The effect of freshwater biofilms on turbulent boundary layers and the implication for hydroelectric canals

The objective of the PhD Thesis titled “The Effect of Freshwater Biofilms on Turbulent Boundary Layers and the Implications for Hydroelectric Canals” is to present the details on multidisciplinary investigations on the effects of freshwater biofilms on hydroelectric canal capacity and turbulent boundary layer structure. Generally speaking, it is well known that biofouling causes significant problems in engineered structures, mostly related to reductions in efficiency, since it can change the wall roughness properties and associated frictional drag. A biofilm is a layer of biological growth that attaches itself to the internal walls and forms an interface with the water. It may any combination of bacteria, algae, protozoa, fungi, mosses and invertebrate organisms. Its composition can have significant impact on roughness and friction properties of the biofouled conduit. Biofouling problems in a hydroelectric cause a reduction in power output generation capacity, lost revenue through reduced available output, and costs for cleaning and removal of biofouling and associated down time for the system affected. In the present study the extent to which the surface roughness affects the structure if the turbulent boundary layer was critically examined, in the context of the Townsend 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. Local skin friction coefficients were also obtained. An artificial biofilm was developed to study the motion of algae streamers under water flow conditions. Each test surface was mapped using digital close-range photogrammetry to provide a three-dimensional surface model. The calculation of the skin friction coefficient and wall shear stress for rough wall turbulent boundary layers is a difficult process. Seven methods for determining the wall shear stress were considered. They are Preston Tube Method, Momentum Integral Method, Bradshaw’s Method, Total Stress Method, Perry and Li Method, Hama’s Method and Log Law Slope Method. Except than for Momentum Integral Method, they were applied and their results were used to assess the validity of Townsend Wall Similarity Hypothesis for compliant natural freshwater biofilms.