe jets. Data were measured in several cross-sections of rectangular free-falling jets downstream of a sharp-crested weir, with a maximum fall distance of 2.0 m. The flow properties were obtained using a conductivity phase-detection probe. Furthermore, a back-flushing Pitot-Prandtl probe was used in order to obtain the velocity profiles. Five specific flows rates were analyzed, from 0.024 to 0.096 m3/s/m. The measurements of the air–water flow allowed us to characterize the increment of the air entrainment during the fall, affecting the flow characteristic distributions, reducing the non-aerated water inner core, and increasing the lateral spread, thereby leading to changes in the jet thickness. The results showed slight differences between the upper and lower nappe trajectories. The experimental data of the jet thickness related to a local void fraction of 50% seemed to be similar to the jet thickness due only to gravitational effects until the break-up length was reached. The amount of energy tended to remain constant until the falling distance was over 15 times greater than the total energy head over the weir crest, a distance at which the entrained air affected the entire cross-section, and the non-aerated core tended to disappear. The new experiments related with air–water properties in free-falling jets allow us to improve the current knowledge of turbulent rectangular jets.

Advances in Spillway Hydraulics: From Theory to Practice

“…Regarding the second topic, spillway transport structures, a diversity of subjects are analyzed, such as examining the physical effects of beveled face steps on various hydraulic design parameters, of relevance to contemporary design and construction practices for stepped chutes (Hunt et al [20]); computing the flow parameters of engineering interest for smooth and stepped spillway chutes using the SpillwayPro program (Wahl et al [17]); evaluating the performance of 3D SPH simulations for nonaerated flow over smooth and stepped spillways with converging walls (Nóbrega et al [18]); estimating bottom-pressure development on stepped chutes caused by an abrupt slope reduction (Ostad Mirza Tehrani et al [22]); CFD modeling of a spillway offset aerator, using the two-phase volume of fluid versus the complete two-phase Euler models included in the OpenFOAM ® toolbox (Mendes et al [26]); and investigating the performance of a deflector aerator at the upstream reach of stepped chutes to prevent cavitation damage, and, therefore, allow high specific discharges (Terrier et al [27]). For the third topic, spillway outlet structures, the contributions include reasonably large-scale physical model studies of the air-water flow properties in rectangular free-falling jets (Carrillo et al [28]), stilling basin performance below 30 • and 50 • inclined smooth and stepped chutes (Stojnic et al [29]), and scour process occurring downstream of a PK weir in the presence of downstream apron (Lantz et al [30]). These papers are novel contributions to the research on spillway hydraulics and constitute a stimulus for further developments on this fascinating subject.…”

“…Three contributions focus on outlet structures. They relate to free-falling jets from linear weirs [28], to stilling basin performance downstream of smooth and stepped spillway chutes [29], and to specific arrangements to prevent erosion at the toe of nonlinear weirs used in low-head projects [30], i.e., without spillway channel and with energy dissipation directly at the weir toe. As such, these contributions highlight the wide variety of problems associated with outlet structure design depending on project configuration and requirements.…”

“…Carrillo et al [28] improve the current knowledge of turbulent rectangular jets by analyzing experimental data gained on a 2.0 m high fall facility with a conductivity phasedetection probe and a back-flushing Pitot-Prandtl tube. In particular, they characterize the evolution of air content along the jet length and show that the whole jet cross-section starts to be affected by aeration for a falling distance higher than 15 times the total energy head over the weir crest.…”

Plunging Circular Jets: Experimental Characterization of Dynamic Pressures near the Stagnation Zone

“…Chanson et al [6] studied scale effects in vertical circular plunging jets, but only with small velocities of up to 4.4 m/s. The desintegration of jets in air due to instabilities and turbulent fluctuations, creating two-phases flows, was studied by several authors, e.g., Chanson [7], Toombes and Chanson [8], Pfister and Schwindt [9], Carrillo et al [10]. Guyot et al [11] studied air entrainment by large-scale plunging jets of up to 10 m fall and assessed that the difficulty predicting air entrainment is due to the huge variety of flow structures in free-falling jets.…”