Comparison of PANS and LES of the flow past a generic ship

Abstract

The prediction of flow past a generic ship has been performed using partially-averaged Navier-Stokes (PANS) turbulence model. The Reynolds number based on the width of the ship is × 8 10 4. A detailed comparison with resolved large-eddy simulation (LES) solution and available experimental data is made for a better understanding of the capability of PANS in predicting the turbulent flow in the wake. The results show that PANS produces similar trends to those predicted by LES. However, a coarse grid resolution will lead PANS to act as unsteady Reynolds-averaged Navier-Stokes (URANS). Although the geometry is symmetric for the incoming flow at a zero yaw angle, bi-stable behaviour is observed in the wake, contributing to an asymmetric flow distribution on the heli-deck.

Qualitative assessment of the bi-stable states in the wake of a finite-width double backward facing step

“…Flow state II (shown by dashed black lines) is transposed to compare directly with flow state I (continuous black lines), and these are compared with the data at identical locations from the numerical study of Syms (2008) (indicated by the continuous blue line) using Lattice-Boltzmann method (LBM), and the experimental data from Aerodynamics Laboratory of the National Research Council, Canada (NRC) at Re ¼ 5:59 Â 10 5 are indicated by the continuous red lines. Despite the larger Reynolds number in the latter study as compared to that of Zhang et al (2018), the flow asymmetry on the top step is well-predicted at the two streamwise locations, further indicating that the bi-stables states are observed at realistic Reynolds numbers, where ships and other bluff bodies operate.…”

“…The length of the recirculation bubble is uniform across AE0:2W of the vertical midplane as seen in Fig. 16 of Zhang et al (2018), and does not vary significantly from that computed in the vertical midplane. The recirculation lengths, X R1 and X R2 in flow state II are approximately 6% and 3.2% longer as compared to flow state I.…”

“…In each of the two flow states, the streamwise vortex on the side of the larger vortex (and closer to the base) on the top step is larger, and persists further downstream as compared to its counterpart across the vertical midplane (the flow topology on the top step is shown in Fig. 17 of Zhang et al (2018) and in Yuan et al (2018)). This asymmetry in the strength of the vortices can be explained based on the isosurfaces of the velocity contours in Fig.…”

“…The spatial resolution and construction of mesh for the base cavity case is based on mesh M1, and consists of approximately 10.4 million hexahedral elements. For details of the mesh construction, distribution of the elements around the ship and spatial resolution studies the reader is referred to Zhang et al (2018). The computational domain has a cross-sectional area of 10W Â 10W, leading to a blockage ratio of less than 1.5% based on the ratio of the frontal area of the ship to the cross-sectional area of the domain.…”

“…Recent investigations by Zhang et al (2018) using large eddy simulations (LES) and partially-averaged Navier-Stokes (PANS) equations, showed the existence of the two flow states in the flight deck region of an idealised ship/frigate model (Mora and Meseguer, 2015). Each of the complementary flow states were observed on meshes of different spatial resolution; M1 and M2 consisting of ' 10 million and ' 21:5 million elements, respectively, with both meshes having adequately resolved the air-wake.…”