Rotor blade–vortex interaction noise

This paper reviews the state of the art in helicopter rotor loads prediction using Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) coupling. The application of CFD to rotorcraft problems has evolved, over the period 1990 to 2005, as a viable means to improve the aerodynamic modeling used in rotorcraft comprehensive analyses (CA). CFD/CSD coupling has the potential to meet the long term objective of a coupled rotor-fuselage analysis that can predict accurate loads and vibration at all critical flight conditions without empirical or semi-empirical inputs. This paper reviews the capabilities and limitations of current CFD analyses to capture the key aerodynamic phenomena driving critical rotor loads: wake, dynamic stall and tip transonic effects. Recent initiatives in CFD/CSD coupling, targeted to improve predictions of rotor loads, are discussed. The high speed (155 kts) forward flight condition of the UH-60A BlackHawk is studied as a test case. The flight condition falls in a critical high vibration regime, where the mechanisms of rotor vibratory loads were not clearly understood. Significant improvements in fundamental understanding and prediction of vibratory loads at this flight condition has been recently demonstrated by researchers using CFD/CSD coupling. The physical mechanisms involved, and the role of CFD in improving predictions is explained.

Review of Rotor Loads Prediction with the Emergence of Rotorcraft CFD

Rotorcraft Aeroacoustics Computations with Overset-Grid CFD Methods

Analytical results of rotor blade‐ vortex interaction airloads are presented with two different wake models in the comprehensive analysis CAMRAD II, and these calculated results are compared with the experimental data obtained from the higher-harmonic-control aeroacoustic rotor test (HART-I) program. The HART rotor was a 40%, Mach-scaled model of the hingeless BO-105 main rotor. Two wake models used in the comprehensive analysis are the single-trailer and the multiple-trailer free-wake models. The multiple-trailer wake model shows good prediction of lift distribution, M 2 cn, as a function of azimuth for baseline, minimum noise, and minimum vibration cases, and shows signie cant improvement relative to the single-trailer model in prediction of the tip vortex wake geometry.

https://rotorcraft.arc.nasa.gov/publications/files/Lim_JAIR2003.pdf

Calculation of Rotor Blade-Vortex Interaction Airloads Using a Multiple-Trailer Free-Wake Model

CFD Investigation of Double-Swept Blade in BVI Noise Reduction

Helicopter rotor simulation codes of different organizations are validated against experimental data obtained in the DNW. The comparison addresses rotor blade loading, wake geometry, blade motion and noise radiation. Although specific differences exist the general prediciton of rotor noise is reasonably well.

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions

As part of an international cooperative program, several aerodynamics and acoustic prediction codes have been recently developed to understand the rotor blade-vortex interaction (BVI) noise. Furthermore, a joint validation activity of these prediction codes was performed with the blade surface pressure and acoustic data taken in the DNW with the 1/7-scale model AH-1 OLS blades. Careful attention has been given to the detailed flow field, especially wake structures during blade-vortex interactionsincluding miss distance between vortex and blade, vortex core size, and tip vortex trajectories. The results of flow field including wake systems from various prediciton codes show some qualitative differences, but the acoustic results using these calculated airload as inputs compare reasonably well with test data.

Pierre Spiegel

Papers

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions: Analysis Capability and its Validation