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Chapter 6 - Analysis of the test results
6.6 Analysis of the flow field
The presence of the macro-roughness influences the scour process by modifying the velocity distribution
in the bend. It is well known that the velocity is higher on the outside of the bend and
that there is a secondary circulation also. Both factors combine to give a large scour on the outside
of the bend. A brief description of the flow field was already given in the paragraph discussing the
preliminary tests (§ 5.2.1/2)e). The analysis of the main tests is presented here.
a) Tangential velocities
The velocities in tangential direction (along the channel axis) are somewhat modified in the bend.
Instead of a classical “log”-velocity profiles as it can be seen on top of Table 6.6 1 undergoes the
following modifications in a curved channel. The maximum velocity is no longer found close to
the free water surface but close to the bed surface, particularly in the scour holes; this can be seen
very well in cross-section 40° in the second profile from the left. The highest tangential velocities
( ) are now found in the outer half of the cross-section.
V θ
B01−Q210−F06
0
0
100
X
200 800
300
600
400
0 200 400 600 800 1000
Distance to outer bank (mm)
400
With macro-roughness, the previously mentioned increase of the water depth in the inlet reach
(§ 6.2.2) is well visible. But this has no influence on the shape of the velocity profile in this zone.
In the bend and especially in the scour holes, the near bed velocity is reduced and the maximum
velocity shifts towards the free water surface (Table 6.6 at 40°, first two profiles at the outer bank).
Appendix 11.1 shows that the highest velocities are located in a straight reach next to the free
water surface. In the bend, this zone of maximum velocity first shifts towards the outer side wall
and then down the wall towards the channel ground. At the maximum scour location, high (tangential)
velocities can be observed near the bottom (Figure 6.22).
100
200
300
400
Figure 6.22: Tangential velocities at 40° without macro-roughness (left) and with ribs spaced every 2° (right); view
in the downstream direction (see also Appendix 11.4)
X = max. velocity
With macro-roughness (Appendix 11.4), the zone of maximum velocity remains all over the bend
close to the surface for the ribs spaced every 4°. Introducing additional ribs (every 2°) the maximum
velocity moves towards the channel ground, but at a significant distance from the outer wall
(about 20 cm, that is about the average flow depth). For the smallest rib-spacing, the high-velocity
zone gets even closer to the ground, but again at about 20 cm from the outer side wall.
Distance to free water surface (mm)
1200
1000
X
B03−Q210−F06
0 200 400 600 800 100
200
Distance to outer bank (mm)
1400
1200
1000
800
600
400
1. Table 6.6 gives the raw velocity profiles. It shows that the first 2 cm from the free water surface
are strongly influenced by the measurement, since the velocity probe had to touch the fluid.
page 138 / November 9, 2002
Wall roughness effects on flow and scouring