CFD computations are conducted at full scale in order to determine the airflow around the above-water hull and the superstructure of the vessel. These studies highlight the local wind conditions and the exhaust dispersion on the relevant open decks (especially helideck) and at other locations (e.g. path of helicopter approach, air intakes for engine and ventilation).

The CFD model retains all components of the vessel above the water line with influence on the airflow, including the major air intakes, while small-scale details are omitted (e.g. railings, antennae). The oncoming wind profile is approximated by a standard atmospheric power law unless the customer specifies a different wind profile.

The computations are performed using HSVA'’s flow solver FreSCo+, a finite-volume code well-suited for the simulation of turbulent flow around complex geometries. For highly unsteady aerodynamics simulations, a hybrid RANS-LES turbulence treatment is applied: In this so-called DES approach, the dominant energetic eddies in the bulk of the flow field are resolved in a LES sense, while the near-wall behaviour is approximated by a RANS k-ω-SST closure. The dispersion of exhaust gases over the decks is included via a two-phase volume-of-fluid (VOF) method.

Time averaged flow field, typical of RANS computation results.

Instantaneous flow field from a DES computation.

These DES simulations are known to be superior to the present industrial standard of RANS-based computational aerodynamics. Especially the simulation of exhaust-gas transport is significantly improved when a DES turbulence treatment is considered instead of a pure RANS approach.

The results indicate potentials of design improvement for efficiency and safety (e.g. funnel design). Typical results include:

• Visualisation of local wind magnitude and direction in planes of interest (e.g. mid-ship plane) by vectors, streamlines and colour plots, highlighting wakes/dead-air regions and vortices
• Monitoring of instantaneous and mean local wind speed and angle at points of interest by “"virtual anemometers"”, distributed over the open deck spaces (especially helideck) as specified by the customer.
• Instantaneous and mean wind forces and moments on the vessel (wind resistance, drift and yaw); longitudinal and lateral wind-load coefficients cx and cy for extrapolation to different wind speeds
• Videos and snapshots of exhaust wash-down from the funnel pipes and transport over the deck areas (helideck, work areas)
• Instantaneous and mean local exhaust concentrations [ppm] measured by “"virtual gas detectors"” at points of interest (e.g. air intakes, helideck). Concentrations [ppm] of single exhaust components (e.g. NOx, SO2) are deduced based on the exhaust composition (customer input or literature data)