Experimental investigations into the effect of at-sea conditions on ship airwake characteristics

Abstract

Shipboard helicopter operations are significantly impacted by the characteristics of the ship airwake which depend on environmental conditions in addition to the ship superstructure. In this paper, the effect of free stream turbulence, wind speed and ship motion on the airwake frequency spectra of two 1:50 scaled ships with stern flight-decks was investigated using wind tunnel testing. Results from a full-scale sea trial are also presented. Using both the wind tunnel and the sea trial data in ship motion conditions corresponding to medium sea states, results showed the ship motion energy was transmitted to the ship airwake at the motion frequencies. For single-frequency ship motions, the energy transfer occurred at the motion frequency only, whereas for more realistic at-sea ship motions with a range of frequency components the energy was transferred at a broadband of frequencies corresponding to the motion frequency range. The way in which realistic motion component effects combine at sea and the amount of energy added to the airwake in realistic conditions is presented. The impact of incoming atmospheric turbulence was also examined.