Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Unsteady RANS Simulation of a Surface Combatant with Roll Motion
Unsteady RANS Simulation of a Surface Combatant with Roll Motion
Pages 110-127
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From page 110... ...
Preliminary results for the surface combatant without forward speed and with free roll decay motion are given where the model is released from an initial angular displacement and the resulting roll motion predicted. The final paper will include simulations with larger roll angles where non-linear effects are important.
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From page 111... ...
Finally, concluding remarks are presented in Section 8. 2 RANS SOLUTION METHODOLOGY Unsteady RANS simulations are performed with CFDSHIP-IOWA, which is a general-purpose, multi-block, high-performance parallel computing code developed for computational ship hydrodynamics and applied to surface ships and complex propulsors without ship motions.
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From page 112... ...
The steady state solution provides a consistent initial condition at t=0, after which the prescribed roll motion is gradually increased to full magnitude over the first period, thus reducing the time required to damp initial transients and reach a periodic response. For simulations with free roll decay, the ship is given an angular displacement as initial condition and then released and allowed to freely rotate about the roll axis.
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From page 113... ...
The free surface boundary conditions are based upon the exact nonlinear kinematic and approximate dynamic free-surface boundary conditions, both of which are applied on the actual free surface. The kinematic equation is a 2D hyperbolic wave equation for the wave elevation ~ (~+U(x+V |
From page 114... ...
i-~ ~ / 2, and aij = JgiiI47k . 2.3.3 Kinematic Free-Surface Boundary Condition Solver (KFSBC)
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From page 115... ...
. Roll motion is predicted by numerically integrating the rigid body equation of motion where the time rate of change of angular momentum about the roll axis is balanced by external roll moments L acting on the ship
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From page 116... ...
Selection of final frequencies for simulations with prescribed roll motion will be based on CFD predictions and EFD measurements of the resonant frequency for 5512 geometry with free roll decay motion. Table 2.
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From page 117... ...
7 RESULTS Discussion and analysis of unsteady results for the surface combatant with prescribed sinusoidal roll motion are provided in this section for boundary layer (Sect.
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From page 118... ...
The wavelength Jv and period tv for the motion of the sonar dome vortices is estimated from the product of the forward speed of the ship UO and period of the roll motion ~ (i.e., by = UOT )
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From page 119... ...
5.00 9.00 3.00 3.00 -9.00 5.00 Fig. 5 Time sequence of axial vorticity contours (x for 5512 simulations with prescribed roll motion (f=2)
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From page 121... ...
The above discussion applies to 2D rolling bodies, while the problem considered here is more complicated due to the effects of varying crosssectional shape with axial distance, presence of transom stern, and forward speed, which generates a steady wave system. Excluding the transom region, the roll motion and variation in cross-sectional shape results in a transverse propagating wave system traveling upstream and originating on the forebody (0.3 < C/L < 0.5)
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From page 122... ...
t=~4; (b) t= ~2; roll motion (f=2)
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From page 123... ...
The pressure component for the afterbody region is roughly constant with mean value equal to the total roll moment MT. Figure 14b shows that the pressure component on the front of the sonar dome lags the roll motion by 90 degrees and that on the rear by 180 degrees.
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From page 124... ...
Present values for the surface combatant and measurements for the fishing vessel show a similar trend of decreasing am with frequency. Roll damping coefficients for the fishing vessel, cargo ship, and surface combatant all show increased damping with frequency.
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From page 125... ...
Preliminary results were presented which demonstrated prediction of free roll decay motion of the surface combatant without forward speed. The final paper will include simulations with larger roll angles where non-linear effects should be important.
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From page 126... ...
and Naito, S., "An Application of Fully Nonlinear Numerical Wave to the Study of Chaotic Roll Motions," Proceedings of the 8th International and Offshore Polar Engineering Conference, Honolulu, Hawaii, Vol.
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From page 127... ...
AUTHORS' REPLY The focus of the current paper is on accurate prediction of near field viscous and pressure forces, which is required for accurate prediction of general 6DOF ship motions. Resolution of the turbulent boundary layer at the ship hull requires fine grid spacing in the wall normal direction with expansion of the grid towards the far-field (see Fig.
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