Measuring turbulence characteristics of artificial biofilms using LDV and high speed photography

The structure of the flow over filamentous biofilms was investigated using an 80 mm long idealised biofilm streamer inserted in a recirculating water tunnel. Previous studies have found that the presence of streamers affects the turbulence structure of the boundary layer including changing the turbulence intensity, Reynolds stress profiles, skin friction coefficient and boundary layer thickness.

To fully quantify these changes high speed photography was coupled with boundary layer profiles obtained using laser Doppler velocimetry at several freestream velocities. The images were processed using photogrammetric techniques to track the motion of a filament and quantify the movement of the tip in relation to the boundary layer. This data was then correlated with the turbulence results from the boundary layer traverses. Results for turbulence behind a single streamer are compared to results for a smooth reference surface.

The maximum displacement of the streamer tip increased with increasing freestream velocity, and three distinct stroke patterns of the streamer tail were observed. Significant disturbances to both the mean velocity profile and the turbulence intensity were measured in the region of streamer movement (1.3–9 mm from the wall) and the presence of the streamer resulted in a large increase in wake strength.