Numerical Modeling of Resistance for a Conceptual Seatrain

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CD-adapco Academic Paper Contest
Thursday, March 31, 2011

A RANS based computational fluid dynamics method incorporating the free-surface Volume of Fluid (VOF) multiphase flow has been employed to compute the resistance forces of two idealized seatrain concepts for fixed flotation as well as for two degrees of freedom (DOF) independent motion of each unit. The motion of each unit is analyzed using Dynamic Fluid Body Interaction (DFBI) morphing mesh technique. Seatrain, a series of hulls connected with rigid (except for pitch and heave) links is a potentially valuable concept for the Army Sealift, the Marine Corps as well as a broad spectrum of commercial marine transportation applications. Several concepts are currently being considered and tested under various US Navy programs. Using STAR-CCM+ v5.06, two idealized seatrain concepts have been modeled for various configurations. The configuration variations are characterized by different hull forms (Wigley hull and barge), number of units, spacing between units, and Froude numbers ranging from 0.267–0.408. In the absence of any available experimental or computational data on seatrains, validation of the methodology is limited to a single-hull case in which such data are available. For the fixed cases of a single Wigley hull, good agreements between the CFD results and experimental towing test have been attained. Using 'DFBI Translation and Rotation' motion specification, 2-DOF (heave and pitch) analysis is then performed to find the hydrodynamic equilibrium sink position for a single hull. This equilibrium position is used for fixed flotation calculation of resistance of several seatrain configurations. Finally, the independent hull motion of the seatrain, which is mainly characterized by individual floating bodies in the same physical domain, is modeled using the 'DFBI Morphing' motion specification in which the 3D mesh deforms in response to the hull and fluid motion. This paper includes the extensive CFD results including drag coefficient and free-surface wave profiles for given seatrain configurations. From the parametric studies, it is observed that for low Froude numbers, the seatrain of Wigley type is 2-14% more efficient than equivalent single hull configuration. However, the drag reduction benefit disappears at high Froude numbers, possibly due to increased in interference drag. Conversely, a barge-type seatrain configuration demonstrates drag reduction advantage (average of about 14%) even for a high cruising speed regime. In summary, a methodology utilizing STAR-CCM+ has been developed which enhances the ability of the designers to balance various technical issues and cost, to fully explore the design space relative to specific requirements, and to efficiently assess feasibility of various potential seatrain designs, and thereby reducing program risks.

Author Name: 
N. Harpal
C. Patel