Comparison of RANS and Potential Flow Force Computations for the ONR Tumblehome Hullfrom in Vertical Plane Radiation and Diffraction Problems

The commercial CFD software STAR-CCM+ is applied as a RANS solver for comparison with potential flow methods in the calculation of vertical plane radiation and diffraction problems.  A two-dimensional rectangular cylinder oscillating in an unbounded fluid is first considered, and the added mass result shown to agree well with the analytical potential flow solution.  Hydrodynamic coefficients are then determined for the cylinder oscillating in heave and sway about a calm free surface.  Predicted values are observed to coincide with available experimental and linear potential flow results for most amplitudes and frequencies of oscillation examined.   A three-dimensional radiation problem is then studied in which 1-DoF heave and pitch motions are prescribed to the ONR Tumblehome hullform in calm water at zero forward speed and Fn 0.3.  Combinations of amplitude and frequency of oscillation ranging from small to large are considered.  Results are compared with several potential flow codes which utilize varying degrees of linearization.  Differences in the force and moment results are attributed to particular code characteristics, and overall good agreement is demonstrated between RANS and potential flow codes which employ a nonlinear formulation.  The ONR Tumblehome is next held static in incident head waves of small and large steepness and zero forward speed or Fn 0.3.  Force and moment time histories of the periodic response are compared with the same set of potential flow codes used in the radiation problem.  Agreement between potential flow and RANS is reasonable in the small wave steepness case.  For large wave steepness, the nonlinear wave response is seen to be important and the RANS solution does not generally agree well with potential flow results.