In this paper, the results of flow-control experiments using single dielectric barrier discharge plasma actuators to control flow separation and unsteady vortex shedding from a circular cylinder in crossflow are reported. This work is motivated by the need to reduce landing gear noise for commercial transport aircraft via an effective streamlining created by the actuators. The experiments are performed at Re D = 3.3 x 104. Using either steady or unsteady actuation, Karman shedding is totally eliminated, turbulence levels in the wake decrease significantly, and near-field sound pressure levels associated with shedding are reduced by 13.3 dB. In the case of unsteady plasma actuation, an actuation frequency of St D = 1 is found to be most effective. The unsteady actuation has the advantage that total suppression of shedding is achieved for a duty cycle of only 25%. However, because unsteady actuation is associated with an unsteady body force and produces a tone at the actuation frequency, steady actuation is more suitable for noise-control applications.

Plasma Actuators for Cylinder Flow Control and Noise Reduction

Phased microphone arrays have become a well-established tool for performing aeroacoustic measurements in wind tunnels (both open-jet and closed-section), flying aircraft, and engine test beds. This paper provides a review of the most well-known and state-of-the-art acoustic imaging methods and recommendations on when to use them. Several exemplary results showing the performance of most methods in aeroacoustic applications are included. This manuscript provides a general introduction to aeroacoustic measurements for non-experienced microphone-array users as well as a broad overview for general aeroacoustic experts.

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A review of acoustic imaging methods using phased microphone arrays

A study was conducted to examine the effects of overall size of directional (or phased) arrays on the measurement of aeroacoustic components An airframe model was mounted in the potential core of an open-jet windtunnel, with the directional arrays located outside the flow in an anechoic environment Two array systems were used; one with a solid measurement angle that encompasses 316 degrees of source directivity and a smaller one that encompasses 72 degrees The arrays, and sub-arrays of various sizes, measured noise from a calibrator source and flap edge model setups In these cases, noise was emitted from relatively small, but finite size source regions, with intense levels compared to other sources Although the larger arrays revealed much more source region detail, the measured source levels were substantially reduced due to finer resolution compared to that of the smaller arrays To better understand the measurements quantitatively, an analytical model was used to define the basic relationships between array to source region sizes and measured output level Also, the effect of noise scattering by shear layer turbulence was examined using the present data and those of previous studies Taken together, the two effects were sufficient to explain spectral level differences between arrays of different sizes An important result of this study is that total (integrated) noise source levels are retrievable and the levels are independent of the array size as long as certain experimental and processing criteria are met The criteria for both open and closed tunnels are discussed The success of special purpose diagonal-removal processing in obtaining integrated results is apparently dependent in part on source distribution Also discussed is the fact that extended sources are subject to substantial measurement error, especially for large arrays

/pdf/effect-of-directional-array-size-on-the-measurement-of-2y0z0wp0xg.pdf

Effect of Directional Array Size on the Measurement of Airframe Noise Components

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

A method is described for the location of moving sources by a microphone array. This method can be applied to out-of-flow measurements in an open jet wind tunnel. For that purpose, an expression is derived for the pressure field of a moving monopole in a uniform flow. It is argued that the open jet shear layer does not form a serious obstacle. A technique is described for reconstruction of power spectra with high signal/noise ratio. The method was implemented for rotating sources, resulting in the computer program ROSI ("Rotating Source Identifier"). Applications of ROSI are given for rotating whistles, blades of a helicopter in hover and wind turbine blades. The test with the rotating whistles demonstrated convincingly the capability to reconstruct the emitted sound. On the helicopter blades, rotating broadband noise sources were made clearly visible. On the wind turbine blades, noise emitted from the leading and trailing edge could be distinguished well.
Paper presented at the 7th AIAA/CEAS Aeroacoustics Conference, Maastricht, The Netherlands, 28-30 May 2001.

Location of rotating sources by phased array measurements

The noise sources of landing commercial aircraft were examined with planar arrays consisting of 96 or 111 microphones mounted on an 8 m by 8 m plate under the glide path on the ground. It is shown that important airframe noise sources can be identified in spite of the presence of engine noise, i.e., landing-gear noise, flap side-edge noise, flap-gap noise, jet-flap interaction noise, slat-horn noise, slat-track noise. A surprising finding is a noise source near the wing tips of some aircraft which is tentatively called wake-vortex wing interaction noise. It is shown to be the by far strongest noise source (6 dB(A) louder than the engines) on a regional jet aircraft.

/pdf/flyover-noise-measurements-on-landing-aircraft-with-a-4yh04926y1.pdf

J M. Hellmig

Papers

Flyover Noise Measurements on Landing Aircraft with a Microphone Array