The use of detached eddy simulation in ship hydrodynamics

Accurate numerical simulation of the flow around the hull of a ship remains a challenging task. This work examines whether this is as a result of the inherent unsteady turbulent flow regime influencing the position and strength of vortex systems shed in the wake. To this end the behaviour of the flow around a truncated cylinder is taken as an exemplar of flow at the stern of a ship. Wind tunnel tests of the flow around a KVLCC2 hull form were made to measure the flow field at the propeller plane using particle image velocimetry. Comparison of the instantaneous and the mean flow field indicate significant low frequency effects with large deviations form the mean flow. Such behaviour is also found in the wake of an aspect ratio 1 truncated cylinder. The approach at capturing the on and off body flow field is compared with detailed experiments and previous calculations of a Reynolds Averaged Navier Stokes with k-e model and Large Eddy Simulation (LES). THe results clearly show significant improvements with the use of DES. Further work is on-going to apply the DES approach to a full ship simulation

An experimental and computational study of three dimensional unsteady flow features found behind a truncated cylinder

The numerical prediction of three-dimensional turbulent separation regions and the resulting unsteady vortical flow patterns within these regions is still poor. This paper presents detailed experimental data for steady onset flow around a truncated cylinder of height/diameter ratio of 1.0, mounted on a ground plane. The performance of Large Eddy Simulation (LES) and Unsteady Reynolds-Averaged Navier Stokes (URANS) methods are compared for this case. It is seen that on an identical grid, the LES simulations predict the separation region more accurately than the URANS model and at 75% of the computational cost

Developments in the use of large eddy simulation for ship hydrodynamics

A summary is given of the work being carried out at the University of Southampton on the use of large-eddy simulation (LES) for marine hydrodynamics. The case of a truncated cylinder on a ground plane has been used to test an in-house code by comparisons with experimental data. This has shown that the LES model performs well in separated and vortical flow regions, but has limitations in its ability to capture boundary layer flow. Further wind tunnel experiments also used particle image velocimetry (PIV) to measure the instantaneous flow field in the stern region of the KRISKO VLCC tanker form. LES simulations of an elliptical body, of similar proportions to the tanker form, illustrate similarities both to the VLCC flow structure and to that of the truncated cylinder. In particular, for all three geometries the instantaneous flow field contains many more large-scale turbulent structures in the wake with a much greater range of vorticity magnitude than in the mean vorticity field. This suggests that LES may give more accurate results than a steady RANS method for ship flows, if the computational cost of resolving the boundary layer can be reduced. Initial simulations of a two-dimensional cylinder flow have been carried out using detached-eddy simulation 9DES), a hybrid RANS/LES technique, which is less demanding of grid resolution than full LES, and which could offer such a route

Measurements of the flow over a low-aspect-ratio cylinder mounted on a ground plane

The flow over a finite-height cylinder of aspect ratio 1, with one end mounted on a ground plane and the other end free, has been studied by means of surface flow visualisation, particle image velocimetry (PIV) and surface pressure measurements. The diameter-based Reynolds number was 200,000. The mean flow topology has been identified in three areas: the horseshoe vortex system, the separated flow over the free-end and the wake region. Evidence is shown for the existence of a three-horseshoe vortex system, while the mean flow over the free-end consists of an arch vortex with its bases on the forward half of the free-end. There are two tip vortices coming off the free-end. The wake region is found to be highly unsteady, with considerable variation from the mean flow

Unsteady simulations of the flow around a short surface-mounted cylinder

The flow around a surface-mounted circular cylinder, of height/diameter ratio 1 with a free end, is simulated using large-eddy simulation (LES) and detached-eddy simulation (DES) at a Reynolds number based on diameter of 200 000. A comparison is made between the abilities of the two models to capture flow features observed in particle image velocimetry (PIV) experiments carried out by the authors. The flow contains three interacting features formed from the junction flow between the cylinder and the ground, separation from the cylinder wall and resultant turbulent wake, and the flow over the free-end of the cylinder. Both LES and DES overpredict the length of the recirculation region by 30%, but the turbulence quantities are close to the measured values. The topology of the flow over the free-end is confirmed as consisting of an arch or 'mushroom' vortex. Due to the high Reynolds number the grid resolution is insufficient to resolve the approaching ground-plane boundary layer flow with LES, leading to inaccuracies in the horseshoe vortex system. The DES model improves this area, though still has grid induced separation effects

Laboratory measurements of vortex-induced vibrations of a vertical tension riser in a stepped current

This paper presents an initial analysis of measurements of the vortex-induced vibrations of a model vertical tension riser in a stepped current. The riser, 28mm in diameter, 13.12m long and with a mass ratio (mass/displaced mass) of 3.0, was tested in conditions in which the lower 45% of it was exposed to a uniform current at speeds up to 1m/s, while the upper part was in still water. Its response in in-line and transverse directions was inferred from measurements of bending strains at 32 points along its length. Transverse vibrations were observed at modes up to the 8th with individual modal amplitudes up to about 80% of the riser’s diameter. However, in most cases the response included significant contributions from several modes, all at the same frequency. Some evidence was found of lock-in