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The effect of freshwater biofilms on turbulent boundary layers and the implications for hydropower canals

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Andrewartha, JM (2010) The effect of freshwater biofilms on turbulent boundary layers and the implications for hydropower canals. PhD thesis, University of Tasmania.

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Abstract

The majority of the electricity supplied in Tasmania, Australia, is produced by hydropower.
Hydro Tasmania, the power generation utility, operates 29 hydropower stations incorporating
170 km of open channels. These open channels are susceptible to extensive biofilm growth
dominated by the freshwater diatoms Gomphonema tarraleahae and Tabellaria flocculosa,
which form a gelatinous biofilm several millimetres thick and cause reductions in flow capacity
of up to 10%.
This thesis presents results of a multidisciplinary study on the effects of freshwater biofilms on
hydropower canal capacity and turbulent boundary layer structure. The extent to which the
surface roughness affects the structure of the turbulent boundary layer was critically examined,
in the context of the wall similarity hypothesis.
A recirculating water tunnel, equipped with a floating element force balance and a twodimensional
Laser Doppler Velocimetry system, was used to obtain detailed measurements on
test plates covered with flow-conditioned freshwater biofilms. Total drag measurements, mean
velocity profiles and turbulent Reynolds stresses were compared for smooth, sandgrain and
biofouled test plates. An artificial biofilm was developed to study the motion of algae streamers
under flow conditions. Each test surface was mapped using digital close-range photogrammetry
to provide a three-dimensional surface model. A logarithmic relationship was found between the
roughness function and the maximum peak-to-valley height from the photogrammetry
measurements for the low-form gelatinous biofilms.
Seven methods for determining the wall shear stress were investigated; however, none of the
methods examined were entirely satisfactory. A method which can be used for both smooth and
rough surfaces with reasonable measurement uncertainty is needed to allow objective
comparison of the structure of smooth and rough wall turbulent boundary layers.
The presence of biofouling caused a significant increase in the local skin friction coefficient and
overall drag coefficient with increases in skin friction of up to 210% measured over smooth plate
values. Results for low-form gelatinous biofilms provided support for the wall similarity
hypothesis. The biofilms modified the structure of the turbulent boundary layer in the near-wall
region. Velocity defect profiles, Reynolds stress profiles and quadrant analyses showed good
agreement for different test plate surfaces in the outer region of the boundary layer. The results
for an artificial filamentous biofilm were less conclusive, and may not support wall similarity.

Item Type: Thesis (PhD)
Keywords: biofilm, boundary layer, drag, skin friction,roughness, turbulent, walls similarity, hydrowpower
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Date Deposited: 28 Apr 2011 02:00
Last Modified: 11 Mar 2016 05:53
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