Air–Water Properties in Rectangular Free-Falling Jets

José M Carrillo,Juan T García,Patricio R Ortega,Luis G Castillo

doi:10.3390/w13111593

José M Carrillo, Juan T García + Show 2 more

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https://doi.org/10.3390/w13111593

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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.

Highlights

During the latter part of the 20th century and into the 21st century, updated hydrological time-series data and new dam safety regulations have required the reevaluation of spillways’ capacity and hydraulic structures in large dams
Air entrainment may affect the behavior of the rectangular jet and its free surface during the fall
The local void fractions measurements were obtained with the conductivity phasedetection probe as the sum of the time the probe tip sensor was in the air ti over the total registered time t (C = Σti /t)

Summary

Introduction

During the latter part of the 20th century and into the 21st century, updated hydrological time-series data and new dam safety regulations have required the reevaluation of spillways’ capacity and hydraulic structures in large dams. Many studies have raised concerns that the current discharge capacity of several hydraulic structures may be insufficient for extreme events, and that those structures may suffer uncontrolled overflow scenarios, compromising the stability and safety of the structure (FEMA [1]). Those new situations create new questions about the hydrodynamic actions at the toe of the dams (Wahl et al [2]). Free-falling water jets in arch dams produce air entrainment and energy dissipation, which change the flow properties Those types of high-velocity air–water flows are characterized by turbulent mixing processes and large amounts of entrained air (Felder and Chanson [3])

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