Investigations of flow characteristics around ship hulls at large drift angle are very important for understanding the motion behavior of ships during maneuvers. At large drift angles, the flow is dominated by strong vortical structures and complex three-dimensional separations. An accurate prediction of these flow structures is still a challenge for modern computational fluid dynamics (CFD) solvers. Hull forms with high block coefficients are blunt and have strong curvatures, which leads to large area flow separations over smooth surfaces. These areas are sensitive to the relative angle between the flow and the ship motion direction. The paper is concerned with a collaborative computational study of the flow behavior around a double model of KVLCC2 at 30 degrees drift angle and Fr=0 condition, including analysis of numerical methods, turbulence modeling and grid resolution, and their effects on the mean flow and separation onset as well as formation of the vortical structures. This research is an outcome of a multi-year collaboration of five research partners from four countries. The overall approach adopted for the present study combines the advantages of CFD and EFD with the ultimate goal of capturing the salient details of the flow around the bluff hull form. The experiments were performed at the low - speed wind tunnel of the Hamburg University of Technology (TUHH). The main features of the global and local flow were captured in the experimental study. To determine the global flow characteristics, two different flow visualization techniques were used. The first one is a smoke test, which allows the visualization of vortex structures in vicinity of the ship model. The second test is a classic oil film method, which yields the direction of the limiting wall streamlines on the surface of the model. The analysis of the experimental results helped identify the separation zones on the ship model. To resolve the local flow-fields, LDA and PIV measurements were carried out in a selected number of measuring sections. Subsequently, the EFD and CFD results for the global and local flow structures were compared and analyzed. The numerical simulations were carried out by 5 institutions: Iowa Institute of Hydraulic Research of the University of Iowa (IIHR), USA, Maritime Research Institute Netherlands (MARIN), The Netherlands, Hamburg University of Technology (TUHH), Germany, Naval Surface Warfare Center, Carderock Division (NSWCCD) West Bethesda, USA and Swedish Defense Research Agency (FOI), Sweden. For the comparison with the experimental results, seven submissions of steady and unsteady CFD results are included in the present study. The participating codes include CFDShip-Iowa, ReFRESCO, FreSCo+, Edge, OpenFOAM (FOI) and NavyFoam. The size of the computational grids varies between 11 and 202 million control volumes or nodes. The influence of turbulence modeling on the predicted flow is studied by a wide variety of models such as isotropic eddy viscosity models of k-w family, Explicit Algebraic Reynolds Stress Model (EARSM), hybrid RANS-LES (DES), and LES. Despite notable differences in the grid resolutions, numerical methods, and turbulence models, the global features of the flow are closely captured by the computations. Noticeable differences among the computations are found in the details of the local flow such as the vortex strength and the location and extent of the flow separations.
Flow characteristics and vortical structures of a free jet issuing from rectangular nozzle with four tapered triangular tubes
