Academic literature on the topic 'NOISE LANDING GEAR'

Noise generated by aircraft landing gears is a major contributor to the overall airframe noise of a commercial aircraft during landing approach. Because of the complex geometry of landing gears, the prediction of landing gear noise has been very difficult and currently relies on empirical tools, which have limited reliability and flexibility on the applications of unconventional gear architectures. The aim of this research is to develop an efficient and accurate numerical method to investigate the generation and far field radiation of the landing gear noise. In this thesis a hybrid approach is developed that combines near field flow computations with an integral radiation model to enable the far field signal to be evaluated without the need to directly resolve the propagation of the acoustic waves. The recent advances in the CAA methods are implemented with high-order finite difference compact schemes and a characteristics-based multi-block interface treatment. Aerodynamic noise from a generic two-wheel landing gear model, provided by Airbus LAGOON (landing gear noise database for CAA validation) program, is predicted by using the hybrid approach and compared with the LAGOON database. The unsteady flow field is computed by using a compressible Navier-Stokes solver based on high-order finite difference schemes. The calculated time history of surface data is used in a FW-H solver to predict the far field noise levels. Both aerodynamic and aeroacoustic results are compared with wind tunnel measurements in good agreement. Individual contributions from three components, i.e. wheels, axle and strut of the landing gear model are also investigated to identify the major noise source component. It is found that strong flow-body interaction noise is generated by the flow separated from tire rim impinging on the axle. Based on the same landing gear model, the comparison study using conventional CFD solver FLUENT is performed with a second-order Navier-Stokes finite volume solver to compute the unsteady near field flow and the built-in FW-H solver to calculate the far field sound propagation. The comparison suggests that although conventional CFD method can obtain good timeaveraged aerodynamic results, its ability of predicting sound radiation is limited by the inherent low-order numerical discretizations. The aerodynamic noise from the isolated undercarriage wheel with detailed hub configuration is also investigated using FLUENT. The asymmetric phenomenon in the mean flow is discovered in the wake region of the wheel, which contributes to a positive lift force for the wheel. It is predicted that the isolated wheel radiates relatively strong noise to the sides with several strong tonal noise.

Wheels are significant landing gear noise sources. In this project, high-order numerical simulations were conducted to investigate the landing gear wheel noise generation mechanisms and noise reduction treatments. The high-order solver solves the Navier-Stokes equations on multiblock structured grids. In this work, a modified solver was developed based on cell-centred formulation, which can provide more accurate solutions than cell-vertex space. This solver applies a finite-difference scheme at interior control points and at block interfaces with smooth grid metrics. At discontinuous block interfaces, a finite-volume method is employed as an interface condition. Two sets of interpolation schemes were developed to apply the finite volume method. This cell-centred high-order solver is accurate and robust for aeroacoustic simulations of complex geometries. The numerical solver was applied to investigate the major noise sources of a 33% scaled isolated landing gear wheel by simulating three different wheel configurations using a hybrid CFD/FW-H approach. The configurations simulated include a baseline wheel with a hub cavity and two rim cavities. Two additional simulations were performed; one with the hub cavity covered and the other with both the hub cavity and rim cavities covered. The tyre is the main low frequency noise source and shows a lift dipole and side force dipole pattern depending on the frequency. The hub cavity is identified as the dominant middle frequency noise source and radiates in a frequency range centred around the first and second depth modes of the cylindrical hub cavity. The rim cavities are the main high-frequency noise sources. The largest noise reduction is achieved by covering both hub and rim cavities in the hub side direction. Simulations of two wheels in tandem were also performed to study the wheel interaction noise at different angles of attack. The interaction noise is greatest at zero angle of attack, radiating towards the two sideline directions with a spectral peak at StW = 0.19, based on the width of the wheel. The dominant interaction noise source is the upstream shoulder of the downstream wheel. The wheel interaction noise is reduced at positive angles of attack, as less of the downstream wheel is immersed in the wake of the upstream wheel. A gap fairing was simulated, and it can significantly reduce the interaction noise by eliminating large-scale turbulent structures in the gap region. The downstream wheel hub and rim cavities do not have large contributions to the far-field acoustics.

A study was performed to investigate and optimize the application of perforated fairings for landing gear noise control. The sparse knowledge about this new subject has necessitated a more fundamental study involving a basic fairing-strut configuration, followed by wind tunnel tests on a simplified landing gear configuration incorporating perforated fairings. For the basic configuration, various exchangeable perforated half-cylindrical shells shrouding a circular cylinder were the subject of aerodynamic and acoustic tests. A qualitative and quantitative description has been given of the influence of perforated fairings on time averaged and unsteady flow and the related acoustics. The bled air through the shell prevents the formation of large scale vortices associated with the shell and thereby reduces low frequency noise. However, a test with a noisy H-beam replacing the circular cylinder has indicated that increasing porosity can result in adverse noise effects due to the bled mass flow washing the strut. Shearing flow past the perforate has been shown to create adverse self-noise of which both intensity and spectral content are dictated by the local velocity past the perforate. The application of perforated fairings to the simplified landing gear model reduces the low frequency noise introduced by the solid fairings to values below the baseline landing gear configuration in both side and ground view directions. Exposing the perforate outside the stagnation area does not yield extra noise reduction but introduces perforate self-noise. The synthesis of the conducted studies has shed new light on the application of perforated fairings for landing gear noise control. In particular the effects of porosity and perforation location have been clarified. However more research is needed for further optimization of these parameters.

