Helicopter Downwash Effects on Ship Airwake: Predictions, Modeling from a Database, and Simulation

摘要

Helicopter downwash impact on ship airwake is addressed from a three-pronged approach: 1.) Predictions of one-point statistics of autospectrum and two-point statistics of cross-spectrum and coherence from a Computational Fluid Dynamics database of flow velocity predictions; 2.) Development of a mathematical framework for extracting interpretive autospectral models in closed form from a database; 3.) Simulation through white-noise-driven filters for the extracted models. The framework begins with an earlier-exercised perturbation series expansion of the autocorrelation for all three velocity components, and the first term of the series has a form of the von Karman longitudinal or lateral correlation function. After transformation into equivalent series of autospectrum, the perturbation coefficients are evaluated by satisfying the theoretical constraints and fitting a curve on a set of selected autospectral data points generated from the database. These constraints ensure that the extracted models and the autospectral data points have the same time scale, intensity, energy, and asymptotic decay according to the Kolmogorov -5/3 law. The framework represents a sensible combination of series expansion, exploitation of a database, and theoretical constraints to provide a foothold on airwake-downwash phenomenon for engineering analysis. The framework's strengths and weaknesses and its advancement over the earlier series-expansion schemes are also included. Downwash increases airwake energy by one order of magnitude, and all of this airwake-downwash energy is concentrated within the bandwidth (0 ＜ f (Hz) ＜ 1.6) that affects flight mechanics.