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4. HYDRAULIC MODEL TESTS
Hydraulic model tests were practiced in order to supply the under sea
information required in the interpretation of the remote sensing imageries.
Based on the coherent structure theory, each of the small scale tidal vortices
can be modellized into a vertical vortex tube, the ends of which terminate at
the sea surface and at the sea bottom. The equipment of the model test is
presented in Fig. 12. In a test basin of 0.6 meter width and 25 meter length,
a uniform flow with the free water surface was generated. The velocity of the
flow was 0.7 m/sec. The vertical vortex tube was produced as follows. A plate
of 0.3 meter width was put suddenly into the uniform flow keeping one edge of it
touched at right angle to the side wall of the test basin. It was kept in the
uniform flow untill a vortex tube with a certain strength is formed at the other
edge of the plate. Then it was drawn out suddenly, and a vertical vortex tube
started into the uniform flow. The most distinguished hydrodynamic phenomenon
was the strong upwelling flow induced around the vortex tube. The vortex tube
was visualized by blue dye which was put into the water surface at the spot of
the vortex tube. Red dye was put on the bottom surface in the course of the
vortex path before the vortex tube was made, in order to visualize the upwelling
flow. Both side view and the overhead view reflected by a mirror set above the
test basin were photographed every one second by a camera. Fig. 13 is an exam-
ple of the succesive pictures of the front of the upwelling flow, which was
mapped from a series of the photos. The upward volume flux induced by the
vortex tube was estimated from such figures.
Observation by the boats in the field suggested that the distinguished
upwelling flow is caused at the spots of the small scale tidal vortices. The
magnitude of the upward volume flux was estimated based on the hydraulic model
tests. With the assumption of the Froude's similitude, the upward volume flux
due to the single tidal vortex was estimated to be about 300 cubic meter per
second. Therefore, about a dozen of the tidal vortices were expected to cause
the total upwelling volume flux of about 3,000 cubic meter per second, which was
expected under the tidal current condition of Fig. 5. Such a direct mass trans-
port from the sea bottom to the sea surface are expected to be playing an impor-
tant role in the sea water mixing in the strait sea area.
Hydraulic model tests using a scaled model of the Naruto Strait were
also performed in order to observe the motion of the large scale vortex-pair
throughout the full tidal cycle. The tidal current was produced in the scaled
model set in the test basin shown in Fig. 14 equipped with a pair of plungers
moving vertically in opposite direction to one another. The sea bottom configu-
ration of the strait was scaled down with a horizontal and vertical scaling
ratio of 1/20,000 and 1/1,667, respectively, under the law of Froude’s simili-
tude. Effective tidal exchange was observed to be caused by the large scale
tidal vortex-pair, because of its self-induced motion into the wide sea area
where it has been formed. Tidal exchange ratio at the Naruto Strait was estima-
ted to be about 60% when the vortex-pair is effectively produced after the amal-
gamation of the small scale tidal vortices.
ACKNOWLEDGEMENTS
This research has been conducted since the spring of 1976. Financial
supports were provided by the Ministry of Education, Science and Culture under
the Grant in Aid for Scientific Research and by the Honshu Shikoku Bridge
Authority.
REFERENCE
(1) Davies P.0.A.L. and A.J. Yule : Coherent structure in Turbulence, Jounal
of Fluid Mechanics, Vol1.69, part 3, pp.513 - 537. 1975.
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