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

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.