Research on a Counter-Rotating Propeller-Type Tidal Stream
Power Unit with Winglets at Rear Blades
Taisei KAWASHITA*, Tengen MURAKAMI** and Toshiaki KANEMOTO***Faculty of Science and Engineering, Saga University
1 Honjo, Saga 840-8502, Japan
E-mail: 21729006@edu.cc.saga-u.ac.jp**Institute of Ocean Energy, Saga University
1 Honjo, Saga 840-8502, JapanAbstract
Tidal stream is expected as a high-integrity energy source in the world. Tidal current energy extraction devices canbe categorized as a horizontal axis system and a vertical axis system. The authors have developed a horizontal axis
counter-rotating type power unit which is applied in the conversion of wind energy and tidal energy. A counter-rotating
tidal stream power unit is composed of tandem propellers driving double rotational armatures, while the rotating torques
are counter-balanced in the power unit. The power unit can be moored with one cable and keep the posture stable withoutrolling motion in the tidal stream, in which the cable is hooked at the balancing point of the moment among the buoyancy,
gravity and drag. The posture, however, may lose balance by a dynamic disturbance. To keep the posture stable in every
circumstance, the winglets were installed on the rear blades. In this research, effectiveness of winglets in case mooringwith a wire near the water surface was verified by the circulation type water channel tests. Besides, the blade performance
was analyzed by the CFD. As the result, it is noticed that the high blade load can be achieved at near the blade tip bysetting the winglets.
Keywords : Tidal Stream, Power Unit, Counter-Rotating, Tandem Propellers, CFD

1. Introduction
The tidal current is expected as an attractive renewable energy source because the time change of flow direction andvelocity is predictable. Tidal current energy extraction devices can be categorized as a horizontal axis system (OrdonezSanchez et al., 2019) and a vertical axis system (Grondeau et al., 2019). The authors have developed a horizontal axis
counter-rotating type power unit which is applied in the conversion of wind energy (Kubo and Kanemoto, 2008) and tidalenergy (Funami et al., 2017). This type of power unit is composed of the tandem propellers driving double rotational
armatures, in which the rotating torques are counter balanced. The performance of the power unit has been validated in
the previous works (Huang et al., 2016). In addition, the offshore test has also verified at Nagasaki Bay, Japan that it not
only can get the excellent power generation but also can guarantee the safety operation (Samura et al., 2019).
In this research, a mooring support structure is applied to the horizontal axis counter-rotating type power unit, which
is not only save the initial cost due to the civil construction but also appropriate for using at narrow strait due to set up
easily. The counter-rotating type power unit can keep the posture stable without rolling motion in the stream when it ismooring with a cable at moment center among the buoyancy, the gravity, and the drag force. In such an arrangement, thedrag force induced from downstream type propellers works effectively to keep the horizontal posture. However, once theunit inclines against the mainstream by unexpected change of local flow velocity, the horizontal posture is disrupted andfurther aggravated by the drag force induced from the flow separation around the casing which includes a nacelle
supporting a generator and two hubs of the front and rear propellers. Therefore, the winglet used in aircraft is set to theblade tip of the rear propeller to help the power unit recover to horizontal posture. In this paper, effectiveness of winglets
in case mooring with a wire near the water surface was verified by the circulation type water channel tests. Besides, theblade performance was analyzed by the CFD. As the result, it is noticed that the high blade load can be achieved at near
the blade tip by setting the winglets.

2. Circulation type water channel tests
Figure 1 shows the model power units with and without winglets at rear propeller. The front and rear propeller blades
were designed based on Blade Element Momentum Theory. The upstream side of model including nose spinner does not
rotate and the tail spinner rotates together with the rear propeller. The generator is not installed in the models. The power
unit consists of front propeller with three blades and rear propeller with five blades. The unit is set in a circulation typewater channel with a cross-sectional area of 0.7 m (water depth) x 1.0 m (width) and moored with a wire at the nosespinner. In the experiments, the posture of the model was analyzed with the images by a camera. Besides, the rotational
speeds of front and rear propeller were measured using a stroboscope (SATOTECH DT-2349JP). In the following
discussion, the power unit model with winglet is called as PUW in comparison with PUO of the model without the winglet.
The flow velocity Vin was changed from 0.7 m/s to 0.8 m/s, 0.9 m/s and 1.0 m/s.

