RANS Sensitivity Study and Global Stability Analysis of the NASA Common Research Model High-Lift Configuration

Following on from NASA’s recent Fourth High-lift Prediction Workshop (HLPW-4), an extensive parametric study is performed for both steady and unsteady 3D Reynolds-averaged Navier-Stokes (RANS) calculations on the high-lift configuration of the NASA Common Re- search Model (CRM-HL). The sensitivity of the solution and convergence characteristics are assessed for a wide range of turbulence closure models on two grid configurations contain- ing 200 and 400 million elements. While HLPW-4 focused mostly on the use of the standard Spalart-Allmaras (SA) turbulence models, we extend the analysis in this work to assess several corrections to the standard SA model and also the more costly Shear-Stress Transport (SST) turbulence model. An overlooked issue during the HLPW-4 was the unsatisfactory convergence of the RANS solutions, that in some instances was poor even at low angles of attack. To estimate the convergence of the RANS solutions, we propose a new physics-based parameter to quantify the numerical quality and uncertainty of RANS simulation results. This parameter is shown to be more sensitive and representative of the reliability of the RANS solutions with respect to the classical L2-norm residuals. Discussions based on this parameter and the comparisons with the available experiments show that the standard SA model with quadratic constitutive relation (QCR) correction does not necessarily improve the accuracy of the results and the standard SA model should be preferred. Good aerodynamic forces predictions are obtained when warm started calculations were performed. Similar to the trends seen during HLPW-4, even though lift coefficients agree well with experiments, we however note the presence of unphysical large regions of flow separation near the wing tips for all simulations at higher angles of attack. The prediction of the pitching-moment coefficient is also unsatisfactory. For all SA-based closure models presented here, the RANS simulations were shown to be insensitive to grid refinement. While SST models perform worse than SA models at lower angles of attack, they exhibit im- proved accuracy at near-stall conditions albeit with slower convergence. Cold-started URANS simulations recover best-practice solutions of steady RANS simulations, giving confidence to the warm started RANS calculations. We also performed some preliminary global stability analyses of RANS baseflows that suggest the existence of an unstable mode associated with vor- tical structures originating from the pylon at a Strouhal number of St = 3.54 (f = 172.5Hz). More investigations are however needed to conclude on the existence of low-frequency instabil- ities associated with buffet or stall phenomena.