Nicolas Molin

Measurements of flow around a flap side edge with porous edge treatment

Wind-tunnel experiments were performed to investigate a flap side-edge vortex, which is a contributor to airframe noise. The flowfield investigation showed that the peak turbulent stresses were contained in the shear layer that rolled up to form the flap side-edge vortex. The wake from the main element was also entrained by the side-edge vortex. The near-field pressure fluctuations where the turbulent shear layer impinged on the flap side edge were broadband in nature from a Strouhal number of 10 to 50. Hot-wire measurements on the downstream vortex identified a broadband instability centered around a Strouhal number of 13.2. A porous side-edge treatment was applied to the half-span flap to modify the flap side-edge flowfield. The effect of applying a porous side edge was to reduce the Reynolds stresses contained within the vortex and the shear layer that formed it. The porous material also had the effect of displacing the vortex further away from the flap surface. This led to a reduction in the broadband pressure perturbations measured at the flap side edge. Compared with the accuracy of the measurements of the aerodynamic forces, the aerodynamic impact of the porous flap side edge was almost negligible.

Bluff body noise control using perforated fairings

Landing gears of commercial aircraft make an important contribution to total aircraft noise in the approach configuration. Using fairings to shield components from high-speed impingement reduces noise. Furthermore, perforating these fairings has been confirmed by flight tests to enable a further reduction. A fundamental study has been performed to investigate and optimize the benefits of bleeding air through the fairing by application of perforations. Experiments have been performed with a simplified fairing–strut combination to clarify the influence of perforations on flow behavior and acoustics. The fairing self-noise is reduced significantly by breakdown of the vortex shedding process, resulting in a reduction of the associated broadband noise level. A redistribution of the velocities is achieved depending on the applied porosity. However, increasing the porosity can result in adverse noise effects due to the bled mass flow washing the strut. Self-noise of the perforations manifests itself at higher frequencies, although scaling of this phenomenon with orifice diameter opens up the possibility to shift it above the upper limit of the audible range.

Control of Noise Sources on Aircraft Landing Gear Bogies

The main landing gears of large commercial aircraft are an important contributor to the total aircraft noise during landing approach. Within a European financed research project called SILENCER (“Significantly Lower Community Exposure to Aircraft Noise”) a study in “advanced low noise landing gear design” was performed to develop operational landing gears that take into account aeroacoustic constraints early in the design stage. In full scale tests a good level of noise reductions was achieved relative to the equivalent conventional landing gears. For the main landing gear however, there was evidence that further noise reductions might be achievable by refining the design of the bogie. This paper describes a follow- on study at 0.25 model scale to investigate in detail some of the design parameters that affect the noise of the bogie. The test results show that the distribution of sources on the bogie is sensitive to the detailed design, but that achieving reductions in total noise is not straightforward. Analysis of flow patterns indicated by the noise data and flow visualization does however lead to a number of ways of adapting the design of the gear to provide significant noise control.

PREDICTION OF AIRCRAFT HIGH{LIFT DEVICE NOISE USING DEDICATED ANALYTICAL MODELS

An exhaustive analytical predictive tool of the aerodynamic noise generated by deected high{lift devices is presented in this paper. Three{dimensional eects are especially investigated. An analytical model of ap side edge noise is rst proposed, on the basis of existing literature. Secondly, another model is proposed to simulate slat horn noise. Both models are comforted by dedicated experiments conducted in the open jet wind tunnel of the Ecole Centrale de Lyon. The corresponding contributions to the far{ eld noise are added to the one from two{dimensional sources spanwise distributed on the slat and ap. The nal results compare favourably with igh t tests of an aircraft with fully deected high{lift devices and retracted landing gears.

Control of landing gear noise using meshes

In previous investigations of aerodynamic noise from landing gears, solid fairings have been used to shield components from the flow so as to reduce the farfield noise. Larger noise reductions have been achieved by designing advanced low noise gears, and in recent studies further reductions of advanced landing gear noise have been achieved by using flowporous fairings. In this study, the application of porous fairings to conventional landing gears is considered to investigate whether their noise reduction potential can be achieved without the potential penalties of re-designing the landing gear with noise as an additional design constraint, as was done for the advanced gears. The results show that porous fairings can provide significantly greater noise reductions than solid fairings, and that porous material wraps can provide useful attenuation of noise with minimal weight penalty.

