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In Figure 5 the trajectories of a spherical parafin ball of diameter of 5mm and density of 900kg/m3,
which is supposed to represent an oil droplet, are shown. The solid curve is the computed trajectory by
Eqn.3 and symbols are the repeated experimental results. The particle release point (x,~ y,,) for Figure
5a is (x,/D. yf/O) = (-2.0, -0.6) and for Figure 5b (-0.8, -0.6). It can be seen from Figure 5 that the
experimentally obtained trajectories from the releasing point nearer to the fence show an inconsistent
behavior behind the fence. The main reason is due to the unsteady nature of the tip vortex emanating
from the tip of the fence. Although no attempt was made to obtain more accurate time-averaged
trajectories from the experiment due to the impracticality of repeating the tests for over at least fifty
times, it is assumed the computed trajectory may represent the reasonable time-averaged values.
Based on the analysis described above, it is thus concluded that the evaluation of the containment
effectiveness of a set of tandem fence by the flow-field computation based on the present Navier-
Stokes solver with k-E turbulence modeling and vanishing velocity condition on the free surface is
reasonable.

Computation for the trajectories of the oil droplet of various sizes is carried out for the draft ratio
(DID2) of 0.5, 1.0, and 2.0 where D, and D, are the drafts of the fore and the aft fence, respectively.
From the results it is concluded that D,/D, = 1 .O is more effective than the other two ratios.

The effect of the water depth on the tandem-fence effectiveness is investigated by computing the
trajectories of the oil droplets for different values of water depth ranging from 2D to 15D. It was found
that the shallower the depth, the longer the oil trajectory between the fences, which is not favorable in
the viewpoint of the oil trapping between the fences.

6 CONCLUSIONS

From the results of the present investigation, the following conclusions are drawn :

1) The present flow-field computation by the Navier-Stokes solver with the k-E turbulence
modeling and the free-surface condition of vanishing vertical velocity together with the
Lagrangian particle-tracking method seems a reasonable tool in evaluating the effectiveness of
tandem fences,
2) The condition of lower current speed, larger oil-droplet size, and smaller fence-skirt deformation
would lead to a lesser chance of oil leakage below the fore fence,
3) An identical draft for both fences of a tandem fence seems more effective than
other combinations of the fore and aft fence drafts, and
4) In the shallow water region, deployment of a tandem fence with larger draft would not
necessarily increase the oil-trapping effectiveness. It is recommended to leave the gap between
the fence tip and the water bottom more than twice the fence draft.

Acknowledgment
The authors express their appreciation for the supports granted by Advanced Fluids Engineering
Research Center of Pohang University of Science and Technology through Korea Science and
Engineering Foundation. They also acknowledge the assistances of their graduate students, Messrs
Min S. Koh and Sang K. Chung in carrying out the present investigation.

References
Comrack, D. (1 983). Response to Oil and Chemical Marine Pollution. Applied Science Publishes. New
York.