The undersea noise of the counter-rotating propellers installed in the tidal stream power unit has been investigated experimentally in a water tunnel and the scale, namely Reynolds number, effect of the propeller blade has been predicted preliminarily by numerical simulation. The counter-rotating propellers have many advantages such as higher efficiency and torque balance but may make the undersea noise increase by the flow interaction between tandem propellers. The noise power induced from the propellers is largely concentrated at the lower frequency and most predominant frequencies are predicted by Hanson's theory. The overall sound pressure level increases as the stream velocity increases and decreases at the best efficiency operation point. According to the 2D numerical simulation results, the total pressure loss decreases with moderate increase of the Reynolds number.
As one of ocean energies, tidal energy is a renewable and predictable resource, and attracts human's attention to cope with the rapid growth of the fuel consumption. Horizontal-axis tidal stream turbines have been proposed (Fraenkel, 2002) to effectively get tidal energy from the ocean. Based on wind turbine technologies (Wei, 2010), Kubo and Kanemoto (2008) invented the tandem wind rotors and the double rotational armature type generator without the traditional stator. The profiles were optimized by Usui, Kubo and Kanemoto (2012) as the intelligent wind power unit and have been provided to the tidal stream power unit (Usui, Kanemoto and Hiraki, 2013; Huang, Zhu and Kanemoto, 2016). The output and the force acting on the pillar of the counter-rotating propellers are affected by the blade setting angles reported by Usui, Kanemoto and Hiraki (2013). The blade setting angles of the counter-rotating propellers can be optimized by response surface methodology and has been validated by experimental method (Huang, Usui, Takaki and Kanemoto, 2016).
As the exploitation and utilization of renewable ocean energy, the potential impacts of human-made underwater noise on marine animals has steadily increased (Slabbekoorn et al., 2010). The frequency range in tidal turbine includes the audiogram of most fish, and the fish can be detected the noise level of 160 dB re 1μPa SELrms 1 meter from the turbine (Halvorsen, Carlson and Copping, 2011). Lloyd, Humphrey and Turnock (2011) reported that the dominant noise source of tidal turbine is due to inflow turbulence at low frequency. The noise prediction of tidal turbine is necessary as part of environmental impact assessment and the limited amount of measurement data and modelling research is identified.