Abstract

Channel expansions provide a transition from a narrow to a relatively wide channelsection,
which is necessary in many hydraulic structures. In the transition, flow tends to separate
from its diverging sidewalls and create turbulent eddies, if the angle of divergence exceeds a
threshold value. This phenomenon can cause undesirable flow energy losses and erosion to the
sidewalls locally and even further downstream. Previously, researchers have tried to optimise the
transition’s horizontal shape in order to reduce flow separation; the results are inconclusive. The
purpose of this study is to extend earlier investigations about fitting a hump in the vertical to
eliminate flow separation. This study uses the CFD modelling approach. This approach permits
an efficient and systematic exploration of the effects of different angles of divergence, crest
height of the hump and the Froude number of subcritical flow. The model results are validated
using existent analytical solutions under simplified conditions and available experimental data
for a limited number of cases. Flow quantities presented in this thesis include details of the
velocity, vorticity, eddy structure, and cross-sectional area of flow reversal; these quantities are
distributed at selected vertical and horizontal planes, and are available for the cases of with and
without a hump. It is shown that the use of a hump effectively reduces flow separation and eddy
motion in the transition. This is because the flow is forced to accelerate over the hump, and as a
result, the otherwise adverse pressure gradient, which is known to be responsible for flow
separation, diminishes. A hump in the vertical can easily be incorporated into the bed of existent
channel expansions, and would be less expensive to construct than to modify the horizontal
shape (or the sidewalls) of existent expansions. The results presented in this thesis are of
practical values for the optimal design of humps.