Thesis-2007-Sun.pdf (41.52 MB)
Flow characteristics in compound channels with and without vegetation
thesis
posted on 2011-02-10, 10:21 authored by Xin SunThe flow characteristics in compound channels with and without vegetation on the
floodplain were investigated experimentally and numerically in this thesis. Detailed
measurements of velocity and boundary shear stress, using a Pitot tube and an
acoustic Doppler velocimeter together with a Preston tube, were undertaken to
understand the flow characteristics in compound channels. Eight no-rod cases, two
emergent-rod cases and two submerged-rod cases were tested. Unsteady large eddies
that occur in the shear layer were explored numerically with Large Eddy Simulation
(LES) to identify its generation and its effects on the flow behaviors. Mean flow
parameters were predicted using the quasi-2D model by considering the shear effect.
Usirgg the data of depth-averaged velocity and boundary shear stress, the contributions
of shear-generated turbulence and bed-generated turbulence to the Reynolds shear
stress were identified, the apparent shear stress was calculated using the modified
method of Shiono and Knight (1991) and the depth-averaged secondary current force
was then obtained. Large eddies were important to the lateral momentum exchange in
shallow non-vegetated compound channels and even in deep vegetated compound
channels. In the compound channel with one-line rods at the floodplain edge, the
secondary current forces were of opposite signs in the main channel and on the
floodplain and the bed shear stress was smaller than the standard two-dimensional
value of yHSo due to the vegetation effect, where y,H,So are the specific weight of
water, water depth and bed slope respectively. In vegetated compound channels, the velocity patterns were different to those and the discharges were smaller than those in
non-vegetated compound channels under similar relative water depth conditions. The
anisotropy of turbulence was the main contribution to the generation of secondary
currents in non-vegetated and vegetated compound channels, but the Reynolds stress
term was more important in the vegetated compound channels. Results of cross
spectra showed the mechanisms of the turbulent shear generation near the main
channel-floodplain junction are due to large eddies in the non-vegetated compound
channel and owing to wakes in the vegetated compound channel. LES results indicated that large eddies caused significant spatial and temporal fluctuations of velocity and water level in the compound channel and the
instantaneousv alues of these flow parameters were significantly higher than the mean values. In vegetated compound channels, the flow moved from the main channel to the floodplain and from the floodplain to the main channel alternately. The characteristic frequencies of the large eddy were less than 1Hz which was consistent
with the experimental data. The capability of the quasi-2D model to predict the 2D mean flow parameters in compound channels were assessed under different flow conditions and also improved by using the mean wall velocity as the boundary condition and appropriate values of the lateral gradient of the secondary current force. In the vegetated compound channels, new approaches were proposed to treat the drag force in the cases of oneline emergent rods at the floodplain edge and submerged rods on the floodplain.
History
School
- Architecture, Building and Civil Engineering
Publisher
© Xin SunPublication date
2007Notes
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.EThOS Persistent ID
uk.bl.ethos.479481Language
- en