Abstract
Wind over the ocean can be divided into two parts: the mean wind and the unsteady, time-varying wind. The unsteady component of the wind profile is not often included when modeling wind and predicting its impact on ship motions, but may have an effect, particularly in extreme cases. The mean velocity profiles for the wind have been thoroughly investigated and documented, however the time varying portion is not as well understood, mostly due to insufficient data.
A portion of the time-varying wind is due to wind gusts, which are typically modeled as a Gaussian stochastic process and can be fully described by a wind gust spectrum [Ocean Engineering 34 (2007), 354–358]. Wind gust spectra measured over land typically have less energy at lower frequencies than measured spectra over the ocean, and this low frequency energy can be important in design of ships and offshore structures. Several wind gust spectra have been proposed, with various dependencies on frequency, height above sea level, and mean wind speed, including Ochi [Proceedings of 20th Offshore Technology Conference, Vol. 2, 1988, pp. 461–467], Wills [Marine Structures 19 (2006), 173–192] and Froya [Marine Structures 19 (2006), 173–192]. These spectra are compared with data collected from the High Resolution Air Sea Interaction (HIRES) research program, sponsored by the Office of Naval Research (ONR), from surface buoy data at 1 m as well as from the R/P FLIP at 30 m above sea level. The various spectra from the literature bound the data at the 1 m level, but all tend to over predict the spectra at the 30 m level.
In addition to gusts, the mean wind profile may be modified by the time-varying wave profile, another aspect of wind behavior that can affect the design of ships and offshore structures. Air flow separation over water waves has been observed by many researchers over the years, though a global criterion for separation and a model for the separated flow have not yet been conclusively determined. Evidence in the literature shows that both the wave age and wave slope play a role in separation (for example, in: Proceedings of the ASME 2010 29th International Conference on Ocean, Offshore, and Arctic Engineering, 2010).
A separation criterion is proposed, based on data from the literature, as well as a model for the adjusted wind profile based on a separation profile developed for land. The adjusted profile compares well with data from the literature.
The criterion and proposed model are also compared with preliminary results from the ONR supported computational fluid dynamics code, Numerical Flow Analysis (NFA). The objective of this numerical effort is to research and develop the capability to couple advanced free-surface hydrodynamic predictions with numerical wind distributions in a marine atmospheric boundary layer. By developing a wind/wave boundary interaction model for the Numerical Flow Analysis (NFA) code, an important step towards carrying out this objective is achieved. It is the final goal of this development to provide a wind/wave interaction prediction capability that includes the possibility of correctly predicting the impact of breaking ambient waves.