Validation of a State-Space Free Wake Model with a Near-Wake Vortex Lattice Method

This paper presents the implementation and validation of a state-space free-vortex wake model with a vortex lattice near-wake formulation, developed for rotorcraft applications. The model is expressed in state-variable form as a nonlinear time-periodic (NLTP) system in first-order structure, enabling linearization and simplification through time-invariant systems theory and model-order reduction techniques. It is applied to a UH-60-like rotor and evaluated in hover, forward flight, and vortex ring state (VRS) conditions. Two configurations are considered: one with tip vortex dynamics only, and another incorporating both tip vortex dynamics and a near-wake vortex lattice model. A parametric study is conducted to determine optimal parameters for solution accuracy. Validation in hover and forward flight is performed against high-fidelity computational fluid dynamics (CFD) results and available experimental data. Validation in VRS includes comparisons with experimental measurements and momentum theory. Overall, the state-space free-vortex wake model accurately predicts blade loads and induced velocities across all three validation cases, with the exception of a slight underprediction observed in fully developed VRS conditions. The inclusion of the near-wake model is essential for accurate blade load prediction in hover and contributes to improved predictions in forward flight.