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Air Entrainment

Designing Spillway Structures

Spillway structures must be designed to handle a wide range of operating conditions. As the flow conditions reach the upper end of the design range, irregularities on the spillway surface can cause flow separation. This in turn can cause the pressures on the spillway surface to become low enough to cause cavitation due to air entrainment. Cavitation is highly detrimental to the strength of the structure and can lead to catastrophic failure.

Air entrainment caused by spillway flow. Analysis by Groupe Conseil Genivar.

Entraining air is a means of reducing the likelihood of cavitation. When air is present in the water it adds a damping effect to the collapsing bubbles of cavitating regions, thus reducing cavitation damage. If the velocities in the spillway are sufficiently high, then aeration devices should be added to entrain air and reduce cavitation.

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Extensions to the Air Entrainment Model

 

Simulation of spillway flow illustrating air entrainmentAir entrainment graph showing steady volume of fluid
Simulations of spillway flow and the prediction of air entrained from aeration device. The image at left is colored by macroscopic density. The graph at right compares the steady volume of fluid entering the spillway versus the quantity of water and air after the aeration device.

FLOW-3D's air entrainment model allows engineers to predict air entrainment in their designs and make appropriate modifications to ensure they function safely. Shown here is a simulation of aeration device where a small deflection ramp allows air to become entrained in the cavity formed before the local nappe. There is a local sub-pressure created that causes air to be drawn into the fluid.

Sustaining Fish Populations

When air is entrained into water, it can help sustain the growth of microorganisms and ensure the survival of healthy fish populations. However, dissolved gas at supersaturated conditions has been identified as a water quality issue negatively affecting aquatic organisms. Another use for the air entrainment model is in marine biology to determine the concentrations of air entrained from spillways that is released into a river downstream.