Efficient CFD simulations for speed and delivered power in waves at full scale

Magazine article, 2023

Computational Fluid Dynamics (CFD) is presently an accepted tool for predicting calm water resistance and power at model scale. The status of CFD for these applications is well covered by the International Workshops on Numerical Ship Hydrodynamics, see Larsson et al. (2014) and Hino et al. (2021). However, like in model testing, the results must be extrapolated to full scale. This extrapolation is burdened by inaccuracies due to many assumptions. Recently, the interest has shifted towards direct full-scale predictions, and the currently most comprehensive validation exercise is performed within the Joint Research Project, ‘‘Development of Industry-Recognised Benchmark for Ship Energy Efficiency Solutions’’, JoRes (2023). Furthermore, ship designers place greater emphasis on real-world conditions, which include environmental factors like wind and waves. Such simulations are still time consuming and computationally expensive when performed using solvers based on the unsteady Reynolds-Averaged Navier Stokes Equations (RANS). However, selecting the most advanced methods is not always feasible or necessary. Through careful verification and validation studies, we can identify and use more efficient methods with confidence. In this article, we introduce a hybrid potential flow/RANS method for predicting the powering performance, including speed loss estimation, of a full-scale ship in irregular waves. The software used is SHIPFLOW, developed and marketed by FLOWTECH International AB in Sweden. FLOWTECH is a spin-off company from Chalmers University of Technology and has provided CFD software to the Marine industry since 1989. A newly developed potential flow Boundary Element Method (BEM), Kjellberg (2023), is used in connection with a steady-state RANS solver, Orych (2021). More than an order of magnitude is saved in computational time compared to unsteady RANS, while maintaining accuracy for sea states typical of model testing. This method should interest ship designers who want to minimize required engine power in waves. Additionally, classification societies may find it valuable due to its capability to assess the speed loss in regulations for the Energy Efficiency Design Index (EEDI).