| Free surface wave-induced separation, caused by interactions of free surface waves and wall boundary layers, has relevance to ship and platform hydrodynamics with regard to resistance and propulsion, stability, and signatures. This study uses 2nd and 3rd order URANS (CFDSHIP-IOWA) with complimentary EFD to investigate the instability mechanisms and vortical/turbulent structures on a surface piercing NACA0024 foil for Fr = 0.37 and 0.55. Instability studies include laminar cases, too. Frequency analysis and vortex core detection reveal the nature of free surface wave induced separation. Normalized Strouhal numbers for shear layer instability (Sttheta = f x theta/U C, theta---momentum thickness at separation, UC---convection velocity), Karman instability (Sth = f x h/US, h---half wake thickness, US---shear layer velocity at separation), and 'flapping' instability (Sth = f x X R/US, XR---reattachment length) show the effects of free surface, Re and Fr on the instability mechanisms.; Shear layer separates without reattachment for all laminar cases simulated (Re = 1500 and 2500, with/without free surface). Turbulent high Re flows without free surface remain steady and attached. With free surface, Fr = 0.37 (Re = 1.5e6) flow exhibits separation with reattachment, and Fr = 0.55 (Re = 2e6) flow exhibits separation without reattachment.; Sttheta (∼0.13) remains constant for all laminar cases simulated. This supports previous findings that Sttheta is Re independent in the laminar regime and may indicate that the free surface does not alter Sttheta in the laminar regime. The value compares well with existing values for laminar flow past airfoils. Sttheta for the turbulent cases have lower values. Sttheta = 0.0077 and 0.0052 for Fr = 0.37 and 0.55, respectively. Sth for all cases with free surface lies in a reduced range (0.065--0.069) compared to universal value for flows without free surface (0.07--0.09). St R (=0.2) lies in similar range as that for backward facing steps and blunt cylinders. A theoretical model constructed based on simple harmonic motion gives a good prediction of the flapping frequency.; Analysis of the separation region for Fr = 0.37 case shows a pattern of periodic vortex merger/breakdown. Breakdown satisfies the necessary criteria. Topological analysis explains the behavior of vortex breakdown in terms of dividing surfaces. Modeled and resolved turbulent stress calculations show the turbulence characteristics and the degree of turbulence actually simulated. Highest resolved stresses occur at the separation starting point. u'w' dominates the resolved shear stresses for both Fr = 0.37 and 0.55.; Rigorous verification and validation of the results ensured reliability and the overall results provide credible description of the flow physics. |
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