https://www.cs.odu.edu/~mln/ltrs-pdfs/NASA-2003-pas-ksb.pdf

Modeling aerodynamically generated sound of helicopter rotors

Density fields of the blade tip vortices from a helicopter in hover flight were visualized by a technique which does not require any installation on the helicopter or close to it. The results illustrate an encouraging prospect for the applicability of the technique. It offers the capability of at least qualitative investigations of unsteady density fields even in full-scale flight tests. The underlying principle is briefly described in this article and an extension to a three-dimensional quantitative technique by using multiple cameras is outlined.

On the applicability of background oriented optical tomography for large scale aerodynamic investigations

Helicopter impulsive noise: Theoretical and experimental status

Rotorcraft Aeroacoustics Computations with Overset-Grid CFD Methods

A two-dimensional, airfoil-vortex interaction experiment was conducted to obtain the aerodynamic behavior of the airfoil during a parallel interaction. The vortex was created by pitching a second airfoil placed upstream and parallel to an instrumented test airfoil. Hot-wire anemometer measurements were taken to determine the vortex velocity distribution. The interaction tests were conducted for a counterclockwise vortex passing above a symmetrical airfoil at zero angle of attack

Airfoil pressure measurements during a blade vortex interaction and a comparison with theory

Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter Averaged model scale data were compared with averaged full scale, inflight acoustic data under similar nondimensional test conditions At low advance ratios (mu = 0164 to 0194), the data scale remarkable well in level and waveform shape, and also duplicate the directivity pattern of BVI impulsive noise At moderate advance ratios (mu = 0224 to 0270), the scaling deteriorates, suggesting that the model scale rotor is not adequately simulating the full scale BVI noise; presently, no proved explanation of this discrepancy exists Carefully performed parametric variations over a complete matrix of testing conditions have shown that all of the four governing nondimensional parameters - tip Mach number at hover, advance ratio, local inflow ratio, and thrust coefficient - are highly sensitive to BVI noise radiation

/pdf/helicopter-model-rotor-blade-vortex-interaction-impulsive-kxzyse30xz.pdf

Helicopter Model Rotor-Blade Vortex Interaction Impulsive Noise: Scalability and Parametric Variations

A 1/17-scale research model of the AH-1 series helicopter main rotor was tested. Model-rotor acoustic and simultaneous blade pressure data were recorded at high speeds where full-scale helicopter high-speed impulsive noise levels are known to be dominant. Model-rotor measurements of the peak acoustic pressure levels, waveform shapes, and directively patterns are directly compared with full-scale investigations, using an equivalent in-flight technique. Model acoustic data are shown to scale remarkably well in shape and in amplitude with full-scale results. Model rotor-blade pressures are presented for rotor operating conditions both with and without shock-like discontinuities in the radiated acoustic waveform. Acoustically, both model and full-scale measurements support current evidence that above certain high subsonic advancing-tip Mach numbers, local shock waves that exist on the rotor blades ""delocalize'' and radiate to the acoustic far-field.

https://ntrs.nasa.gov/api/citations/19840001953/downloads/19840001953.pdf

Model helicopter rotor high-speed impulsive noise: Measured acoustics and blade pressures

Helicopter model rotor-blade vortex interaction impulsive noise - Scalability and parametric variations

Model rotor high-speed impulsive noise - Parametric variations and full-scale comparisons

Two aeroacoustic facilities--the CEPRA 19 in France and the DNW in the Netherlands--are compared. The two facilities have unique acoustic characteristics that make them appropriate for acoustic testing of model-scale helicopter rotors. An identical pressure-instrumented model-scale rotor was tested in each facility and acoustic test results are compared with full-scale-rotor test results. Blade surface pressures measured in both tunnels were used to correlated nominal rotor operating conditions in each tunnel, and also used to assess the steadiness of the rotor in each tunnel's flow. In-the-flow rotor acoustic signatures at moderate forward speeds (35-50 m/sec) are presented for each facility and discussed in relation to the differences in tunnel geometries and aeroacoustic characteristics. Both reports are presented in appendices to this paper. ;.);

K. J. Schultz

Papers

Helicopter Model Rotor-Blade Vortex Interaction Impulsive Noise: Scalability and Parametric Variations

Model helicopter rotor high-speed impulsive noise: Measured acoustics and blade pressures

Helicopter model rotor-blade vortex interaction impulsive noise - Scalability and parametric variations

Model rotor high-speed impulsive noise - Parametric variations and full-scale comparisons

A comparison of the acoustic and aerodynamic measurements of a model rotor tested in two anechoic wind tunnels