Closed Section Wind Tunnel Research Articles

Evaluation of the effect of microphone cavity geometries on acoustic imaging in wind tunnels

Aeroacoustic measurements performed by flush-mounted microphone arrays on the walls of closed-section wind tunnels are contaminated by the hydrodynamic pressure fluctuations of the wall’s boundary layer. This study evaluates three different microphone cavity geometries for mitigating this issue. Their improvement to the signal-to-noise ratio (SNR) and the accuracy of their acoustic imaging results are compared to a flush-mounted microphone array. The four geometries include: (1) an array of flush-mounted microphones as the baseline, (2) a cylindrical hard-plastic cavity with a countersink, (3) a conical cavity made of melamine acoustic absorbing foam, and (4) a conical cavity with star-shaped protrusions, also made of melamine. The three arrays with cavities were covered with a steel-wire cloth to reduce the boundary layer fluctuations at the microphone while the baseline array was uncovered. Two sound sources were tested in an aeroacoustic wind tunnel for assessing the performance of the different cavities: a speaker placed outside the flow and a distributed sound source generated by a flat plate inside of the flow. When using conventional frequency domain beamforming, both cavities made of melamine offer up to a 30 dB increase in SNR with respect to the flush-mounted case, followed by the hard-walled cavity with up to a 20 dB increase. This is a 20 dB improvement when compared to the single microphone cases. The melamine cavities also provide cleaner acoustic source maps and accurate spectral estimations for a wider frequency range. The effect of cavity placement and geometry on the coherence, which affects the beamforming analysis of the acoustic signal was negligible for all cases. Distributed sound source measurements using the three arrays agreed with predictions using the Brooks, Pope, and Marcolini (BPM) model, showing that the cavities could detect vortex shedding that was undetectable by the flush array.

A probabilistic approach for cross-spectral matrix denoising: Benchmarking with some recent methods

Array measurements can be contaminated by strong noise, especially when dealing with microphones located near or in a flow. The denoising of these measurements is crucial to allow efficient data analysis or source imaging. In this paper, a denoising approach based on a Probabilistic Factor Analysis is proposed. It relies on a decomposition of the measured cross-spectral matrix (CSM) using the inherent correlation structure of the acoustical field and of the flow-induced noise. This method is compared with three existing approaches, aiming at denoising the CSM, without any reference or background noise measurements and without any information about the sources of interest. All these methods make the assumption that the noise is statistically uncorrelated over the microphones, and only one of them significantly impairs the off-diagonal terms of the CSM. The main features of each method are first reviewed, and the performances of the methods are then evaluated by way of numerical simulations along with measurements in a closed-section wind tunnel.

Improvements in closed-section wind-tunnel beamforming experiments of acoustic sources distributed along a line

Phased-array microphone techniques are widely used tools for the measurement and analysis of aeroacoustic noise sources. Nevertheless, beamforming results of sources distributed along a line, such as airfoil trailing-edge and slat, have not been entirely understood. This paper investigates the performance of beamforming methods in representing sources distributed along a line and estimating their spectral level. Conventional beamforming and the deconvolution techniques DAMAS and CLEAN-SC were employed for both synthetic and physical sources. The physical source is the slat of an MD30P30N high-lift model tested in a closed-section wind-tunnel with array microphones mounted flush to the tunnel wall. The synthetic source consisted of a large number of uncorrelated aligned monopole point sources. DAMAS was found to be the most accurate approach for both synthetic and physical test cases. The physical source results were not as accurate as the synthetic source ones, nevertheless, they were substantially improved by array shading and by an acoustic treatment (foam coating) on the working section walls. The most effective array shading methodology was based on the mean coherence level of each microphone, here referred to as CW (Coherence Weighting shading).

