**Krzysztof Sobczak \* , Damian Obidowski , Piotr Reorowicz and Emil Marchewka**

Institute of Turbomachinery, Lodz University of Technology, 90-924 Lodz, Poland;

damian.obidowski@p.lodz.pl (D.O.); piotr.reorowicz@p.lodz.pl (P.R.); emil.marchewka@dokt.p.lodz.pl (E.M.)

**\*** Correspondence: krzysztof.sobczak@p.lodz.pl; Tel.: +48-42-631-2362

Received: 9 June 2020; Accepted: 16 July 2020; Published: 19 July 2020

**Abstract:** Savonius wind turbines are characterized by various advantages such as simple design, independence of wind direction, and low noise emission, but they suffer from low efficiency. Numerous investigations were carried out to face this problem. In the present paper, a new idea of the Savonius turbine with a variable geometry of blades is proposed. Its blades, made of elastic material, were continuously deformed during the rotor revolution to increase a positive torque of the advancing blade and to decrease a negative torque of the returning blade. In order to assess the turbine aerodynamic performance, a two-dimensional numerical model was developed. The fluid-structure interaction (FSI) method was applied where blade deformations were defined by computational solid mechanics (CSM) simulations, whereas computational fluid dynamics (CFD) simulations allowed for transient flow prediction. The influence of the deformation magnitude and the position of maximally deformed blades with respect to the incoming wind direction were studied. The aerodynamic performance increased with an increase in the deformation magnitude. The power coefficient exceeded *Cp* = 0.30 for the eccentricity magnitude of 10% and reached 0.39 for the highest magnitude under study. It corresponded to 90% improvement in comparison to *Cp* = 0.21 in the case of the fixed-shape Savonius turbine.

**Keywords:** vertical axis wind turbine (VAWT); Savonius turbine; deformable blades; power coefficient; blade load; computational fluid dynamics (CFD); fluid-structure interaction (FSI)
