Stall cells over an airfoil. Part 1: Three-dimensional flow organisation and vorticity dynamics
This study investigates the three-dimensional organisation and evolution of stall cells in the separated flow region over an airfoil. Using a hybrid RANS/LES approach based on the DDES-SST turbulence model, we characterise the formation and development of these structures, which remain challenging to capture experimentally. Initial validation confirms accurate reproduction of global loads when comparing with both experimental data and RANS simulations. The complex three-dimensional flow organisation is analysed through investigating the vorticity, revealing that spanwise variation of the separation location leads to non-uniform load distribution along the airfoil span. The mid-span experiences premature separation due to flow bifurcation, while flow attraction at $\pm1$ chord length successfully sustains attached flow further along the chord. The separated flow generates a shear layer culminating in a separation vortex tube, which exhibits a Crow-type instability when interacting with the counter-rotating trailing edge vortex tube. This instability induces a wave-like bending of the vortex tubes and shear layer, generating significant vertical vorticity (y-vorticity) that drives spanwise flow. We identify a previously unreported phenomenon where the maxima of spanwise velocity structures exhibit rotation around fixed spanwise axes, with the rotation angle evolving linearly with downstream distance according to $\zeta = 14.5(x/c) - 0.8$. This study provides new insights into the mechanisms underlying stall cell formation and highlights the importance of three-dimensional effects in separated flows, which has implications for aerodynamic load prediction and control strategies.