On closed-section wind-tunnel aeroacoustic experiments with a two-dimensional lifting body

Slat noise experiments are sometimes performed to provide data for the validation of numerical procedures. Under such circumstances, closed-section wind-tunnels, in contrast with open-section ones, offer well-defined boundary conditions for simulations. Nevertheless, many difficulties arise in aeroacoustic experiments with two-dimensional lifting body tests in such wind-tunnels. Reverberation and acoustic images can also interfere with the measurements. This paper addresses the effects of the aforementioned issues on the MD30P30N airfoil slat aeroacoustic measurements. Conventional beamforming and DAMAS deconvolution techniques were employed for the acoustic data post-processing. Pressure taps were used in the assessment of spanwise and chordwise pressures on the high-lift airfoil model surfaces. The interaction between the airfoil and the wind-tunnel wall boundary-layer was mitigated by the application of suction on the tunnel wall and rendered a more two-dimensional flow. Acoustic polyurethane -based foam was applied on the test-section walls for reducing reverberation effects on the slat noise source maps. The results showed the error in the noise spectrum associated with the three-dimensional effect at the model ends may correspond to a decrement of 4 deg or higher of the entire wing angle of attack. The acoustic treatment over the wind-tunnel closed-section walls mitigated the mirror sources, nevertheless, the effects of such source images on both noise source maps and noise spectra in the region of interest were considered small.

Airframe noise measurements in a large hard-walled closed-section wind tunnel

The airframe noise of a 1:2 scaled aircraft model is investigated through a series of experiments in the hard walled closed test section of a large non-anechoic wind tunnel. A surface mounted phased microphone array is used to acquire the wall pressure fluctuations generated by the interaction of the wind tunnel flow and the propagating noise. Data from two measurement campaigns are analysed. The wind tunnel acoustic behaviour and the array performance are first assessed evaluating the results obtained from a reference sound source installed in the centre of the test section. The combination of different airflow speeds, array positions and sound sources are tested. The airframe noise is evaluated acquiring pressure fluctuations in different configurations. The main parameters varied during the tests are the flight configuration, the speed of the airflow, the incidence angle of the model and the position of the microphone array. Data are processed and then analysed in the frequency domain and using a conventional beamforming algorithm to retrieve the sound source maps over the regions of interest. Source maps are deconvolved with a CLEAN-SC routine and single source contributions determined through integration of auto-powers. The resulting spectra are finally scaled determining the exponents of the pertinent scaling laws.

Slat Noise from an MD30P30N Airfoil at Extreme Angles of Attack

This study investigates the slat noise of a two-dimensional scaled, unswept, and untapered MD30P30N high-lift model. The experimental data refer to aeroacoustic and aerodynamic measurements in a closed-section wind tunnel for a wide range of angles of attack (from up to the stall; approximately at 18 deg) and Mach numbers between 0.07 and 0.1. Three slat configurations (the original MD30P30N, another with a higher slat deflection, and one with smaller slat gap and overlap) are studied experimentally. The signal processing applied to the acoustic data involves conventional beamforming enhanced by two deconvolution algorithms, namely, DAMAS and CLEAN-SC. An original variation of the beamforming cluster approach that is based on the coherence level among microphone pairs is introduced, and it improves the results obtained by DAMAS. Below and above 12 deg angles of attack, the slat noise is very small and mostly below the wind-tunnel background noise for all configurations. Between and 12 deg angles of attack, the slat noise spectra are substantially affected by the slat configuration, although it always contains a dominant low-frequency content, a midfrequency broadband noise, and a single high-frequency broad peak. Within this range, the lower angles of attack display the strongest low-frequency narrowband peaks. In fact, at lower angles of attack, the low-frequency narrowband peaks scale with a Mach power above 10.

Experiment Evaluation of Skin Friction Drag by Surface Tailoring

Reduction of drag is an important role of aerodynamic specialist in real time world. The performance of forward moving object improved when the drag is reduced. Skin friction drag caused when the fluid tending to shear along the surface of the body and it is dependent on energy expenditure. Initial research concluded that nearly 20 to 40% of total drag is skin friction drag, based on flight forward velocity. This means a lot of fuel burned. In this paper we investigate a methodology to reduce the skin friction drag by implementing different kinds of exterior treatments. The ideology inspired from the world fastest moving oceanic creature. Structures are fabricated based on the replica of scales of the oceanic creature. The outer skin of the aerofoil NACA0012 is modified like shark scales. Then it is tested using open type sub sonic wind tunnel. In addition to that, the leading edge thickness effect also studied. The turbulent flow phenomenon is validated at different velocities and compared with numerical results using STAR CCM+. From the plots and graphical results, it is found that the skin friction drag is generated less due to reduction of transverse shear stress present in turbulent flow and skin friction drag depends on boundary layer thickness and on the percentage of chord of flow separation. In addition to this, the result delivers that the ordinary polished surface produces more drag than the modified scales. The outlook of this technology is excrescence for different applications. This open section wind tunnel testing produces 10-15% reduction in drag and can be turn to high values when the experiment is conducted in closed section wind tunnel with real time atmospheric conditions, which can be done as a future work.

