Evaluation of resistance, sinkage, trim and wave pattern predictions for JBC

Kapitel i bok, 2021

JBC predictions are presented for four cases: towed and self-propulsion for the bare hull and a hull with an Energy Saving Device (ESD). A statistical evaluation is made based on the 88 resistance predictions submitted. The comparison error is defined as the difference between the measured data and the numerically predicted value. This error is analyzed in different ways. It is seen that the mean signed error is as small as the measurement accuracy, while the mean absolute error is about twice as large. This represents the typical error in the prediction. A similar accuracy was found in the 2010 Workshop. The number of grid cells has increased since 2010 and the required grid size for a given uncertainty of 4% has increased from 3 M cells to 10 M cells. Reasons for this are discussed. As in the previous workshop the two-equation turbulence models produce more accurate results than the more advanced models, although the more and more popular Explicit Algebraic Stress Model (EASM) is close. A surprising result of the analysis is that methods with wall-functions give significantly smaller resistance errors than those with a wall-resolved flow. Grid convergence is discussed and it is shown that the vast majority of results converge with grid refinement, but that the achieved order of accuracy is often far from the theoretical one. Sinkage is much better predicted than in 2010 and the trim results are also improved. Finally, the wave pattern prediction is discussed. The best methods in 2010 predicted the waves extremely well, in fact better than in the present workshop. A consistent shift in the predicted wave phase relative to the data could indicate a measurement error.