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Hydrodynamics in Hydropower Tailraces

The McNary Dam on the Columbia River (photo courtesy of the U.S. Army Corps of Engineers).

The McNary Dam on the Columbia River (photo courtesy of the U.S. Army Corps of Engineers).

The flow in the tailrace of a hydropower dam is usually very complex. The large amount of energy introduced by spillway flows, mostly dissipated in the stilling basin and adjoining tailwater channel, introduces massive amounts of bubbles and creates energetic waves and sprays.

Spillway deflectors redirect spilled water horizontally, forming a surface jet that prevents the bubbles from plunging to depth in the stilling basin, and thus reducing the air dissolution. The flow pattern in the tailrace is completely modified after installation of spillway deflectors, since spillway surface jets attract water toward the jet region, a phenomenon called water entrainment.

The change in the flow pattern affects the performance of fish passage, sedimentation processes, and distribution of TDG, among other effects. The main mechanisms causing water entrainment are:

  • Acceleration of the surrounding fluid as the jets decelerates;
  • Surface currents;
  • The Coanda effect; and
  • The presence of bubbles.

Numerical studies and field and model observations indicate that the presence of bubbles has a strong effect on the water entrainment. Bubbles reduce the effective density (and pressure), viscosity, and affect the liquid turbulence.

Model-Scale Experiments

Model-scale experiments fail to reproduce the water entrainment observed in the prototype, thus preventing flow studies under some spillway operational conditions. The scaling is performed based on the Froude number, and therefore the Reynolds and Weber numbers are not honored, resulting in smaller levels of turbulence and fewer and bigger bubbles (in dimensionless terms) than in the prototype.

The inadequate representation of the turbulence, bubble residence time, and gas volume fraction leads to weaker surface jets and less entrainment for the physical model.

Numerical Prediction

Numerical prediction of the water entrainment in tailraces has received considerable attention. Isotropic single-phase models grossly underpredict the water entrainment. A multidimensional anisotropic model that takes into account the effect of the bubbles on the flow field and the attenuation of fluctuations at the free surface is needed to predict the entrainment of flow toward the spillway region. Numerical predictions of water entrainment and TDG distribution with TDG-MP3 are shown in Projects.

Last modified on March 2nd, 2015
Posted on March 14th, 2012