Reduction of Landing Gear Noise using Meshes

Acoustic and resistance measurements were performed in NLRs Small Anechoic Wind Tunnel on a large number of meshes intended for landing gear noise reduction. The meshes were tested on generic bluff bodies which simulated single and combined landing gear struts. An out-of-flow microphone array was used to localize and quantify the noise sources on the model. It is found that the meshes yield a drastic broadband noise reduction for a wide range of mesh materials and mesh shapes. The noise reduction occurs for all tested angles of attack and for all tested models. For the combined bodies it is found to be sufficient to treat only the upstream component with a mesh.

Noise Sources Control of an Aircraft Landing Gear

3 Landing gears are one of the major sources of airfr ame noise at approach and previous studies have shown that fairings can be used succes sfully to reduce broadband noise radiation in the mid- and high-frequency ranges 1 . In flight tests 2,3 , however, the overall benefit of the fairings was offset by an increase i n low frequency broadband noise and a new tonal noise source that was believed to originate f rom the leg-door or hinge-door of the gear. This paper describes an experimental study on a 1/4 scale model of the main landing gear of the aircraft to control the low-frequency broadband noise and to identify and eliminate the source of the tone, whilst retaining the noise redu ction capability at higher frequencies. The aeroacoustic and aerodynamic characteristics of the model were assessed in a low-speed closed-loop wind tunnel using phased microphone arrays on the roof and wall of the tunnel and surface pressure transducers on the gear. The t one could be readily suppressed or reduced by a number of different treatments. Control of low frequency noise was less straightforward, although some preliminary solutions have been identified.

The use of a fairing and split plate for bluff body noise control

Landing gears have been identified as major noise source contributors during the aircraft’s approach to landing phase. Techniques such as using fairings to alter the flow around the landing gear components have shown to reduce noise . This study investigates methods to further improve the fairing performance. A fundamental study has been performed to understand the influence of using a fairing on a simple cylindrical strut when a split plate is placed in the cavity between the fairing and the strut. Aerodynamic and acoustic tests are performed. Wind tunnel tests equipped with a phased microphone array as well as far field measurements have been conducted out on the fairing-cylindrical strut configuration. The results show that the splitter plate either reduces or eliminates the vortex shedding which in turn reduces noise. This is due to the dividing plate blocking the interaction between the two opposing shear layers aft of the fairing’s trailing edge and reducing their interaction with the downstream strut. Lower velocities around the shell and the strut are observed with the use of the splitter plate as well as reduction of recirculating flow within the shell strut cavity. This alteration in the flow characteristics results in a broadband noise reduction and hence reducing the self-noise generated by the fairing.

Modelling landing gear noise with installation effects

This paper describes continued development and validation of a semi-empirical model of the aerodynamic noise generated by aircraft landing gears at approach. The model has been developed with Airbus funding over a number of years1,2 , but recent studies carried out as part of the EC funded SILENCER project have led to the introduction of a number of new effects that occur on aircraft in flight. The core of the noise model is based on nondimensional source spectra derived from a database of full-scale tests on landing gears installed in the DNW wind tunnel. The new installation effects have been derived using the results of small-scale model tests3 and by analysis of flight test data4. The model is used to examine differences between the performance of noise control fairings installed on a landing gear in a wind tunnel and similar fairings installed on a real aircraft in flight.

Perforated Fairings for Blufi Body Noise Control

Landing gears of commercial aircraft make an important contribution to total aircraft noise in the approach conflguration. Using fairings to shield components from high speed impingement reduces noise. Furthermore, perforating these fairings has been conflrmed by ∞ight tests to further enable noise reduction. A fundamental study has been performed to investigate and optimize the beneflts of bleeding air through the fairing by application of perforations. Wind tunnel tests have been performed with a simplifled fairing-cylindrical strut combination to clarify in∞uence of perforations on ∞ow behaviour and acoustics. The results show that for this speciflc case, the fairing self noise is reduced signiflcantly by breakdown of the vortex shedding process. The perforated fairings, exhibiting difierent porosities between 33% and 55%, appear to be equally e‐cient in removing the spectral peak associated with the vortex shedding. A redistribution of the velocities is achieved depending on the applied porosity. Perforate selfnoise manifests itself at higher frequencies, although scaling of this phenomenon with oriflce diameter opens up the possibility to shift it above the upper limit of the audible range.