Coherent turbulent structures in the boundary layer resulting in turbulent bursts penetrating the sublayer, have been proposed as a mechanism for the removal of fouling deposits. The interaction between the bursts and representations of particulate and biological fouling deposits was studied using dye to highlight the flow structure and a twin laser system to provide illumination for filming and video recording. Over a smooth plate the dimensionless spanwise low speed streak spacing (λ+s = λsU*/v) was found to be 93 which is in excellent agreement with the literature. Grooved plates and polysaccharide gums were used as test surfaces and microglass beads and biofilms were also employed. The mean cleaning efficiency of bursts over a particulate deposit was found to be 0.023 percent and it was considered that a burst event had insufficient energy to remove fouling. Dimensionless streak spacings over the grooves ranged from 83.1 to 196.7 for longitudinal grooves and from 75.9 to 209.9 for transverse grooves. This increase in the spacing was expected from the literature. A change in the scaling parameters from inner variables over a smooth surface to outer variables over a grooved plate was recorded. Burst counts over the grooves, both longitudinal and transverse, and the gums were found to increase from the smooth surface. However, the spanwise spacing of low speed streaks also increased Over the grooves bursts were noted to display shallower ejection angles than over a smooth surface. A typical burst angle over longitudinal grooves was 3.70 compared to 4.30 for a smooth surface. This together with an increase in streak persistence and a delay in transition to turbulence suggested that the surfaces representing fouling deposits reduced the energy of burst events so making removal by burst action unlikely. Further work is recommended to investigate the flow over biofilms grown under flowing conditions, and over random roughnesses. The determination of the burst duration and intensity and the skin fiction over these surfaces would be a useful undertaking.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:502990 |
| Date | January 1991 |
| Creators | McKee, Craig John |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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