Forward Speed Calm Water Roll Decay for Surface Combatant 5415: Global and Local Flow Measurements

Global and local flow measurements for forward speed calm water roll decay are performed in a towing tank for surface combatant model 5415 for both bare hull (BH) and bilge keel (BK) conditions. Roll motion is oscillatory with underdamped exponential decay and linear with respect to initial roll angles less than the average initial roll angle (9°). For larger mean roll angles (> 3°), damped natural frequency is a few percent below the hydrostatic natural roll frequency with larger values for BH than BK condition and increasing Froude number (Fr), whereas for smaller mean roll angles, it sharply increases toward the hydrostatic natural roll frequency. For larger mean roll angles, logarithmic decrement and linear damping linear increase with larger values for BK than BH condition and increasing Fr, whereas for smaller mean roll angles, they sharply increase. For increasing Fr, mean and initial roll angle averaged mean roll angle decreases by 50%; damped natural frequency increases by 6% with larger values for BH than BK condition; and logarithmic decrement/linear damping coefficient increase by a factor of three/four with larger values for BK than the BH condition. Logarithmic decrement and Himeno method linear damping coefficients are qualitatively similar. Nonlinear damping coefficient is two orders of magnitude smaller than linear damping coefficient. Roll reconstruction errors are smallest for Himeno with linear and nonlinear damping. The phase-averaged wave pattern and velocity and axial vorticity fields at x/L = 0.675 initially show larger amplitudes followed by oscillatory exponential decay. Alternating vortex pairs are shed from the bilge keel tip with damped magnitudes for decreasing mean roll angles. The local flow indicates lower frequencies and larger damping than the roll motion.