The wave influence on wind pressure fluctuation of drifting rescue units

Abstract

In the article the stochastic model of wave influence on wind loading fluctuation of drifting rescue units is presented. Wind speed is the single most important factor when trying to determine basic wind pressure. Wind motion is turbulent, and it is difficult to give a concise mathematical definition of turbulence. However, it is known that wind turbulence exists due to the lower viscosity of air in comparison with water. Any air motion faster than 4 km/h is turbulent; i.e., air particles move erratically in all directions. Basic wind pressure is affected by the uncertainty effect caused by the likelihood of the wind hitting the drifting rescue unit from any given direction. This parameter is known as the directionality effect. The pressure exerted by strong winds on the life raft is a function of the dynamic part of Bernoulli’s equation, known as basic pressure, which is modified by the following factors: wind direction according to a life raft axis system, wind speed, the life raft height, wave height, wave slope angle. Two random factors were considered. The first one is the drifting rescue unit heeling angle to the horizontal plane. The second one is connected with the position of drifting rescue unit on wave slope. The results obtained during laboratory tests in wind tunnel were used to model wind pressure on a life raft. The measurements of a life raft movement on waves obtained during sea experiments were used to estimate the distribution of pitch and roll angle. The position of drifting rescue unit on a wave slope has uniform distribution. The wind load coefficients for life rafts presented in this paper are derived from wind tunnel tests in uniform flow obtained at the Aviation Institute in Warsaw. Data of life raft movements on waves have been collected during full size experiments at sea. Data from wind tunnel test are the basis of knowledge of wind loads on drifting rescue units.