Compressible Air Effects in CFD Simulations of Free Fall Lifeboat Drop

Abstract

A free fall lifeboat is typically dropped from heights between 30 and 40m, but during full scale testing, drop heights have been exceeding 60 m. During a drop, the free fall lifeboat is going through several phases; sliding on the skid, rotation on skid, free fall, water entry, ventilation, maximum submergence, resurfacing and the sailing phase. During impact and submergence the lifeboat will go through a ventilation phase. In this phase the lifeboat creates an air cavity behind the pilot house and a larger one behind the stern. The air cavity formed on top of canopy, aft of the pilot house is compressed and then starts to oscillate in time giving rise to large oscillating pressures. Computational Fluid Dynamics (CFD) simulations with compressible air flow model are compared with full scale experimental results. The results compare very well, both in oscillating frequency and amplitude. Later in the ventilation phase, an air bubble aft of the vessel will be drawn down several meters below the water surface. This bubble collapses in an imploding manner and slams with large pressures on the aft bulkhead of the lifeboat. This is simulated in CFD with compressible air model and compares very well to the full scale experimental results. Using incompressible air flow will not capture these effects, and the calculated pressures on the aft bulkhead are a fraction of the real pressures. The water surface in the cavity eventually hits the aft bulkhead with a high slamming velocity. The structure of the air pocket does not have any symmetry, and seem to be chaotic in nature. The local pressures can be very high and distributed over a small area. The effects of the air cavity also influence the motion and acceleration of the lifeboat in the ventilation phase; this is shown and compared to full scale experimental results.