*2.1. Experimental Set Up*

Experiments were conducted in Marine Renewable Energy Laboratory, State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University. The experiments used recirculating flume to simulate the tidal current flow and a 3D printing facility to create the tidal current turbine. Sandbox was used to simulate the seabed condition with the selected sand size to simulate the tidal turbine-induced seabed scour. A laser distance meter was used to measure the scour depth at designated positions of the scour regions. The overall experimental installations are shown in Figure 1. To make the experimental designs more clear, the overall diagram is shown in Figure 2.

**Figure 1.** Experimental installation. (**a**) Overall setup of recirculating water flume [22] and (**b**) local setup of scour experimental part.

In the turbine scour experiment, the flume is 0.6 m in height and 0.35 m in width and the central cross-section is 0.8 m in width and 0.6 m in height. The turbine models were created by 3D printing. During the experiment process, flow circulates in a flume. Water is driven by propeller and moves towards the experimental side of flume. Flow moves through the flow-equalizing equipment and becomes more uniform. Then, the flow passes the tidal current turbine and erodes the sediment around supporting piles of turbine. The rotated turbine rotor is driven by Miniature DC rotor to ensure stable speed; the profiles of real-time seabed were measured by laser distance meter with 0.2 mm accuracy. During the experiment, the measurement grid is 5 mm in either the x-axis or y-axis. The seabed profile is measured in real time: the laser distance meter is installed above the water. It can launch laser and measure the distance between target and source by calculating the time di fference. During the experiment, the flow moving is stopped by turning o ff the propeller after certain operational time. The overall sand bed profile is measured by Laser distance meter. Then reopening the propeller and

continuing scour experiment until the scour equilibrium. Laser-based instruments provide accurate results when they are utilized to acquire scour pattern. For instance, Dodaro et al. (2014) [23] and Dodaro et al. (2016) [24] employed a 3D laser scanner to measure scour profile downstream of a rigid bed.

 **Figure 2.** Overall design diagrams of scour experiment: (**a**) front view and (**b**) overhead view.

Various parameters were considered in the series of experiments to investigate the influence factor of turbine scour. The constant parameters are supporting pile diameter, rotor height, blade chord length, water depth, inlet velocity, sediment diameter, and rotational speed. The variable parameters are rotor radius and tip-bed clearance. These two parameters are main variables in our research. The specific value of each parameter is shown in Table 1. It is worth noting that this recirculating flume was used in the previous works to propose empirical model for equilibrium scour depth induced by turbine [22].


**Table 1.** Experimental parameters.
