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Chapter 6 - Analysis of the test results
b) Velocities in the cross-section - secondary current
The flow field in the cross-section (radial and vertical components combined) shows the evolution
of the secondary current (Appendix 11.2 and 11.5). Just after the inlet (10°) the flow field does not
show a clear structure. Upstream the first scour, a small secondary current starts developing near
the bed. At the maximum scour location (Figure 6.23), a big secondary cell (main cell) has developed
in the scour hole. At the inner bank, another secondary cell (with the same rotation direction)
emerges and grows if we move downstream. Towards the second scour hole, the main cell
gets again more important, but not as important as for the first scour. The near bed radial velocities
are of the same order of magnitude over the whole channel (about 10% of the tangential
velocity components).
B01−Q210−F06
0
Distance to free water surface (mm)
Distance to free water surface (mm)
100
200
300
200 mm/s
400
0 100 200 300 400 500 600 700 800 900 1000
Distance to outer bank (mm)
B03−Q210−F08
0
100
200
300
3
1
1
200 mm/s
400
0 100 200 300 400 500 600 700 800 900 1000
Distance to outer bank (mm)
Figure 6.23: TOP: Main (1) and inner bank (2) secondary cells in the first scour hole at 40° (no MR)
BOOTOM: Main (1) and inner bank (2) secondary cells and outer bank secondary cell (3) protecting
the wall of the channel (rib spacing 2°) at 70°
Placing ribs at the outer side wall (every 4°), a secondary current of almost constant intensity over
the whole bend is well visible.
At the outer bank at the free water surface a very small secondary cell (outer bank cell) gets visible
(Figure 6.23). This cell was described by BLANCKAERT & GRAF (2001). For the performed tests,
this cell keeps the same intensity over the whole bend. If more ribs are added (2°), the cell
1
2
2
page 140 / November 9, 2002
Wall roughness effects on flow and scouring