Experimental measurements and numerical simulations were conducted on a simplified main landing gear model that consists of a leg-door, and a main strut in a parallel configuration. The effects of varying the leg-door angle of attack, and the gap distance between the two elements, were initially studied by two-dimensional and low-order numerical simulations, using the unsteady Reynolds-Averaged Navier-Stokes equations. The strut diameter was specified to the same diameter as a full-scale main landing gear, and simulated under a free-stream Mach number of 0.2, and a Reynolds number based on the cylinder diameter of 1:7 x 106. Further three-dimensional and high-order numerical simulations were conducted on models with a constant gap distance of 8.7% of the cylinder diameter. The high-order solver evaluates the three-dimensional Navier-Stokes equations in the full-conservation form, with the Zonal Detached-Eddy Simulation model. The fidelity of the numerical solver was improved in two parts. Firstly, an Eigenvalue analysis for a multiple-block environment was developed to optimise the combination of spatial and filtering schemes for maximum grid resolution that is numerically stable. Secondly, a grid quality metric, which correlates strongly to the solution accuracy, was developed. A validation database of experimental measurements on a tripped 26% scale interaction model, at a free-stream Mach number of 0.09, and a Reynolds number based on the cylinder diameter of 2 x 105, was developed at the 2:1 m x 1:5 m wind tunnel at the University of Southampton. The experimental and numerical results show that the wake generated by the interaction model is dominated by low frequencies that correspond to the vortex shedding modes of the cylinder, and the door. As the door angle is increased from 0 to 10.7 degrees, the intensity of the cylinder shedding mode decreased. The sound pressure levels of the radiated noise were calculated using the FW-H method. The dominant noise source is a compact dipole, which reduced in strength as the door angle was increased.

A novel concept in noise control devices for landing gears is presented. These devices consist of elastic membranes creating a fairing around the major noise sources. The purpose of these devices is to reduce wake interactions and to hide components from the flow, thus, reducing the noise emission. The design of these fairings was focused on the major noise sources identified in a 777 main landing gear. To find the major noise sources, an extensive noise source identification process was performed using phased arrays. To this end, phased array technologies were developed and a 26%-scale 777 main landing gear model was tested at the Virginia Tech Stability Wind Tunnel. Since phased array technologies present some issues leading to misinterpretation of results and inaccuracy in determining actual levels, a new approach to the deconvolution of acoustic sources has been developed. The goal of this post-processing is to "simplify" the beamforming output by suppressing the sidelobes and reducing the sources mainlobe to a small number of points that accurately identify the noise sources position and their actual levels. To this end, the beamforming output is modeled as a superposition of "complex" point spread functions and a nonlinear system of equations is posted. Such system is solved using a new 2-step procedure. In the first step an approximated linear problem is solved, while in the second step an optimization is performed over the nonzero values obtained in the previous step. The solution to this system of equations renders the sources position and amplitude. The technique is called: noise source Localization and Optimization of Array Results (LORE). Numerical simulations as well as sample experimental results are shown for the proposed post-processing.<br>Ph. D.

An aeroacoustic study was conducted on a 26%-scale Boeing 777 main landing gear in the Virginia Tech (VT) Anechoic Stability Wind Tunnel. The VT Anechoic Stability Wind Tunnel allowed noise measurements to be carried out using both a 63-elements microphone phased array and a linear array of 15 microphones. The noise sources were identified from the flyover view under various flow speeds and the phased array positioned in both the near and far-field. The directivity pattern of the landing gear was determined using the linear array of microphones. The effectiveness of 4 passive noise control devices was evaluated. The 26%-scale model tested was a faithful reproduction of the full-scale landing gear and included most of the full-scale details with accuracy down to 3 mm. The same landing gear model was previously tested in the original hard-walled configuration of the VT tunnel with the same phased array mounted on the wall of the test section, i.e. near-field position. Thus, the new anechoic configuration of the VT wind tunnel offered a unique opportunity to directly compare, using the same gear model and phased array instrumentation, data collected in hard-walled and semi-anechoic test sections. The main objectives of the present work were (i) to evaluate the validity of conducting aeroacoustic studies in non-acoustically treated, hard-walled wind tunnels, (ii) to test the effectiveness of various streamlining devices (passive noise control) at different flyover locations, and (iii) to assess if phased array measurements can be used to estimate noise reduction. As expected, the results from this work show that a reduction of the background noise (e.g. anechoic configuration) leads to significantly cleaner beamforming maps and allows one to locate noise sources that would not be identified otherwise. By using the integrated spectra for the baseline landing gear, it was found that in the hard-walled test section the levels of the landing gear noise were overestimated. Phased array measurements in the near and far-field positions were also compared in the anechoic configuration. The results showed that straight under the gear, near-field measurements located only the lower-truck noise sources, i.e. noise components located behind the truck were shielded. It was thus demonstrated that near-field, phased-array measurements of the landing gear noise straight under the gear are not suitable. The array was also placed in the far-field, on the rear-arc of the landing gear. From this position, other noise sources such as the strut could be identified. This result demonstrated that noise from the landing gear on the flyover path cannot be characterized by only taking phased array measurement right under the gear. The noise reduction potential of various streamlining devices was estimated from phased array measurements (by integrating the beamforming maps) and using the linear array of individually calibrated microphones. Comparison of the two approaches showed that the reductions estimated from the phased array and a single microphone were in good agreement in the far-field. However, it was found that in the near-field, straight under the gear, phased array measurements greatly overestimate the attenuation.<br>Master of Science

