2023/01/06 by Siddharth Gupta, Jisheng Zhao, Atul Sharma +3 · 4 citations
Engineering · #Fluid Dynamics and Vibration Analysis #Fluid Dynamics and Turbulent Flows #Lattice Boltzmann Simulation Studies
paper · pdf · doi:10.1017/jfm.2022.958
Flow transitions are an important fluid-dynamic phenomena for many reasons, including the direct effect on the aerodynamic forces acting on the body. In the present study, two-dimensional (2-D) and three-dimensional (3-D) wake transitions of a NACA0012 airfoil are studied for angles of attack in the range 0^∘ ≤ α ≤ 20^∘ and Reynolds numbers 500 ≤ \textit Re ≤ 5000 . The study uses water-channel experiments and 2-D and 3-D numerical simulations based on the nodal spectral-element method, level-set function-based immersed-interface method and Floquet stability analysis. The different wake states are categorised based on the time-instantaneous wake structure, non-dimensional frequency and aerodynamic force coefficients. The wake states and transition boundaries are summarised in a wake regime map. The critical angle of attack and Reynolds number for the supercritical Hopf bifurcation (i.e. steady to periodic wake transition) varies as α 1 ∼ \textit Re-0.65 , while the critical angle of attack for the onset of three dimensionality varies as α 3D ∼ \textit Re-0.5 . Over the entire Reynolds number range, the transition to 3-D flow occurs through a mode C (subharmonic) transition. Beyond this initial transition, further instabilities of the 2-D periodic base flow arise and are investigated. For instance, at \textit Re=2000 and α 3D,2=11.0^∘ , mode C coexists together with modes related to modes A and QP seen in a stationary circular cylinder wake. In contrast, at \textit Re=5000 and α 3D,2=8.0^∘ , the dominant mode C coexists with mode QP. Three-dimensional simulations well beyond critical angles indicate that 2-D vortex-street transitions are approximately maintained in the fully saturated 3-D wakes in a spanwise-averaged sense.