Phase-averaged organized oscillation velocities (U,V,W) and random fluctuation Reynolds stresses (uu¯,vv¯,ww¯,uv¯,uw¯) are presented for the nominal wake of a surface ship advancing in regular head (incident) waves, but restrained from body motions, i.e., the forward-speed diffraction problem. A 3.048×3.048×100m towing tank, plunger wave maker, and towed, 2D particle-image velocimetry (PIV) and servo mechanism wave-probe measurement systems are used. The geometry is DTMB model 5415 (L=3.048m, 1∕46.6 scale), which is an international benchmark for ship hydrodynamics. The conditions are Froude number Fr=0.28, wave steepness Ak=0.025, wavelength λ∕L=1.5, wave frequency f=0.584Hz, and encounter frequency fe=0.922Hz. Innovative data acquisition, reduction, and uncertainty analysis procedures are developed for the phase-averaged PIV. The unsteady nominal wake is explained by interactions between the hull boundary layer and axial vortices and incident wave. There are three primary wave-induced effects: pressure gradients 4%Uc, orbital velocity transport 15%Uc, and unsteady sonar dome lifting wake. In the outer region, the uniform flow, incident wave velocities are recovered within the experimental uncertainties. In the inner, viscous-flow region, the boundary layer undergoes significant time-varying upward contraction and downward expansion in phase with the incident wave crests and troughs, respectively. The zeroth harmonic exceeds the steady-flow amplitudes by 5–20% and 70% for the velocities and Reynolds stresses, respectively. The first-harmonic amplitudes are large and in phase with the incident wave in the bulge region (axial velocity), damped by the hull and boundary layer and mostly in phase with the incident wave (vertical velocity), and small except near the free surface-hull shoulder (transverse velocity). Reynolds stress amplitudes are an order-of-magnitude smaller than for the velocity components showing large values in the thin boundary layer and bulge regions and mostly in phase with the incident wave.
Towing-tank experiments of coupled pitch and heave motions are presented for a surface combatant advancing in regular head waves. The data include ballasting parameters, time histories, fast Fourier transform (FFT), Fourier series amplitudes, and pitch and heave transfer functions and phases for a range of speeds, wave steepnesses, and wave frequencies. The geometry is David Taylor Model Basin (DTMB) model 5512, which is a 1/46.6 scale geosim of DTMB model 5415 (DDG-51) with Lpp = 3.048 m. The experiments are performed in a 3.048 × 3.048 × 100 m towing tank equipped with a plunger-type wave maker. The test program is undertaken to provide a validation data set for unsteady Reynolds-averaged Navier-Stokes and other computational fluid dynamics (CFD) codes, including rigorous uncertainty assessment of the experimental results following standard procedures. Results indicate that the regular head waves are linear with second- and third-order magnitudes consistent with third-order Stokes waves. Pitch and heave responses and phases show expected trends for long and short wavelengths and are linear or Ak independent for all test conditions. Maximum response occurs for frequency of encounter equal to pitch and heave natural frequencies and Lpp / λ = 0.75. Under these conditions, an equation is derived that predicts the Froude number for maximum response as a function of ship geometrical coefficients.
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.
From the thrusters on smaller, but numerous, harbour support vessels through to the pod-drives on cruise ships and ocean going liners, azimuth control has rapidly established itself in the maritime industry. From the design of the ship, to the training of personnel and the development of operational procedures, the industry has risen to meet the demand. However, this rapid evolution has not allowed sufficient time for the propagation of knowledge throughout the different disciplines. On a day-to-day basis, maritime pilots must deal with such ships, coping as they do, with an as yet unstandardized environment. This paper presents the findings of an EU project (AZIPILOT) considering accidents and incidents and concerning the training and operational practice of ships equipped with Azimuth Control Devices (ACD’s).
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