Poststall Airfoil Performance and Vertical-Axis Wind Turbines

The sensitivity of vertical-axis wind turbine startup to airfoil aerodynamic performance characteristics is analyzed. Existing poststall airfoil data and the corrections used in acquiring them are critiqued. Uncertainties in the existing data that are large enough to impact on vertical-axis wind turbine startup modeling are established, with wind-tunnel blockage identified as a potential cause of the discrepancies. Using a conventional closed-section wind tunnel with blockage corrections applied to data and a passive, tolerant test section design, tests are conducted on five NACA 0015 airfoils with a range of chord lengths. The passive tunnel is found to be better at minimizing the effects of blockage, and it makes no use of correction formulas that are not strictly applicable when flow is separated because they are based on potential flow theory. Preliminary results are presented from a study varying Reynolds numbers and showing little variation in poststall performance for the NACA 0015 in the range of Reynolds numbers from to .

Experimental investigation on the effect of slat geometrical configurations on aerodynamic noise

Relatively few studies have addressed the slat noise dependence on gap, overlap and deflection angle, although slat noise level and spectral content are topics of increasing concern in the aeronautical industry. This paper presents new experimental data on the subject. An array of microphones placed in a closed-section wind tunnel was used in the experiments. Beamforming signal processing enhanced by DAMAS (Deconvolution Approach for the Mapping of Acoustic Sources) was applied to the data. The experimental data covered a range of angles of attack and Mach numbers, for which the typical slat noise signature features high-level narrow-band peaks, broadband noise and a single broad tone. The narrow-band peaks often dominate the slat noise spectra and arise at Strouhal numbers up to approximately 5. The broadband noise is well characterized for Strouhal numbers between 5 and 20, whereas the broad tone arises for Strouhal above 20. A total of 10 different slat configurations, including variations in gap, overlap and deflection angle were tested. The slat noise dependence on gap, overlap and slat deflection angle was assessed by letting each of them vary separately. The slat configuration exerted a substantial effect on all slat noise components, with variations of up to 20dB/Hz in the level of the narrow-band peaks. In general, the level of the narrow-band peaks reduced and the high frequency content increased as the gap, overlap, angle of attack and slat deflection increased separately or in combination. The aerodynamically optimized baseline configuration is very distinct from the lowest noise one.

Slat Noise: Aeroacoustic Beamforming in Closed-Section Wind Tunnel with Numerical Comparison

Experiments in closed-section wind tunnels are more suitable for comparisons with numerical results than corresponding experiments in open-section tunnels because well-defined boundary conditions far from the model can be established for the numerics. This paper reports acoustic experimental results on the slat region of a two-dimensional high-lift model. An objective was the production of experimental data to be used in the testing of numerical procedures. Conventional beamforming combined with a deconvolution technique was used. The Reynolds number was approximately 1 million, whereas Mach number and angle of attack ranged from ~0.07 to and 2 to 10 deg, respectively. To prevent interaction between wind-tunnel-wall boundary layers and the model, which causes undesirable three-dimensionality, suction was applied on the wind-tunnel walls, and its effect was investigated. Experimental results for two angles of attack were compared with computational results given by a lattice–Boltzmann solver for the validation of the experimental procedure based on side-wall suction and deconvolved source map computations. The results agreed, in particular regarding the dominant parts of the spectra, and were considered a cross validation of both experimental and numerical approaches. The numerical results were used in the characterization of the flow in the slat cove.

Aeroacoustic Imaging Experiments of Airframe Noise in Lined Wall Closed-Section Wind Tunnel

AbstractAcoustic imaging is an indispensable experimental aid in the design of silent aircraft. In this work, acoustic imaging is performed in a classical closed-section wind tunnel that has been extensively modified using specifically designed liners on the walls of the test section. The details of the associated acoustic modification and the aeroacoustic imaging method are introduced in this paper. The acoustic test performance is carefully evaluated by examining background noise reductions and acoustic beamforming results. The experimental outcomes suggest that the proposed acoustic modification could suppress background noise by 5 dB at 40–80 m/s test flow speeds. The beamforming results clearly identify the dominant airframe noise sources at landing gears and high-lift devices. The experimental results agree fairly well with the preceding computational and empirical predictions. The proposed acoustic imaging technique has been shown to be an effective experimental technique in identifying airframe n...

