*Article* **Viscous Damping Identification for a Wave Energy Converter Using CFD-URANS Simulations**

**Marco Fontana \*,†, Pietro Casalone \*,†, Sergej Antonello Sirigu †, Giuseppe Giorgi †, Giovanni Bracco † and Giuliana Mattiazzo †**

Department of Mechanical and Aerospace Engineering, Polytechnic of Turin, C.so Duca degli Abruzzi, 24, 10129 Turin, Italy; sergej.sirigu@polito.it (S.A.S.); giuseppe.giorgi@polito.it (G.G.); giovanni.bracco@polito.it (G.B.); giuliana.mattiazzo@polito.it (G.M.)

**\*** Correspondence: marco\_fontana@polito.it (M.F.); pietro.casalone@polito.it (P.C.)

† These authors contributed equally to this work.

Received: 19 April 2020; Accepted: 14 May 2020; Published: 17 May 2020

**Abstract:** During the optimization phase of a wave energy converter (WEC), it is essential to be able to rely on a model that is both fast and accurate. In this regard, Computational Fluid Dynamic (CFD) with Reynolds Averaged Navier–Stokes (RANS) approach is not suitable for optimization studies, given its computational cost, while methods based on potential theory are fast but not accurate enough. A good compromise can be found in boundary element methods (BEMs), based on potential theory, with the addition of non-linearities. This paper deals with the identification of viscous parameters to account for such non-linearities, based on CFD-Unsteady RANS (URANS) analysis. The work proposes two different methodologies to identify the viscous damping along the rotational degree of freedom (DOF) of pitch and roll: The first solely involves the outcomes of the CFD simulations, computing the viscous damping coefficients through the logarithmic decrement method, the second approach solves the Cummins' equation of motion, via a Runge-Kutta scheme, selecting the damping coefficients that minimize the difference with CFD time series. The viscous damping is mostly linear for pitch and quadratic for roll, given the shape of the WEC analysed.

**Keywords:** wave energy; computational fluid dynamics; identification; viscous damping; URANS
