Helicopter Rotor Aerodynamic Noise Prediction Method

Combining CFD technology and aeroacoustic theory in time domain, the high-speed impulsive (HSI) noise of helicopter rotor in hover and in forward flight was predicted and investigated. By means of dual-time stepping method and moving grids technique, we solved the 3-D unsteady Euler'equations for helicopter rotor in forward flight, and obtained the near-field HSI noise. Taking the near-field solution as the inputting data for aeroacoustic formulas, we solved the far filed HSI noise of rotors by several methods, which include Farassat method solving FW-H(Ffowcs-William/Hawkings) equation, Kirchhoff method solving general wave equation and a combined method(K-FWH method). Thus, we developed a series of methods predicting the aeroacoustic noise for transonic rotor in hover and in forward flight based on CFD/Aeroacoustics methodology. The quantitative calculations for a rotate point source, UH-1H rotor and AH-l/OLS rotor showed that the results were in good agreements with the experimental data and other researcher's.The main aspects of this dissertation are as follows:1. The unsteady flow around helicopter rotor in forward flight was numerically simulated by solving 3-D Euler's equations. Moving grid-strategy and multi-block grids technology were used in the calculation. The grid cells moved and deformed during the time stepping. The velocity of the grid cell boundary was added into the governing equations and no source term appeared because the reference coordinate system was the inertial one. The flux terms in the governing equations was directly calculated in the inertial coordinate system without projection to the rotate one.2. Using general function theory, we derived out the FW-H(Ffowcs-Williams and Hawkings) equation, general wave equation(called Kirchhoff equation), and their solutions, known as Farassat 1A formula and Kirchhoff formula which were also derived in detail. Then, the acoustic formulas in common use were presented in the paper.3. We put forward a combine algorithm for predicting the aerodynamically generated noise using different methods mentioned above, and developed a computer code named "NOSPRE" to predict acoustic noise generated by rotors4. The acoustic noise generated by the rotate point source, UH-1H rotor in hover and AH-l/OLS in forward flight were calculated, and the computed results from several methods were compared with each other and with the experimental data. The comparison showed that: (l)Farassat 1A formula is the simplest one and work well for the subsonic rotor.(2)both Kirchhoff formula and K-FWH formula can predict the nonlinear noise generated by transonic rotor;(3) When the surface is not positioned in the linear region , Kirchhoff approach for rotating integration surface could lead to substantial error, but K-FWH formula can still work well.