**Preface to "Aero/Hydrodynamics and Symmetry"**

The existence of symmetry and its tendency to break in aero/hydrodynamics applications are two of the most important aspects of many engineering fields, such as mechanical, aerospace, chemical and process engineering. For instance, the existence of symmetry breaking at a critical Reynolds number confirmed the existence of a bifurcation in expansion pipe flows. Such a symmetry breaking mechanism may cause the appearance of turbulence, which in return increases the mixing, as well as the required pumping power, for several process engineering design applications. Meanwhile, in aerospace applications, the receptivity of symmetric laminar flow to internal/external perturbations may cause flow transition and dramatic change in a local drag coefficient, and heat removal from the surface. The latter needs to be considered in the design step for choosing proper materials that can also bear the unbalanced thermodynamics loads.

The applications of symmetry and its breaking are usually inter-disciplinary, and prior knowledge of them is crucial for many real-life applications. Therefore, the current Special Issue, "Aero/Hydrodynamics and Symmetry", invites original and review works in the field for participation. The scope of this Special Issue includes, but is not limited to, the state of the art computational, theoretical and experimental works that deal with symmetry and its breaking, that are in line with aero-hydrodynamics applications. Recent advances in numerical, theoretical and experimental methodologies, as well as finding new physics, new methodological developments and their limitations, are within the scope of the current Special Issue. Potential topics which are deemed suitable for publication include, but are not limited to:


**Mostafa Safdari Shadloo** *Editor*

*Article*

### **Numerical Investigation of the Savonius Vertical Axis Wind Turbine and Evaluation of the E** ff**ect of the Overlap Parameter in Both Horizontal and Vertical Directions on Its Performance**

#### **Mohammad Ebrahimpour 1, Rouzbeh Shafaghat 1,\*, Rezvan Alamian 1 and Mostafa Safdari Shadloo 2**


Received: 28 May 2019; Accepted: 19 June 2019; Published: 21 June 2019

**Abstract:** Exploiting wind energy, which is a complex process in urban areas, requires turbines suitable for unfavorable weather conditions, in order to trap the wind from di fferent directions; Savonius turbines are suitable for these conditions. In this paper, the e ffect of overlap ratios and the position of blades on a vertical axis wind turbine is comprehensively investigated and analyzed. For this purpose, two positive and negative overlap situations are first defined along the X-axis and examined at the di fferent tip speed ratios of the blade, while maintaining the size of the external diameter of the rotor, to find the optimum point; then, the same procedure is done along the Y-axis. The finite volume method is used to solve the computational fluid dynamics. Two-dimensional numerical simulations are performed using URANS equations and the sliding mesh method. The turbulence model employed is a realizable K-ε model. According to the values of the dynamic torque and power coe fficient, while investigating horizontal and vertical overlaps along the X- and Y-axis, the blades with overlap ratios of HOLR = +0.15 and VOLR = +0.1 show better performances when compared to other corresponding overlaps. Accordingly, the average C m and Cp improvements are 16% and 7.5%, respectively, compared to the base with a zero overlap ratio.

**Keywords:** Savonius vertical axis wind turbine; horizontal overlap ratio; vertical overlap ratio; torque coe fficient; power coe fficient