This study investigates the flow field and acoustics of a generic four wheel main landing gear. The landing gear is an important airframe noise source during the approach phase. The characteristics of the flow field around the bogie area of the main landing gear are largely unknown. CFD simulations using the DES turbulence model have been used to calculate the unsteady flow field around a generic landing gear model. The surface pressure data has been sampled and used in a FW-H solver to determine far field noise levels. Two different landing gear models have been used, a simplified geometry and a more realistic complex geometry. Three different bogie angles have been simulated: horizontal bogie aligned with the flow, 10⁰ toe up and 10⁰ toe down. Strong streamwise vortices are generated at the front wheels of the landing gear. The trajectory of these vortices determines where the turbulent flow interacts with the downstream components. This interaction leads to surface pressure fluctuations which are a major noise source. The flow field of the simplified configurations shows a consistent trend of the trajectory of the streamwise vortices with respect to changes in bogie angle. The far field noise levels generated by the different components of the simplified configurations are related to the distance at which the streamwise vortices pass. The additional components of the complex landing gear geometry change the characteristics of the flow field. The strong streamwise vortices persist but they do not show the same trend as for the simplified configurations. The wake of the articulation link generates a turbulent in flow for the other components. The different characteristics of the flow field of the complex configurations lead to significant changes in the far field noise levels of the components compared to the simplified configurations.

Due to the recent development of quieter turbofan engines, airframe noise has started to emerge asthe most important noise source. This is particularly true during the approach/landing phase, whenthe engines are operated at low-thrust levels. In order to meet future noise level regulations, thecharacterization and subsequent reduction of landing gear induced noise is necessary. Wind-tunnelaeroacoustic tests have always been the favoured method for assessing and studying the noise generatedby landing gears, but their prohibitive cost has steered the attention towards numerical methods.Since direct flow noise simulations are still too demanding in computer resources, there is astrong interest in developing coupled CFD-CAA simulations as a tool to model and identify flownoise sources. More recently, they have been coupled with phased array methods in order to conductaeroacoustic studies on scaled-down, or simplified, aircraft components. This project investigates theaerodynamic sound sources on a realistic nose landing gear using numerical phased array methods,based on array data extracted from compressible Detached Eddy Simulations of the flow. Assumingmonopole and dipole modes of propagation, the sound sources are identified in the source regionthrough beamforming approaches: conventional beamforming, dual linear programming (dual-LP)deconvolution, orthogonal beamforming and CLEAN-SC. To assess the accuracy of the employedmethods, beamforming maps from flyover, sideline and forward point of views are obtained andcompared to experimental ones originating from wind-tunnel experiments performed on the samenose landing gear configuration by industrial and academic partners of the ALLEGRA project. Anarray design metric is defined to quantitatively assess the fitness of the employed arrays with respectto the different frequencies and distances separating the beamforming and array planes. A geneticalgorithm based on the Differential Evolution method is used to generate optimized arrays for selectedfrequencies in order to reduce the computational size of the problems solved. The modelledsources are used to generate far-field spectra which are subsequently compared to the ones obtainedwith the FfowcsWilliams and Hawkings acoustic analogy. The results show a good concordance betweenthe numerical phased array beamforming maps and the experimental ones, and a good matchbetween the far-field spectra up to a certain frequency threshold corresponding to the quality of themesh used. The presence of specific noise sources has been validated and their contribution to theoverall generated noise has been quantified. The results obtained demonstrate the potential of numericalphased array methods as a legitimate tool for aeroacoustic simulations in general and as atool to gain insight into the noise generation mechanisms of landing gear components in particular.

This diploma thesis deals with the landing gear design of the small two-seat aircraft. The introduction focuses on the conceptual gear design and shock absorber computational dynamic characteristic model. The practical part is focused on the landing gear construction according to the selected parameters based on the conceptual and computational model. The diploma thesis ends with strength calculations and production documentation.