Statistical Analysis on The Near-Wake Region of RANS Turbulence Closure Models for Vertical Axis Tidal Turbine

Muhammad Wafiuddin Abd Rahim (Faculty of Mechanical Engineering and Technology, Universiti Malaysia Perlis (UniMAP), Pauh Putra Main Campus, 02600 Arau, Perlis)
Anas Abdul Rahman (Faculty of Mechanical Engineering and Technology, Universiti Malaysia Perlis (UniMAP), Pauh Putra Main Campus, 02600 Arau, Perlis)
Ayu Abdul-Rahman (Department of Mathematics and Statistics, School of Quantitative Sciences, Universiti Utara Malaysia, 06010 UUM, Sintok, Kedah)
Muhammad Izham Ismail (Institute of Engineering Mathematics, Universiti Malaysia Perlis (UniMAP), Pauh Putra Main Campus, 02600 Arau, Perlis)
Mohd Shukry Abdul Majid (Faculty of Mechanical Engineering & Technology, Universiti Malaysia Perlis (UniMAP), Pauh Putra Main Campus, 02600 Arau, Perlis)
Nasrul Amri Mohd Amin (Faculty of Mechanical Engineering & Technology, Universiti Malaysia Perlis (UniMAP), Pauh Putra Main Campus, 02600 Arau, Perlis)

International Journal of Renewable Energy Development
Vol 12, No 1 (2023): January 2023

The flow field in the near wake region (up to six turbine diameters downstream) of a tidal current turbine is strongly driven by the combined wake of the device support structure and the rotor. Accurate characterisation of the near-wake region is important, but it is dominated by highly turbulent, slow-moving fluid. At present, limited number of research has been undertaken into the characterisation of the near-wake region for a Vertical Axis Tidal Turbine (VATT) device using the Reynolds Averaged Navier Stokes (RANS) model in the shallow water environment of Malaysia. This paper presents a comprehensive statistical analysis using the Mean Absolute Error (MEA), Mean Squared Error (MSE) and Root Mean Squared Error (RMSE) on the near-wake region for shallow water application by comparing numerical solutions (i.e., different types of RANS turbulence models using Ansys Fluent) with published experimental data. Seven RANS turbulence models with a single VATT, represented by using a cylindrical object, were employed in the preliminary study. The statistical analysis performed in this study is essential in exploring and giving a detailed understanding on the most suitable RANS turbulence model to be improved, specifically on its near-wake region. In this study, the near wake region is defined as D ≤ 6, where D is the device diameter. The analysis shows that the RANS numerical solutions are unable to accurately replicate the near-wake region based on large statistical errors computed. The average RMSE of near-wake region at z/D = [2, 3, 4, 6] are 0.5864, 0.4127, 0.4344 and 0.3577 while the average RMSE at far-wake region z/D = [8, 12] are 0.2269 and 0.1590, where z is the distance from the cylindrical object along the length of domain. The statistical error values are found to decrease with increasing downstream distance from a cylindrical object. Notably, the standard k–ε and realizable k–ε models are the two best turbulent models representing the near-wake region in RANS modelling, yielding the lowest statistical errors (RMSE at z/D = [2, 3, 4, 6] are 0.5666, 0.4020, 0.4113 and 0.3455) among the tested parameters