An advanced digital filter method is presented to generate divergence-free synthetic turbulence with homogeneous anisotropic velocity spectra. The resulting fluctuating velocity field is obtained through a superposition of anisotropic Gaussian eddies. This method is used to generate a two-dimensional turbulent flow with the key statistics of homogeneous axisymmetric turbulence. This type of turbulence has been reported in aero-engine intakes, fan wakes and open-jet wind tunnel experiments. The advanced digital filter method is implemented in a linearized Euler solver in order to investigate potential effects of anisotropic turbulence on leading edge noise. Computational aeroacoustic simulations are performed for anisotropic turbulence with streamwise-to-transverse length scale ratios ranging from 0.33 to 3 on a number of isolated airfoil configurations, including variations in mean flow Mach number, airfoil thickness and angle of attack. Noise reduction due to airfoil thickness is assessed on a NACA 0012 airfoil at zero angle of attack, showing similar trends for both isotropic and moderately anisotropic turbulent flows. Effects of anisotropic turbulence on noise become evident for airfoil configurations at non-zero angle of attack.

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Leading edge noise predictions using anisotropic synthetic turbulence

The first airloads measurements were made in the 1950s at NACA Langley on a 15.3-ft-diameter model rotor, stimulated by the invention of miniaturized pressure transducers. The inability to predict higher harmonic loads in those early years led the U.S. Army to fund airloads measurements on the CH-34 and the UH-1A aircraft. Nine additional comprehensive airloads tests have been done since that early work, including the recent test of an instrumented UH-60A rotor in the 40× 80-ft Wind Tunnel at NASA Ames. This historical narrative discusses the 12 airloads tests and how the results were integrated with analytical efforts. The recent history of the UH-60A Airloads Workshops is presented, and it is shown that new developments in analytical methods have transformed our capability to predict airloads that are critical for design.

Rotorcraft Airloads Measurements: Extraordinary Costs, Extraordinary Benefits The 31st Alexander Nikolsky Honorary Lecture

Study of a low-dispersion finite volume scheme in rotorcraft noise prediction

The helicopter acoustics program at NASA Langley has included technology for elements of noise control ranging from sources of noise to receivers of noise The scope of Langley contributions for about the last decade is discussed Specifically, the resolution of two certification noise quantification issues by subjective acoustics research, the development status of the helicopter system noise prediction program ROTONET are reviewed and the highlights from research on blade rotational, broadband, and blade vortex interaction noise sources are presented Finally, research contributions on helicopter cabin (or interior) noise control are presented A bibliography of publications from the Langley helicopter acoustics program for the past 10 years is included

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Recent Langley helicopter acoustics contributions

The techniques of helicopter rotor noise prediction attempt to describe precisely the details of the noise field and remove the empiricisms and restrictions inherent in previous methods These techniques require detailed inputs of the rotor geometry, operating conditions, and blade surface pressure distribution The Farassat noise prediction techniques was studied, and high speed helicopter noise prediction using more detailed representations of the thickness and loading noise sources was investigated These predictions were based on the measured blade surface pressures on an AH-1G rotor and compared to the measured sound field Although refinements in the representation of the thickness and loading noise sources improve the calculation, there are still discrepancies between the measured and predicted sound field Analysis of the blade surface pressure data indicates shocks on the blades, which are probably responsible for these discrepancies

/pdf/limits-on-the-prediction-of-helicopter-rotor-noise-using-3385hoh2ax.pdf

G. P. Succi

Papers

Limits on the prediction of helicopter rotor noise using thickness and loading sources: Validation of helicopter noise prediction techniques