A detailed computational study of a high-lift configuration was conducted to understand the source mechanism behind a dominant acoustic tone observed in recent experiments on slat noise The unsteady Reynolds-averaged Navier-Stokes computations focused on accurate simulation of the local flowfield of a slat with a blunt trailing edge At a slat deflection angle of 30 deg relative to the main element, the simulations revealed the presence of strong vortex shedding behind the slat trailing edge The resulting flow unsteadiness produced large-amplitude acoustic waves propagating away from the trailing-edge region The local spatial resolution of the computed solution was sufficiently fine to capture both the near-field structure and propagation direction of the generated sound The calculated shedding frequency is in good agreement with the measured acoustic frequencies obtained at NASA Langley Research Center's Low Turbulence Pressure Tunnel In contrast, computational results at a slat deflection angle of 20 deg indicated that the shedding process was severely damped, and, therefore, in agreement with the corresponding acoustic measurements during the experiment There was no evidence of a strong acoustic source near the trailing edge at this lower slat deflection

Unsteady Flow Computations of a Slat with a Blunt Trailing Edge

An acoustic analysis based on the Ffowcs Williams and Hawkings equation was performed for a high-lift system. As input, the acoustic analysis used un- steady flow data obtained from a highly resolved, time-dependent, Reynolds-averaged Navier-Stokes calculation. The analysis strongly suggests that vor- tex shedding from the trailing edge of the slat results in a high-amplitude, high-frequency acoustic signal, similar to that which was observed in a correspond- ing experimental study of the high-lift system.

/pdf/computational-aeroacoustic-analysis-of-slat-trailing-edge-1zpdviw1os.pdf

Computational Aeroacoustic Analysis of Slat Trailing-Edge Flow

An acoustic analysis based on the Ffowcs Williams and Hawkings equation was performed for a high-lift system. As input, the acoustic analysis used unsteady flow data obtained from a highly resolved, time-dependent, Reynolds-averaged Navier-Stokes caclulation. The analysis strongly suggests that vortex shedding from the trailing edge of the slat results in a high-amplitude, high-frequency acoustic signal, similar to that which was observed in a corresponding experimental study of the high-lift system.

/pdf/computational-aeroacoustic-analysis-of-slat-trailing-edge-1ktpf95kvp.pdf

Unsteady computational simulations of a multi-element, high-lift configuration are performed. Emphasis is placed on accurate spatiotemporal resolution of the free shear layer in the slat-cove region. The excessive dissipative effects of the turbulence model, so prevalent in previous simulations, are circumvented by switching off the turbulence-production term in the slat cove region. The justifications and physical arguments for taking such a step are explained in detail. The removal of this excess damping allows the shear layer to amplify large-scale structures, to achieve a proper non-linear saturation state, and to permit vortex merging. The large-scale disturbances are self-excited, and unlike our prior fully turbulent simulations, no external forcing of the shear layer is required. To obtain the farfield acoustics, the Ffowcs Williams and Hawkings equation is evaluated numerically using the simulated time-accurate flow data. The present comparison between the computed and measured farfield acoustic spectra shows much better agreement for the amplitude and frequency content than past calculations. The effect of the angle-of-attack on the slat's flow features radiated acoustic field are also simulated presented.

/pdf/time-accurate-simulations-and-acoustic-analysis-of-slat-free-gykaj6y84k.pdf

Time-Accurate Simulations and Acoustic Analysis of Slat Free Shear Layer

Flap-edge aeroacoustic measurements and predictions

An extensive numerical and experimental study of airframe noise mechanisms associated with a subsonic high-lift system has been performed at NASA Langley Research Center (LaRC). Investigations involving both steady and unsteady computations and experiments on a small-scale, part-span flap model are presented. Both surface (steady and unsteady pressure measurements, hot films, oil flows, pressure sensitive paint) and off-surface (5 hold-probe, particle-imaged velocimetry, laser velocimetry, laser light sheet measurements) were taken in the LaRC Quiet Flow Facility (QFF) and several hard-wall tunnels up to flight Reynolds number. Successful microphone array measurements were also taken providing both acoustic source maps on the model, and quantitative spectra. Critical directivity measurements were obtained in the QFF. NASA Langley unstructured and structured Reynolds-Averaged Navier-Stokes codes modeled the flap geometries excellent comparisons with surface and off-surface experimental data were obtained. Subsequently, these meanflow calculations were utilized in both linear stability and direct numerical simulations of the flap-edge flow field to calculate unsteady surface pressures and farfield acoustic spectra. Accurate calculations were critical in obtaining not only noise source characteristics, but shear layer correction data as well. Techniques utilized in these investigations as well as brief overviews of results will be given.

/pdf/fundamental-investigations-of-airframe-noise-tpdeoksyci.pdf

M G Macaraeg

Papers

Fundamental Investigations of Airframe Noise