Optimization of Microphone Array Wall Mountings in Closed-Section Wind Tunnels

The objective of the present paper is to improve the implementation of microphone arrays on the wall of closed-section wind tunnels. A test campaign has been conducted in the ONERA F2 1.4 m × 1.8 m wind tunnel to investigate the performance of thin and smooth metallic clothes to protect the microphones from the hydrodynamic fluctuations in the turbulent boundary-layer flow and, still, to preserve the transmitted acoustic signal. Three microphone mountings have been considered : individually recessed microphones in (i) cylindrical and (ii) conical apertures and (iii) a recessed, screened, large-scale microphone mounting plate (microphones flush with the plate). It was found that by covering the top of the cylindrical and conical microphone apertures with a wire-mesh, the strong self-noise due to the interaction of the boundary-layer flow with the cavities can be suppressed. The most significant noise reduction is obtained when the large-scale microphone mounting plate is recessed behind the metallic wire-mesh. A reduction by up to 20 dB is then obtained with respect to a flush-mounted, unscreened, microphone. This microphone mounting out-performs the cone-nose-protected and Kevlar-protected microphone mountings, two conventional low-noise devices. Thanks to an original calibration technique, the acoustic response of the tested microphone mountings could be evaluated in presence of flow (� 80 m/s). The recessed, screened, microphone mounting plate has an oscillating acoustic response (in function of the frequency) by less than 5 dB deviation. This attenuation can be post-corrected by using a very simple acoustic model.

Measurement and control of aircraft landing gear broadband noise

Noise generated by aircraft landing gear is a major contributor to the overall airframe noise during landing approach of a commercial aircraft. Fairings covering geometrically complex areas of the gears have proved to be very effective in noise reduction, but in tests on an A340 main landing gear the overall benefits of the fairings were offset by an increase in low-frequency noise. In this study the measurement and control of broadband noise throughout the frequency range have been investigated using a 1/4 scale A340 main landing gear model that has been tested using a range of different measurement methods and wind facilities. Experiments were firstly conducted in a conventional closed-section wind tunnel using two phased microphone arrays mounted on the ceiling and sidewall of the tunnel respectively, and noise control strategies with modified fairings were developed. Noise tests were then performed in an open-jet aeroacoustic facility using far-field microphones to verify the wind-tunnel test results. Comparison between the two tests showed that the relatively smaller noise-reduction potential of the fairings measured in wind tunnel was due to the higher flow blockage of the tunnel. Nevertheless, the experiments indicated that the phased microphone arrays could be applied to measure the noise reductions achieved by noise control treatments with reasonable accuracy, especially in mid- and high-frequency range. The aeroacoustic facility test results were finally projected to full scale and compared to the full-scale model wind-tunnel test. Both results were shown to agree over a wide frequency range, indicating the validity of the tests on the 1/4 scale model.

Obtaining absolute acoustic spectra in an aerodynamic wind tunnel

Cost effective methods for identifying and reducing sources of noise have become essential in the design of many modern transport vehicles. Whilst closed-section wind tunnels can readily evaluate aerodynamic performance, obtaining accurate acoustic spectra is often a major challenge because of the poor signal to noise ratios available. In this paper, methods of obtaining absolute spectra from the non-acoustically treated Markham wind tunnel at the University of Cambridge are discussed. Initial measurements with a small monopole source compare well with simulations and show that it is possible to obtain similar spectra from two nested acoustic arrays. However, a series of further experiments with simplified landing gear models show very different spectra from each array. By comparing measurements with simulations, it is shown that negative side lobes affect beamforming source maps. Measurements of an ‘empty tunnel’ cross spectral matrix allow the removal of sidelobes, providing much greater consistency between spectra. Finally, a dipole beamforming vector is used to account for the directivity of the landing gear noise, leading to good agreement between absolute spectra from the differently sized arrays. This analysis demonstrates that data from a phased array in a hard-walled, aerodynamic wind tunnel can provide meaningful acoustic spectra from low-noise models.

