Environmental Engineering Reference
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the axis z (height) orientation is equal to or greater than ten percent of inlet flow velocity. A
clockwise rotation vortex is generated at the rear of the artificial reef and is called the back
vortex flow field. Compared with the inlet flow velocity, the flow velocity in the back vortex
flow field is slower. Therefore, the back vortex flow field is also called the slow flow region.
The water current reattachment point is an important characteristic quantity and is located
downstream of the artificial reef where the flow velocity is on the verge of zero. All of the
arguments for the upwelling current and back vortex flow are on the vertical two-
dimensional central plane.
4.1 Flow field of a single hollow cube artificial reef
4.1.1 A single hollow cube artificial reef at a 90 degree angle
Figs. 4(a) and 4(b) show the experimental and numerical results for the flow field velocity
vector diagram of a single hollow cube artificial reef with a 90 degree angle, respectively. In
front of the artificial reef, the flow velocities slow down due to the reef resistance causes.
Several narrow regions with different values of flow velocities spread in the upstream field,
and the flow velocities increase gradually with height. Within the reef, the upper and
bottom regions of the flow field have a low velocity, while the regions in the opening have a
high velocity, which is approximately the incoming flow velocity. The reason is that the flow
into the reef is obstructed by the two vertical members on both sides of the cube side face
but can enter freely through the opening located in the center of the face. Figs. 4(a) and 4(b)
also show that a low velocity flow field is formed in the area downstream of the reef.
Experimental and simulation results show a similar velocity distribution around the
artificial reef. The overall velocity vector diagrams of the two figures are in good agreement.
(a) Experimental results
(b) Simulation results
Fig. 4. Flow field velocity vector diagram of a hollow cube artificial reef with a 90 degree
angle
Fig. 5 compares the scale and intensity of upwelling and back vortex flow field between the
experimental and simulation results at a 90 degree angle. The height ( H up ) and the area ( S up )
of upwelling field by simulation are close to the experimental data. The computational
average upwelling current velocities ( V a ) are slightly higher than the experimental results. A
major gap exists in the length of the back vortex flow ( L e ), especially with a lower inlet flow
velocity. In the field of the back vortex flow, the flow velocity is very slow and is on the
verge of zero at the water current reattachment point, such that the performance of the
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