Unsteady free surface wave-induced separation: Vortical structures and instabilities |
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Authors: | M Kandasamy T Xing F Stern |
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Institution: | 1. Institute of Numerical Mathematics of the Russian Academy of Sciences, Moscow, Russia and Keldysh Institute of Applied Mathematics of the Russian Academy of Sciences, Moscow, Russia;2. Department of Mathematics, University of Houston, Houston, TX, USA;3. School of Earth, Energy & Environmental Sciences, Stanford University, Stanford, CA, USA;4. Moscow Institute of Physics and Technology, Dolgoprudny, Russia |
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Abstract: | Vortical structures and instability mechanisms of the unsteady free surface wave-induced separation around a surface-piercing NACA0024 foil at a Froude number of 0.37 and a Reynolds number of 1.52×106 are studied using an unsteady Reynolds-averaged Navier–Stokes (URANS) code with a blended k?ε/k?ω turbulence model and a free surface tracking method. At the free surface, the separated flow reattaches to the foil surface resulting in a wall-bounded separation bubble. The mean and instantaneous flow topologies in the separation region are similar to the owl-face pattern. The initial shear-layer instability, the Karman-like instability, and the flapping instability are identified, and their scaling and physical mechanisms are studied. Validation with experimental fluid dynamics (EFD) and comparison with complementary detached-eddy simulation (DES) indicate that URANS resolves part of the organized oscillations due to the large-scale unsteady vortical structures and instabilities, thereby capturing the gross features of the unsteady separation. The URANS solutions show an initial amplitude defect of 30% for the free surface oscillations where the shear layer separates, and the defect progressively increases downstream as URANS rapidly dissipates the rolled up vortices. |
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