*2.1. The Geometry of the Wingsail*

The geometry is a simplified configuration of a two-element wingsail with 1/20th model-scale (as Figure 3 and Table 1). The nondimensional number for characterizing the flow of fluid Reynolds number is defined by Equation (1):

$$Re = \frac{\rho v c}{\mu} \tag{1}$$

where <sup>µ</sup> is the viscosity coefficient of air. The results of naca0018 airfoil with *Re* <sup>=</sup> <sup>5</sup> <sup>×</sup> <sup>10</sup><sup>5</sup> (the wind speed is assumed to be 20 m/s) are compared with the existing experiments to ensure the accuracy of the numerical results. The general view of the geometry and its characteristic parameters are as follows:



The aim of this paper is to understand the influence of geometric parameters on aerodynamic performance. The parametric design of the wingsail is selected and described in Figure 4. In order to simplify the general problem, it has been decided to focus on the flap deflection angle *d*, the position of flap rotation axis in the direction of the wing chord Xr, and flap thickness e2/c2. The angle of attack (α) of the wing represents the angle of attack (AOA) of the wingsail, as seen in Figure 4.

Based on the preliminary simulation and previous research [11,18], the initial wingsail configuration has chord ratio c1/c<sup>2</sup> = 3:2, wing thickness e1/c<sup>1</sup> = 18%, flap thickness e2/c<sup>2</sup> = 15%, one flap rotation axis Xr/c<sup>1</sup> = 85%, slot width g/c<sup>1</sup> = 2.4%. Its name will be r1.5t1815 X85g2.4. Adding the angle of attack (α) and flap deflection angle (*d*) after this name, and the full configuration name of the wingsail will be r1.5t1815 X85g2.4α6 d15, for α = 6 ◦ , *d* = 15◦ .

**Figure 4.** Wingsail geometry and parameterization. **Figure 4.** Wingsail geometry and parameterization. *J. Mar. Sci. Eng.* **2019**, *7*, x FOR PEER REVIEW 5 of 16
