Simulation of SFS2 using Embedded LES: Part-II Understanding the asymmetry in bistable airwake at zero yaw

Abstract

The turbulent flow field of simplified frigate model SFS2 is numerically studied using an embedded Large Eddy Simulation (ELES) turbulence model in Ansys Fluent® 19.0 environment for its characterization. The focus of this research work is to understand the underlying reasoning of the generation of bistable airwake of the superstructure and to ascertain the influence of its upstream flow field such as bow region turbulent vortical structures, if any, on the nature and characteristics of airwake. The asymmetry in the mean airwake behind superstructure was attributed to “locking” and/or differential momentum flux on sides of superstructure in literature, however no concrete evidence of causation was reported. In this research, the in-depth analysis of turbulent boundary layer buildup, vorticity distribution, relative size and orientation of vortices on and along starboard and port sides of superstructure provide clear understanding and evidence of the influence of bow region flow field on airwake. It has been observed that dynamics of bow leading edge vortices and horseshoe vortex generated upstream of superstructure forward facing step, and their interaction are the main contributor towards the generation of asymmetrical airwake. The asymmetrical bias towards starboard side, which has also been observed in the previous experimental and numerical studies, can also be explained from this finding. That is the relatively larger turbulent eddies on starboard side generates more entrainment of the freestream upon reaching the flight deck compared to the portside. Later, the correlation and spectral analyses of the velocity measurements recorded on bow region and in the superstructure airwake supplements and validates the influence of bow region turbulent vortical structures on the airwake as dominant frequencies associated with bow vortical structures are also observed in airwake spectrum.