Comparison of NATO-GD and SFS2 Ship Airwakes Using Low-Rank Models*

Spectral analysis and reduced-order modeling were used to compare the airwake characteristics across multiple frequency bands of two generic Navy ship shapes, SFS2 and NATO-GD. Measurements were carried out in a low-speed wind tunnel at a Reynolds number of 3.2 million based on ship length. Two different incident wind conditions, namely a uniform headwind and a simulated atmospheric boundary layer (ABL) were considered. In the case of the uniform headwind, both airwakes exhibited similar turbulent kinetic energy (TKE) levels at low frequencies (St < 0.5 based on ship height) distributed among the first ten modes. When an ABL was introduced, the NATO-GD airwake distributed more TKE among higher frequencies but kept a uniform TKE distribution among the modes. The SFS2 airwake showed a significant TKE increase only in the first two modes at the lowest frequency band (St < 0.1), indicating a more substantial contribution from coherent structures. These opposite effects highlight significant differences in the sensitivity of each airwake to changes in upstream conditions.