Chee Tung

Effect of wake structure on blade-vortex interaction phenomena: Acoustic prediction and validation

The higher harmonic control aeroacoustic rotor test conducted at Duits-Nederlandse Wind Tunnel has made extensive measurements of the rotor aerodynamics, far-field acoustics, wake geometry, and the blade motion for powered descent flight conditions. These measurements have been used to validate and improve the prediction of blade-vortex interaction (BVI) noise. The improvements made to the BVI modeling after the evaluation of the test data are discussed. The effect of these improvements on the acoustic-pressure predictions is shown. These improvements include restructuring the wake, modifying the core size, incorporating the measured blade motion into the calculations, and attempting to improve the dynamic blade response. A comparison of four different implementations of the Ffowcs Williams and Hawkings equation is presented. A common set of aerodynamic input has been used for this comparison.

Aerodynamic aspects of blade-vortex interaction (BVI)

This paper presents a review of the blade-vortex interaction (BVI) phenomenon. It covers both analytical and experimental studies of two-dimesional parallel airfoil-vortex interaction, three dimensional BVIs, and helicopter rotor BVIs. The important parameters that affect the BVI airloads are discussed.

Control of Massive Separation on A Thick-Airfoil Wing: A Computational and Experimental Study

The objective of this research is mutual validation of computational analysis and experimental measurements of baseline and controlled flow over a blunt configuration. A rectangular wing with a 36 percent thick airfoil was selected as a test case, representing a rotor pylon fairing. However, the results are relevant to thick airfoils in general, and contribute to the study of turbulent boundary layer separation and control. Despite the apparently simple geometry, previous measurements on this NACA 0036 airfoil proved challenging for computational analysis. Computational analysis, using DES, provided accurate prediction of the the massive separation and the effects of the oscillatory flow control. Both the aerodynamic coefficients variation with angle of attack, and the pressure distribution on the wing were in good agreement with measurements. These results stand in contrast to the previous failure of incompressible RANS solutions to reproduce the massive separation, its effect on aerodynamic coefficients, and the impact of flow control from t different slots.

Rotor Wake Vortex Definition-Evaluation of 3-C PIV Results of the HART-II Study:

An evaluation is made of extensive three-component (3-C) particle image velocimetry (PIV) measurements within the wake across a rotor disk plane. The model is a 40 percent scale BO-105 helicopter main rotor in forward flight simulation. This study is part of the HART II test program conducted in the German-Dutch Wind Tunnel (DNW). Included are wake vortex field measurements over the advancing and retreating sides of the rotor operating at a typical descent landing condition important for impulsive blade-vortex interaction (BVI) noise. Also included are advancing side results for rotor angle variations from climb to steep descent. Using detailed PIV vector maps of the vortex fields, methods of extracting key vortex parameters are examined and a new method was developed and evaluated. An objective processing method, involving a center-of-vorticity criterion and a vorticity “disk” integration, was used to determine vortex core size, strength, core velocity distribution characteristics, and unsteadiness. These parameters are mapped over the rotor disk and offer unique physical insight for these parameters of importance for rotor noise and vibration prediction.

The Effects of Vortex Modeling on Blade-Vortex Interaction Noise Prediction,

Abstract : The use of a blade vortex interaction noise prediction scheme, based on CAMRAD/JA, FPR and RAPP, quantifies the effects of errors and assumptions in the modeling of the helicopters shed vortex on the acoustic predictions. CAMRAD/JA computes the wake geometry and inflow angles that are used in FPR to solve for the aerodynamic surface pressures. RAPP uses these surface pressures to predict the acoustic pressure. Both CAMRAD/JA and FPR utilize the Biot-Savart Law to determine the influence of the vortical velocities on the blade loading and both codes use an algebraic vortex model for the solid body rotation of the vortex core. Large changes in the specification of the vortex core size do not change the inplane wake geometry calculated by CAMRAD/JA and only slighty affect the out-of-plane wake geometry. However, the aerodynamic surface pressure calculated by FPR changes in both magnitude and character with small changes to the core size used by the FPR calculations. This in turn affects the acoustic predictions. Shifting the CAMRAD/JA wake geometry away from the rotor plane by 1/4 chord produces drastic changes in the acoustic predictions indicating that the prediction of acoustic pressure is extremely sensitive to the miss distance between the vortex and the blade and that this distance must be calulated as accurately as possible for acceptable noise predictions. The inclusion or exclusion of a vortex in the FPR-RAPP calculation allows for the determination of the relative importance of that vortex as a BVI noise source. (AN)