The Investigation of the Hydrodynamics of an Artificial Reef - Aquaculture and the Environment: A Shared Destiny - page 87

Fig. 5. H up /h, S up / S , L e / h and V a of experimental and simulation results at a 90 degree angle

tracing particles is weakened and causes a larger error. The mean error of upwelling and
back vortex flow between the experiment and simulation is lower than 11%, and both of the
results have an identical variation trend with the increase of the incoming flow velocity.

4.1.2 A single hollow cube artificial reef at a 45 degree angle

Figs. 6(a) and 6(b) present the experimental and numerical results obtained for the velocity
vector diagrams of a hollow cube artificial reef with 45° angle. The flow structures observed
at this angle are not the same as those for a 90° angle, especially inside the artificial reef. At
this angle, the water flow is obstructed by the side face of the artificial reef, and no fluid can
directly enter the reef in the middle. In the upstream, the slow flow velocity field is mainly
distributed at the bottom or close to the reef side face. The flow velocity, both in the
experiment and simulation, in most regions within the reef, is less than 2.0 cm/s, and the
maximum velocity is only half of the inlet flow velocity. There is a large difference for the
flow field structure in the rear of the artificial reef between the two arrangements. A large
and complicated vortex is formed immediately downstream of the reef. The low flow
velocity conditions in this region indicate the presence of a desirable shading effect, i.e., fish
can rest and seek shelter in this suitable environment. For Fig. 6 (a) and (b), the numerical
simulation predicts a flow field that is comparatively similar with the experimental results.

(a) Experimental results (b) Simulation results

Fig. 6. Flow field velocity vector diagram of a hollow cube artificial reef with a 45 degree
angle

Fig. 7 shows the comparisons of the scale and intensity of the upwelling and back vortex
flow field that are derived from the experiment and numerical simulation at a 45 degree
angle. A good agreement was found between the two results. Nonetheless, there is a large