*Article* **3D Numerical Study of the Impact of Macro-Roughnesses on a Tidal Turbine, on Its Performance and Hydrodynamic Wake**

**Ilan Robin \*, Anne-Claire Bennis and Jean-Claude Dauvin**

Morphodynamique Continentale et Côtière (UMR CNRS 6143), Université de Caen Normandie, Rouen Normandie, 14000 Caen, France; anne-claire.bennis@unicaen.fr (A.-C.B.); jean-claude.dauvin@unicaen.fr (J.-C.D.)

**\*** Correspondence: ilan.robin@unicaen.fr

**Abstract:** Biofouling is an important factor to consider when calculating the energetic efficiency of tidal farms. Despite the fact that biofouling effects have been widely investigated in the past for naval applications, very few studies concern tidal turbines. This paper proposes a numerical approach to assess the impact of biofouling on tidal turbines, which is efficient for testing many configurations. Two turbulence models are tested (RANS k-*ω* SST and LES Smagorinsky) for the motionless blade case to validate them. Then we chose to use the Smagorinsky model for the case of a complete tidal turbine rotor with realistically fouled blades. The pressure coefficient is strongly affected by the barnacle in the motionless blade case and the power coefficient is slightly degraded in the complete rotor case. Motionless blade cases do not represent the real biofouling behaviour for two reasons. First, sessile species settle in the down flow part of the chord where their impact is less important. Then, the surrounding turbulence provoked by the blades rotation in the rotor case reduces the impact of biofouling. In the wake, biofouling generates small vortexes that propagate into the larger ones, causing them to spread their energy.

**Keywords:** marine renewable energy; tidal energy; fluid–structure interaction; biofouling; turbulence; numerical modelling
