Resolvent-based estimation of a turbulent wake

Abstract: We present a resolvent-based framework for estimating turbulent velocity fluctuations in the wake of a spanwise-periodic NACA0012 airfoil at Mach 0.3, Reynolds number 23,000, and an angle of attack of 6 degrees. Building on the methodology of Jung et al. (J. Fluid Mech. 2025, vol. 0, A1), we extend the approach to the more complex regime of a turbulent wake, which involves three primary challenges: (1) globally unstable modes in the linearized Navier-Stokes operator, (2) multi-scale turbulent structures, and (3) high-dimensional datasets. To address these challenges, we employ a data-driven approach that constructs causal resolvent-based estimation kernels from cross-spectral densities obtained via large-eddy simulations. These kernels are derived using the Wiener-Hopf method, which optimally enforces causality, thereby enhancing real-time estimation accuracy. The framework captures the spectral signatures of coherent structures and incorporates the colored statistics of nonlinear forcing terms acting on the linear system. To handle the computational demands of the high-dimensional estimation problem, we utilize parallel algorithms developed within the same framework. We further investigate sensor placement by analyzing single-sensor estimation error and coherence with target flow quantities. Results demonstrate accurate causal estimation of streamwise velocity for the spanwise-averaged, spanwise-Fourier-transformed, and mid-span flow using limited shear-stress measurements on the surface of the airfoil. This study underscores the potential of the resolvent-based framework for efficient estimation in compressible, turbulent environments.