The flow field inside and downstream of an open channel placed near the surface of a free flow (such as the tail water of a turbine) is characterized in detail. The channel cross-section is U-shaped and in the downstream end is placed a ramp on the bottom which accelerates the flow passing through the channel. This flow is intended to catch the attention of fish and improve their entrance to fishways, which has also been successfully demonstrated in field tests.
The flow through the channel is subcritical and the ramp thus blocks some water from passing through. To find the optimum vertical position of the channel, a down-scaled model channel is placed in a water flume and flow fields in vertical planes directed along the flow are visualized using particle image velocimetry. Results show that increasing the depth over the ramp has little effect on the maximum velocity while it makes the accelerated water more perceptible downstream the channel. This will most likely improve the channel's ability to attract fish. It is also shown that a recirculation zone is formed in the channel for the small depths tested. Finally, it is shown that a modest tilt of the channel will not affect the flow field in any significant way. Copyright © 2010 John Wiley & Sons, Ltd.