Identification and Attenuation of a Tonal-Noise Source on an Aircraft's Landing Gear

Aircraft landing gear noise is a major contributor to the overall airframe noise during the landing approach of a commercial aircraft. Fairings covering geometrically complex areas of the gears have proved to be very effective in noise reduction. However, in tests on an A340’s main landing gear, the overall benefit of the fairings was offset by both a slight increase in low-frequency broadband noise and a strong tonal noise. In this study, the identification and attenuation of the tonal-noise source has been carried out using a one-quarter-scale A340 main-landing-gear model. Aeroacoustic and aerodynamic tests were conducted in a closed-section wind tunnel, using a phased microphone array on the ceiling of the test section, flush-mounted pressure transducers on the model surface, and the particleimage- velocimetry technique. Far-field noise tests were then taken in an open jet aeroacoustic facility, using far-field microphones to verify the wind-tunnel test results. The experiments demonstrated the tonal-noise mechanism and a number of different control methods.

Implementation of a Phased Microphone Array in a Closed-Section Wind Tunnel

This paper describes the design and implementation of a phased microphone array system for aeroacoustic measurements in a closed-section wind tunnel at the Department of Engineering at the University of Cambridge. The tunnel has a test-section area of 1.67 x 1.22 m and can generate wind flows up to 60 m/s. The research started by conducting a feasibility study to find out whether an array could be used in the presence of the tunnel background noise. The boundary-layer pressure fluctuations were found to be correlated according to the Corcos model (Corcos, G. M., Journal of Fluid Mechanics, Vol. 18, No. 3, 1964, pp. 353-378.). Their level can be reduced by recessing microphones away from the flow, but the benefits of such an approach in this facility were found to be insufficient for it to be followed. Two different size arrays, for frequency ranges 650 ∼ 6500 Hz and 5 ∼ 50 kHz, were designed and installed. A range of characterization and shakedown experiments are described; in particular, it is shown that averaging can increase signal-to-noise ratio and that the array can detect sources up to 6 dB below the mean cross-spectral level of the background noise.

Experimental study of cavity resonance suppression methods

Flow-excited cavity resonance is a common, generally undesirable, sound-generating phenomenon that occurs in many engineering applications. In this study, low-speed cavity resonance suppression techniques were investigated. The focus was on two methods: leading edge spoilers and leading edge air mass injection. Experiments were conducted in a closed section wind tunnel, which included a secondary blowing apparatus for controlling boundary layer thickness. The cavity, attached to the wind tunnel test section, acted as a side-branch resonator. Smoke flow visualization was performed for various operating conditions. For example, a nondimensional spoiler length of 0.156 (based on cavity length) inclined at a 50-deg angle of attack typically provided a 12 dB (re: 20 μPa) reduction of the 140-dB, 124-Hz tonal cavity pulsation. Air mass injection through a perforated array at a rate of 2.8 liter/s yielded a 27-dB reduction. In general, the results suggested that such leading edge suppression techniques yield greater orifice shear layer thicknesses, similar to that of increasing the boundary layer thickness. The thickened shear layer may cause greater vortex diffusion, a key parameter establishing the level of the cavity pressure fluctuations. Diffuse vortices lead to weaker cavity pressure oscillations and lower vortex convection velocities.

Investigation of Wedge Probe Wall Proximity Effects: Part 2—Numerical and Analytical Modeling

An experimental study of wedge probe wall proximity effects is described in Part 1 of this paper. Actual size and large-scale model probes were tested to understand the mechanisms responsible for this effect, by which free-stream pressure near the outer wall of a turbomachine may be overindicated by up to 20 percent dynamic head. CFD calculations of the flow over two-dimensional wedge shapes and a three-dimensional wedge probe were made in support of the experiments, and are reported in this paper. Key flow structures in the probe wake were identified that control the pressures indicated by the probe in a given environment. It is shown that probe aerodynamic characteristics will change if the wake flow structures are modified, for example by traversing close to the wall, or by calibrating the probe in an open jet rather than in a closed section wind tunnel. A simple analytical model of the probe local flows was derived from the CFD results. It is shown by comparison with experiment that this model captures the dominant flow features.