Transition-sensitive hybrid RANS–LES simulation of subcritical circular-cylinder flow with modified shielding and grid-length scale

• An algebraic transition model is reformulated to satisfy Galilean invariance. • A transition-sensitive DDES framework is developed for subcritical cylinder flow. • Modified shielding function and grid length scale yield a more consistent RANS–LES partition. • Improved predictions are demonstrated across Re D = 3,900–140,000 in the subcritical regime. Subcritical flow past a circular cylinder involves laminar separation (LS), boundary-layer transition, secondary attachment (SA), secondary separation (SS), turbulent attachment (TA), wake vortex shedding, and multiscale vortex interactions. Traditional hybrid Reynolds-averaged Navier-Stokes (RANS)–large eddy simulation (LES) models, which are based on fully turbulent closures, cannot capture LS and boundary-layer transition, leading to substantial deviations in the predicted aerodynamic parameters, such as drag and pressure coefficients, for subcritical cylinder flow. To account for these coupled phenomena, a transition-coupled delayed detached eddy simulation (DDES) formulation is developed based on an improved Galilean-invariant (GI) algebraic transition model. The conventional DDES formulation is modified in terms of the baseline RANS closure, shielding function, and grid length scale to improve the prediction of transition–separation-coupled flow and wake development. The proposed formulation is primarily assessed for circular-cylinder flow at R e D = 46 , 000 , with additional evaluations at R e D = 3 , 900 and 140 , 000 to examine its robustness across the subcritical regime. Compared with the baseline SST-DDES model, the proposed formulation improves the prediction of the principal aerodynamic and wake quantities and reproduces the characteristic separation–reattachment topology and secondary vortex formation more consistently. The modified RANS–LES partition reduces excessive modeled eddy viscosity in the separated shear layer and facilitates the resolved development of shear-layer instability and three-dimensional wake structures. A probe-based spectral analysis further identifies a dominant high-frequency shear-layer mode at approximately 38.8 kHz for the R e D = 46 , 000 case, corresponding to S t KH ≈ 5.74 and f KH / f K ≈ 29.5 . The instability is amplified in the upstream separated shear layer and progressively loses coherence downstream, consistent with Kelvin–Helmholtz-type instability growth and breakdown. Lagrangian coherent structure (LCS) analysis further reveals the recurrent material-transport organization associated with Kármán vortex formation, shedding, and downstream convection. These results demonstrate that transition-sensitive treatment improves hybrid RANS–LES predictions of subcritical bluff-body flows.