Title:
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The interaction between turbulent bursts and fouled surfaces
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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.
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