Shear-layer instabilities and vortex evolution in disk wakes at subcritical Reynolds numbers

This study investigates the shear-layer instabilities and vortex evolution at subcritical Reynolds number (Re=3000) using an advanced anisotropic hybrid Reynolds-Averaged Navier–Stokes and Large-Eddy Simulation model, enhancing the theoretical framework of disk wake dynamics by systematically resolving the spatial and spectral signatures of three interrelated instability mechanisms. Quantitative analysis reveals a radial variation in vortex shedding intensity (StV=0.146–0.148), with the most prominent spectral peaks consistently observed at an intermediate radial location (y/D=0.31). This radial asymmetry highlights the spatially inhomogeneous response of the near wake to shear-layer excitation. Subsequently, detailed spectral analysis confirms and extends previous observations of ultra-low-frequency oscillations (StL=0.01–0.07), demonstrating their established but complex relationship with recirculation bubble dynamics, adverse pressure gradient modulation, and synchronized force fluctuations. Finally, combined vorticity–spectral analysis provides the first quantitative determination of Kelvin–Helmholtz instability extent (Ls≈0.7D) while revealing higher-order harmonics (up to StKH4=5–6.5), offering new evidence of nonlinear energy transfer in separating shear layers. These findings advance the theoretical framework for subcritical wake transitions by establishing quantitative relationships between multi-scale instabilities and their governing physical mechanisms, providing insights for practical wake control and industrial design strategies.