3.1.5. Remark

In general, the maximum scour hole depth occurs at the back side of the foundation. The location is approximately x/*D* = −0.5, y/*<sup>D</sup>* = 0.5. A scour hole is developed around the supporting pile and the deposition mound occurs behind the turbine. The depositional wedge moves further downstream. The hole size shows an inverse correlation with the space between turbine and seabed. The deposited mount moves faster with lower installation height. Furthermore, when the tip clearance is high enough, the turbine has little impact on seabed sour development. The evolution speed of scour shows grea<sup>t</sup> increase with a lower installation height. And this influence increases until the appearance of live bed scour. The scour development shows little change with the continuous decrease of tip clearance in live bed scour process. This process shows the same trend with live bed scour at piers [30].

#### *3.2. Temporal Seabed Scours at Various Rotor Radius*

The influence of rotor radius is investigated at radii of 37.4 mm, 45.9 mm, and 56.3 mm, as shown in Figures 7 and 8. Figure 7 shows temporal scour profiles at y = 0, and Figure 8 shows temporal scour profiles at y = 0.5 *D*.

As discussed in the previous section, tip-bed clearance gives significant temporal impacts to the size of the scour hole, maximum scour depth, and position of the maximum scour. The constant tip-bed clearance *C*/*H* = 0.5 was chosen with the changes of turbine radius. A tip-bed clearance of *C*/*H* = 0.5 is commonly used distance in the installation.

**Figure 7.** Temporal seabed profiles at different times with different turbine radius at location y/0. (**a**) *R* = 56.3 mm, (**b**) *R* = 45.9 mm, and (**c**) *R* = 37.4 mm.

Rotor radius is an important factor for hydrodynamic performance and wake structure of Darrieus-type tidal current turbine [26]. The energy extraction efficiency is closely related to the tip speed ratio which is a function define by rotor radius, inlet velocity, and rotational speed. The scour process around the turbine foundation is greatly influenced by the turbine energy extraction [18]. Therefore, the rotor radius has significant impact on the temporal evolution of scour depth induced by tidal current turbine.

**Figure 8.** Temporal seabed profiles at different times with different turbine radius at location y/<sup>D</sup> = 0.5. (**a**) *R* = 56.3 mm, (**b**) *R* = 45.9 mm, and (**c**) *R* = 37.4 mm.

## 3.2.1. Size of Scour Hole

The scour process is live bed scour in all the tested cases with different rotor radius. The size of scour hole increases with the time until equilibrium state at 150 min. The temporal seabed profiles at 30 min, 90 min, and 150 min can be seen in Figures 7 and 8. The size of scour hole shows a nonlinear relationship with rotor radius. For instance, the maximum size of the scour hole is obtained at *R* = 37.4 mm, and the minimum size of scour hole is at *R* = 45.9 mm. In the case of *R* = 56.3 mm, a medium-sized equilibrium scour hole is obtained.
