Cavitation On Spillways
When water flows over a dam spillway, the irregularities on the spillway surface will cause small areas of flow separation in a high speed flow, and, in these regions, the pressure will be lowered. If the velocities are high enough the pressure may fall to below the local vapour pressure of the water and vapour bubbles will form. When these are carried downstream into high pressure region the bubble collapses giving rise to high pressures and possible cavitation damage.
Experimental investigations show that the damage on concrete chute and tunnel spillways can start at clear water velocities of between 12 to 15 m/s, and, up to velocities of 20 m/s, it may be possible to protect the surface by streamlining the boundaries, improving the surface finishes or using resistant materials.
When some air is present in the water the resulting mixture is compressible and this damps the high pressure caused by the bubble collapses. If the velocities near the spillway invert are sufficiently high, aerators (or aeration devices) must be introduced to prevent cavitation. Although these have been installed for some years, the mechanisms of air entrainment at the aerators and the slow movement of the air away from the spillway surface are still challenging.
The spillway aeration device design is based upon a small deflection of the spillway bed (or sidewall) such as a ramp and offset to deflect the high velocity flow away from the spillway surface. In the cavity formed below the nappe, a local subpressure beneath the nappe is produced by which air is sucked into the flow. The complete design includes the deflection device (ramp, offset) and the air supply system.
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