Jm Andrewartha

Drag force and surface roughness measurements on freshwater biofouled surfaces

The detrimental effect of biofilms on skin friction for near wall flows is well known. The diatom genera Gomphonema and Tabellaria dominated the biofilm mat in the freshwater open channels of the Tarraleah Hydropower Scheme in Tasmania, Australia. A multi-faceted approach was adopted to investigate the drag penalty for biofouled 1.0 m × 0.6 m test plates which incorporated species identification, drag measurement in a recirculating water tunnel and surface characterisation using close-range photogrammetry. Increases in total drag coefficient of up to 99% were measured over clean surface values for biofouled test plates incubated under flow conditions in a hydropower canal. The effective roughness of the biofouled surfaces was found to be larger than the physical roughness; the additional energy dissipation was caused in part by the vibration of the biofilms in three-dimensions under flow conditions. The data indicate that there was a roughly linear relationship between the maximum peak-to-valley height of a biofilm and the total drag coefficient.

Succession and physiological health of freshwater microalgal fouling in a Tasmanian hydropower canal

Freshwater microalgal biofouling in hydropower canals in Tarraleah, Tasmania, is dominated by a single diatom species, Gomphonema tarraleahae. The microfouling community is under investigation with the aim of reducing its impact on electricity generation. Species succession was investigated using removable glass slides. Fouled slides were examined microscopically and for chlorophyll a biomass. Chl a biomass increased steeply after 8 weeks (0.09–0.87 mg m−2), but increased much earlier on slides surrounded by a biofouled inoculum. Succession began with low profile diatoms such as Tabellaria flocculosa, progressing to stalked diatoms such as Gomphonema spp. and Cymbella aspera. Few chlorophytes and no filamentous algae were present. Pulse amplitude modulated fluorometry was used to measure the physiological health of fouling on the canal wall. Maximum quantum yield (F v/F m) measurements were consistently <0.18, indicating that the fouling mat consisted of dead or dying algae. The succession and physiological health of cells in the fouling community has broad implications for mitigation techniques used.

The Effect of Freshwater Biofilms on Skin Friction Drag

A study is underway at the University of Tasmania, in partnership with Hydro Tasmania, to investigate the skin friction drag generated by different surfaces, including smooth, rough and biologically fouled surfaces. A purpose built recirculating water tunnel has been developed to allow the detailed measurement of boundary layer flow over a variety of surfaces. Mean velocity boundary layer profiles and turbulence intensity profiles are measured using Laser Doppler Velocimetry (LDV) to determine local skin friction coefficients and boundary layer parameters in an attempt to better understand the mechanisms of drag production. An artificial biofilm was developed consisting of wool strands glued to a smooth surface in a regular 100 mm grid with the aim to mimic filamentous algae streamers observed in hydroelectric power system canals. Results were compared with a smooth painted reference plate. Measurements directly downstream of a streamer showed elevated streamwise turbulence intensity in the vicinity of the streamer motion path. No significant difference in local skin friction coefficient was measured between the artificial biofilm and the smooth plate.