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

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The Effect of Freshwater Biofilms on

Turbulent Boundary Layers and the

Implications for Hydropower Canals

by

Jessica Maree Andrewartha

BE (Hons)(Civil) GradIEAust

Submitted in fulfilment of the requirements for the Degree of

Doctor of Philosophy

School of Engineering

University of Tasmania

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Declaration of Originality

This thesis contains no material which has been accepted for a degree or diploma by the University or any other institution, except by way of background information and duly acknowledged in the thesis, and to the best of my knowledge and belief no material previously published or written by another person except where due acknowledgement is made in the text of the thesis, nor does the thesis contain any material that infringes copyright.

Date: Jessica Maree Andrewartha

Statement of Authority to Access

This thesis may be made available for loan and limited copying in accordance with the Copyright Act 1968.

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Abstract

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 two-dimensional 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.

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Acknowledgements

ACKNOWLEDGEMENTS

This project was funded by an Australian Research Council Linkage Grant (LP0667925), with significant financial and in-kind support from Hydro Tasmania.

This thesis would not have been possible without the assistance of a great number of people. I would like to particularly thank Dr Jane Sargison for her supervision, support, friendship, and inspiration over the past five years, including during the honours year of my undergraduate degree. My co-supervisors Dr Greg Walker and Dr Alan Henderson have shared their knowledge and enthusiasm for fluid dynamics, and provided much guidance over the course of this study. Prof Chris Letchford, the Head of the School of Engineering, has also provided support.

This project was of a multidisciplinary nature, and involved two PhD programs: my own, and that of Kathryn Perkins from the School of Plant Science. Kate and I shared many trips to Tarraleah, and provided advice and input into each other’s projects. All of the species identification and microscope photographs of the biofilms in this thesis are attributed to Kate. My thanks goes to Tony Denne at Hydro Tasmania, now retired, who gave me the opportunity to work on this project in the first place and was one of the driving forces behind it. Norm Cribbin, Michael Sylvester, Joshua Kline and Angus Swindon have also been great supporters. Brian Knowles and Stephen Kelly provided invaluable support in the field at Tarraleah. I worked for Hydro Tasmania Consulting on a casual basis during the first 18 months of my study, and I would like to thank Fraser White for allowing me to do this, as it expanded my knowledge of hydroelectricity generation.

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Acknowledgements

iv I inherited the UTAS Water Tunnel from Dr Andrew Barton, who spent many hours teaching me how to use it, and also provided research papers for background reading. Associate Professor Paul Brandner and his team from the Australian Maritime College assisted with the instrumentation.

The photogrammetry methods used in this study were developed by Dr Jon Osborn and Timothy Bendall from the School of Geography and Environmental Studies. All of the photographs and surface topography models presented in this thesis were obtained by Alex Leith.

My brother, Tristan, spent many hours proofreading this thesis, and I would like to thank him for his thoughtful suggestions. The critical discussion provided by the two anonymous examiners was appreciated. Dr Colin Terry also provided thoughtful comments on my thesis defence and response to the examiner’s reports.

On a personal note I would like to acknowledge the love and support from my parents, James and Helen, and my siblings Tristan and Melissa, who have always encouraged me to do my best, and without whom I would have never reached this level. Wendy and Trevor Walker have also provided much support over the past four years.

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Contents

NOMENCLATURE

Symbols

A Cross-sectional area [m2] B Velocity defect law intercept

BN Rough wall Nikuradse log law constant (value depends on flow regime)

C Smooth wall log law constant = 5.0 CD Total drag coefficient

D Drag force [N]

Ed Young modulus for pressure transducer diaphragm [Pa]

G Velocity defect parameter =

2 c

_f

(

1 −

1 H

)

H Boundary layer shape factor = δ*/θ or Hyperbolic hole size for quadrant analysis P Pressure [Pa]

Q Volumetric flow rate [m3/s]

Ra Mean of absolute deviation of roughness profile from mean line [mm]

Re Reynolds number

ReD Reynolds number based on a diameter = Ud/υ

Red Pitot probe Reynolds number = Udp/υ

Rek Roughness Reynolds number = u*ks/υ

Rek,rough Flows with Rek > Rek,rough are hydraulically rough

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Nomenclature

xii Rel Test plate Reynolds number = Ul/υ

Reθ Momentum thickness Reynolds number = Uθ/υ

RH Hydraulic radius [m]

Rp Max height of profile above mean line of sample length [mm]

Rq Root mean square parameter [mm]

Rt Max peak to valley height of profile of sample length [mm]

Rv Max depth of profile below mean line of sample length [mm]

So Slope of channel invert

St Strouhal number = fD/U T Temperature [oC]

U Freestream streamwise velocity [m/s] b Width of test plate [m]

cf Local skin friction coefficient

d Diameter [m]

dp Pitot probe diameter [m]

df Spacing of interference pattern for LDV [m]

f Frequency [Hz]

fd Natural frequency of pressure transducer diaphragm [Hz]

fD Doppler shift frequency [Hz]

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Nomenclature

hd Thickness of pressure transducer diaphragm [m]

k Roughness height [mm] kentry Entry loss coefficient

kexit Exit loss coefficient

k+ Roughness Reynolds number = u*Rt/υ

ks Equivalent sandgrain roughness height [mm]

l Length [m]

l+ Length of LDV measuring volume in viscous units = lu*/ υ n Manning’s n friction coefficient

nd Poisson’s ratio

r Radius [m]

rd Radius of pressure transducer diaphragm [m]

t Thickness of test plate [m] ti Transit time of i

th

particle through LDV measuring volume [s] u Local streamwise velocity [m/s]

u* Wall shear velocity = (τw/ρ)½ [m/s]

u+ Normalised velocity = u/u* ∆u+ Roughness function

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Nomenclature

xiv w(y/δ) Wake function

x Horizontal distance from leading edge of test plate [m] xp Laser penetration depth [m]

y Distance from wall [m]

y+ Normalised distance from the wall = yu*/ υ α Rotation of LDV in xy plane (degrees)

β Rotation of LDV in yz plane (degrees)

γ Light extinction coefficient [m-1] ∆ Change in a variable

δ Boundary layer thickness at 99% of freestream velocity [mm] δw Wall correction for Pitot probe [mm]

δ* Displacement thickness [mm] ε Virtual origin error [mm]

ηi Transit time weighting factor (defined Equation 5.12)

θ Momentum thickness [mm] or angle κ von Karman constant

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Nomenclature

ρ Density [kg/m3]

ρd Density of pressure transducer diaphragm [kg/m3]

τ Shear stress [Pa] τw Wall shear stress [Pa] +

Normalised by u* or u*/υ Time mean value

‘ _{Instantaneous fluctuation about the time mean value}

Abbreviations

ADV _{Acoustic Doppler Velocimetry} EPS _{Extracellular polymer substances} LDV _{Laser Doppler Velocimetry} PIV _{Particle image velocimetry}

UTAS _{University of Tasmania}

UVP _{Ultrasonic velocity profiling} 1D _{One-dimensional}

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Nomenclature

xvi Test Plate Nomenclature

SP Lab _{Clean reference plate that remained in the laboratory with a smooth, painted}

substrate

RP Lab _{Clean reference plate that remained in the laboratory with a sandgrain}

roughened substrate

RP1 F1 _{RP – indicates substrate type (RP = rough plate; SP = smooth plate)}

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