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Friction, roughness and boundary layer characteristics of freshwater biofilms in hydraulic conduits

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Barton, Andrew F (2006) Friction, roughness and boundary layer characteristics of freshwater biofilms in hydraulic conduits. PhD thesis, University of Tasmania.

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Abstract

Hydraulic conduits are vulnerable to deterioration in their carrying capacity over time
due to biofilm development and accumulation on internal surfaces. It is generally
recognised that this is brought about by an increase in the effective roughness of the
surface in contact with water. However, the detailed mechanisms by which biofilms
affect the flow are poorly understood. This thesis presents the findings from a multifaceted
research program to investigate wall friction, roughness and boundary layer
characteristics of freshwater biofilms in hydraulic conduits.
Field investigations included a series of headloss tests before and after cleaning of
biofilm material from pipelines in three different hydroelectric schemes. Bacteria made
up the majority of biofilm biomass in the pipelines studied. Results of the headloss
testing show that improvements to hydraulic efficiency can be achieved from the
removal of biofilms in the pipelines tested. Identifying the flow velocities at which
hydraulically smooth, transitional or rough conditions occur was found to aid
optimisation of conduit operating characteristics. The friction law for conduits
roughened by biological growths did not always follow a Colebrook-White type
relationship.
A purpose designed water tunnel was built and used to undertake total drag and
boundary layer measurements of freshwater biofilms grown and conditioned in the field
on test plates immersed in open channels. These measurements allowed a comparison
between the flow behaviour of smooth and rough plates covered with biofilms with the
same smooth and rough plates in their clean state. Instrumentation used in conjunction
with the water tunnel included total pressure probes to determine the mean velocity
boundary layer structure, a floating load-cell arrangement to measure total drag, wall
tappings for static pressures, and pressure transducers. Turbulence measurements were
undertaken using hot-film constant temperature anemometry, though results were
inconclusive due to fouling problems with the probe. Unsteady pressure measurements
were instead made to measure the turbulence character of the water tunnel working
section. Observations of flow effects on biofilm behaviour were also made using the
water tunnel. An increase in drag and local friction was measured for all fouled plates
from their initially clean condition. Smooth plates coated with biofilms created
comparable levels of drag to rough plates coated with biofilms. The greatest relative
increase in drag from the clean state was measured for a fouled smooth plate.
The increase in drag was related to the thickness and coverage of the biofilm over the
test plate, and also the type of biofilm present. Low form gelatinous and filamentous
algae made up the majority of biofilm biomass in these test plates. The greatest levels of
drag were measured on test plates covered with filamentous biofilms. The non-uniform
biofilm thickness encountered over the test plates affected the state of the boundary
layer and complicated the roughness characterisation of the test plates. In all cases, the
physical roughness of the biofilms was less than the effective roughness measured in
the water tunnel.
An investigation into the physical character of freshwater biofilms was undertaken
using data generated from close range photogrammetry. Biofilm surfaces were mapped,
and the data were subsequently compared to the roughness information derived from
water tunnel measurements. It was found that low form gelatinous biofilms are most
amenable to close range photogrammetry studies. Filamentous biofilms lay flat when
out of water, and so little value can be gained by studying their physical character in
this condition. It was found that biofilms are able to change a rough surface to a
physically smoother surface by growing in the gaps and valleys between roughness
elements. It was also found that in some instances biofilms on coarse rough surfaces
form peaks on the roughness elements. On heavily fouled test plates, the biofilm formed
ripples transverse to the flow, not unlike a compliant or erodible surface. Algae made
up the majority of biofilm biomass investigated in the photogrammetry studies.
This research shows that it is the uppermost surface of the biofilm exposed to the flow
that most influences surface friction. This has implications for whether or not to
refurbish concrete canals with a smooth coating. An appropriate smooth surface coating
must also have good antifouling properties if gains in hydraulic efficiency are to be
fully realised.

Item Type: Thesis (PhD)
Keywords: Channels (Hydraulic engineering), Pipelines, Hydraulic structures
Copyright Holders: The Author
Copyright Information:

Copyright 2006 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. CD-ROM contains Appendix A. Thesis (PhD)--University of Tasmania, 2007. Includes bibliographical references. Ch. 1. Introduction -- Ch. 2. Literature review: the problem of biofouling in hydraulic conduits -- Ch. 3. Field headloss studies and paint trials -- Ch. 4. Test plate details, deployment strategy and schedule for testing -- Ch. 5. Characterising surface roughness using close range photogrammetry -- Ch. 6. Water tunnel details, instrumentation and calibration -- Ch. 7. Boundary layer, drag and roughness measurements for clean and fouled smooth plates -- Ch. 8. Boundary layer, drag and roughness measurements for clean and fouled rough plates -- Ch. 9. Results synthesis and discussion -- Ch. 10. Conclusions and recommendations

Date Deposited: 25 Nov 2014 00:55
Last Modified: 11 Mar 2016 05:54
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