Numerical Sensitivity of the Slat Brackets Wake on Fixed Grid RANS Simulations of the High-Lift Configuration Aircraft

During the AIAA High Lift Prediction Workshop 5 (HLPW-5), a distinctive flow-separation pattern referred to as the "pizza-slice" separation was repeatedly observed in steady RANS simulations behind the slat brackets despite the absence of such separation in wind-tunnel experiments. This discrepancy indicates that the phenomenon may originate from numerical or modeling limitations associated with current fixed-grid RANS practices. Although various factors were considered in the workshop discussions, the detailed mechanism responsible for the pizza-slice separation remained unresolved. The present study investigates the source of this artificial separation by focusing on the sensitivity of the bracket-induced vortices to grid resolution. Numerical simulations are performed for the NASA CRM-HL high-lift configuration using the TAS solver with the Spalart-Allmaras turbulence model on unstructured grids generated by MEGG3D. A series of refined grids are constructed to examine how well the vortices shed from the slat brackets are resolved and how they interact with the boundary layer over the main wing.The results show that coarse grids fail to resolve the vortices shed from the brackets, leading to inaccurate prediction of their downstream development and their influence on surface separation. As the grid is refined, the bracket-induced vortices become more clearly captured, and the interaction between the vortices and the separation region is represented with increasing fidelity. Overall, the findings demonstrate that the pizza-slice separation observed in steady RANS simulations originates from bracket-induced vortical structures whose prediction is highly sensitive to grid resolution. The study highlights the importance of appropriate grid refinement strategies around slat-bracket geometries and provides insights toward improving the reliability of steady RANS simulations for complex high-lift configurations.