Enhancing the shear-stress-transport turbulence model with symbolic regression: A generalizable and interpretable data-driven approach

Turbulence modeling within the Reynolds-averaged Navier-Stokes (RANS) equations' framework is essential in engineering due to its high efficiency. Field-inversion and machine-learning (FIML) techniques have attempted to improve RANS turbulence models' predictive capabilities for separated flows. However, FIML-generated models often lack interpretability, limiting physical understanding and manual improvements based on prior knowledge. Additionally, these models typically struggle with generalization in flow fields distinct from the training set. This study addresses these issues by employing symbolic regression (SR) to derive an analytical relationship between the correction factor of the baseline turbulence model and local flow variables, enhancing the baseline model's ability to predict separated flow across diverse test cases. The shear-stress-transport (SST) model undergoes field inversion on a curved backward-facing step case to obtain the corrective factor field beta, and SR is used to derive a symbolic map between local flow features and beta. The SR-derived analytical function is integrated into the original SST model, resulting in the SST-SR model. The SST-SR model's generalization capabilities are demonstrated by its successful predictions of separated flow on various test cases, including 2D-bump cases with varying heights, periodic hill case where separation is dominated by geometric features, and the three-dimensional Ahmed-body case. In these tests, the model accurately predicts flow fields, showing its effectiveness in cases completely different from the training set. The Ahmed-body case, in particular, highlights the model's ability to predict the three-dimensional massively separated flows. When applied to a turbulent boundary layer with Re-L = 1.0 x 10(7), the SST-SR model predicts wall-friction coefficient and log layer comparably to the original SST model, maintaining the attached boundarylayer prediction performance.