Fig. 1 Models with and without winglet.

Figure 2 shows the tilt angle  of the experimental result and Fig. 3 is the counter-rotational speed Nt. The abscissa
Vin is the flow velocity. As shown in Fig. 2, the  in case PUW was about 8 degrees lower than the case PUO regardless ofthe flow velocity. Besides, in case PUW, the counter-rotational speed Nt became 1.3 times higher than the case PUO
because of the low tilt angle as shown in Fig. 2.

Fig. 2 Changes in tilt angle due to flow velocity.

Fig. 3 Changes in counter-rotational speed.

3. Numerical simulations
The 3-D turbulent flow was simulated at the steady state condition by the commercial CFD code of ANSYS-CFX

ver.19 with SST turbulent model to understand the propeller performances with and without winglet. The simulation
domain is composed of 1 pitch passage areas of front and rear propellers. Besides, the condition that the tangential
averaged parameters are transferred was adopted at the interface between front and rear propellers. The number of thegrids for the model PUO is 1953847 and that of the model PUW is 2036927.
Figure 4 shows changes in the ratio of torque, where the abscissa  is the tip speed ratio. The torque T was normalized
by the maximum torque Tref at  = 6.4 giving the maximum power coefficient Cp in case PUO. Furthermore, Fig. 5 is theratio of power coefficient Cp. The power coefficient Cp was divided by the maximum power coefficient Cp ref at  = 6.4
in case PUO. As shown in Fig. 5, the numerical simulation result of PUO is in good agreement with the experimental one
although the power coefficient ratio of simulation is slightly high at around  = 10.2 of high tip speed ratio. In comparisonof simulation results, the power coefficient ratio in case PUW almost corresponds to the one of PUO in a range of  lower
than  = 6.4 though the ratio of PUW at  = 6.4 is 0.94. Besides, the trends of Cp are based on the torque as shown in Fig.
4.

Fig. 4 Changes in torques due to tip speed ratio.

Fig. 5 Changes in ratio of power coefficient.

Figure 6 shows the rear blade load at 90% span location, midspan and 10% span location in case  = 3.9. It is noticed
that there is little difference in blade load in a range between hub to midspan. On the other hand, it was confirmed thatthe high blade load can be achieved at 90% span location around the blade tip in case PUW.

4. Conclusions
In this research, effectiveness of winglets at rear propeller of counter-rotating tidal stream power unit was verified
by the circulation type water channel tests. In the experiments, the flow velocity was varied from 0.7 m/s to 0.8 m/s, 0.9m/s and 1.0 m/s. Besides, the blade performance was analyzed by the CFD. The concluding remarks are as follows.
(1) The tilt angle in case PUW with winglets is about 8 degrees lower than the case PUO regardless of the flowvelocity.
(2) The counter-rotational speed in case PUW with winglets became 1.3 times higher than the case PUO without
winglet bases on the lower tilt angle.
(3) The power coefficient ratio in case PUW almost corresponds to the one of PUO in a range of tip speed ratio lower than  = 6.4 though the ratio of PUW at  = 6.4 is 0.94.
(4) At lower tip speed ratio , the high blade load can be achieved at 90% span location around the blade tip bysetting the winglets.

5. Acknowledgments
A part of this research was supported by Adaptable and Seamless Technology transfer Program through Target-driven
R&D (A-STEP 2019) from Japan Science and Technology Agency (JST) and research grant of 2021 from Haradafoundation.

(b) Midspan.

(a) 90% span location.

(c) 10% span location.
Fig. 6 Rear blade load at  = 3.9.

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