Experimental study of truncated-cylinder struts for noise reduction of large-scale landing gears

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Abstract

Landing gears are an important noise source of aircraft during approach and landing, which makes the research on their aerodynamic noise reduction a topical problem. This paper experimentally studies the method of noise reduction developed by the authors, which consists in using truncated-cylinder elements for landing gears instead of circular-cylinder parts (e.g. struts, links, actuators, etc). The method stems from a theoretical interpretation of the experimental results obtained for bluff body noise measured with the help of azimuthal decomposition technique, and its efficiency has been many times demonstrated in tests with small-scale models. The present work continues these studies in application to large-scale generic landing gears and natural Reynolds numbers (up to Re = 2.4·106); it reports the results of experimental tests performed in a large anechoic wind tunnel. The large-scale experiments have shown that the truncated-cylinder strut could lead to broadband noise reduction of landing gear noise by up to 2 dB, which thus validates the noise reduction method demonstrated earlier for the small scales only. Beamforming source localization shows that the noise source in the frequency region where noise reduction is observed is associated with the strut. These results obtained for the large-scale models at natural Reynolds numbers allow the noise reduction method of cross-section shape modification to be recommended for implementation in low-noise landing gear design for struts and other cylinder elements.

Introduction

Aircraft noise is one of the major concerns for the aviation industry, as well as an important source of annoyance for the population in the vicinities of airports. With the advent of low-noise high-bypass ratio engines, airframe noise sources have now become dominant noise components during approach phases. Landing gears are considered as one of the major sources of airframe noise [1,2]; as a result, landing gear noise and methods of its reduction were extensively studied in the last decades [1,[3], [4], [5]].

There are various methods to reduce landing gear noise that include solid and porous fairings [6,7], meshes [8], wheel hub caps [9], and flow blowing [10], among others. These methods have different technology readiness levels, from proof-of-concept studies in the laboratory experiments, e.g. water injection [11] or plasma actuators [12], to demonstrations in flight tests, e.g. fairings [13], [14], [15]. An overview of most landing gear noise reduction methods developed to date is presented in [5]. Nevertheless, the problem of landing gear noise reduction is still formidable, because proposed solutions might not be practically interesting due to multiple constraints, including safety, maintainability, mass, integration, system complexity, and cost.

This paper considers a novel method for landing gear noise reduction; namely, the truncated-cylinder geometry for streamlined bluff bodies [16]. This method of noise reduction for a circular cylinder in a cross-flow consists in changing the shape of its cross-section from round to truncated (see Fig. 1), while the radius R of the cylinder remains the same. The truncation point (defined by truncation angle ϑ) is typically located downstream of the separation point so that the mean flow around the cylinder does not experience significant changes, as PIV measurements confirmed [17], see Fig. 1.

The method under consideration is based on a theoretical interpretation of experimental results obtained for bluff body noise measured with the help of the azimuthal decomposition technique [18,19]. The method has been validated in small-scale tests with isolated cylinders at Reynolds numbers up to Re ≈ 8·104 (based on mean velocity and cylinder diameter) [20], inserted in a turbulent round jet, which showed that the truncated cylinder leads to ~5 dB reduction in aerodynamic noise for the cylinder in both turbulent and laminar cross-flows (the mixing part and the initial part of the jet, respectively) in a wide frequency range. The latter point should be emphasized: unlike a number of other methods for cylinder noise reduction that result in mitigation of the narrow-band component of cylinder noise [21], [22], [23], [24], [25] the experiments with truncated cylinders demonstrated broadband noise reduction as well.

The Reynolds effect on the efficiency of the method of truncated-cylinder cross-section has been further studied in [17] for isolated cylinders of different diameters, corresponding to Reynolds numbers from 8·104 to 2.6·105, although for the highest Reynolds numbers there have been concerns regarding the blockage effect of the jet flow by the cylinders with large diameters (the largest cylinder diameter was 10 cm and the diameter of the cross-flow jet was 40 cm). These experiments have shown that the effect of noise reduction with the truncated shape of the cylinder cross-section persists with the increase in Reynolds number. In [26] the method has been studied in application to the main leg of realistic small-scale (1:10) models of nose and main landing gears, and noise reduction of up to 2 dB has been observed.

Although these experimental studies have shown that the method does indeed lead to noise reduction, there still have been concerns regarding its applicability to full-scale landing gears with larger Reynolds numbers and with the presence of other elements of landing gears. There have also been concerns that the small-scale experiments were performed for round cross-flow jets, whereas the rectangular cross-flow would be more relevant and could have lead to different results. Consequently, this work studies the proposed method in application to a full-scale generic landing gear model in a rectangular cross-flow.

It should be noted that there are several studies on the effect of non-circular cross-section shape on cylinder aerodynamic noise [27], [28], [29], [30], [31], [32], [33], [34], where experiments with cylinders of different cross-sections at small and medium Reynolds numbers (up to 1.2·105) were performed. These experiments have not intended to account for the effect of cross-section shape modification on the mean flow and aerodynamics, which raises the question of whether the smaller noise levels for some shapes are due to noise source mitigation or due to change in the aerodynamic characteristics, e.g. different values of critical Reynolds number for different cross-section shapes. Therefore, these studies would also require experimental validation for large Reynolds numbers.

The present work for the first time reports the results for the noise reduction method of the truncated-cylinder shape in application to full-scale landing gears and investigates its efficiency at large Reynolds numbers. It should be noted that it is of interest to study the effect of the truncated cross-section shape not only for the noise of the main leg but also for the other landing gear parts, such as struts, which can be located in the wake from the other elements (e.g. the main leg). It is the latter situation that is realized in this study; the noise reduction method has been applied to a strut in the wake from the main leg of the full-scale landing gear. Therefore, the method is studied in application to a cylinder located in a turbulent cross-flow that in a sense is similar to a cylinder located in the mixing part of the jet, for which the efficiency of the noise reduction method was demonstrated [20] in small-scale tests.

The paper is organized as follows. The next section contains a theoretical background for the truncated cylinder concept. Section 3 describes the landing gear models with circular and truncated-cylinder struts. Experimental facility FL-17 CARDC where the tests were performed is described in Section 4. Section 5 provides the results of the experiments with some analysis of the obtained data.

Section snippets

Theoretical background for the truncated cylinder concept

The truncated cylinder concept for noise reduction stems from the approach to noise generated by a cylinder in low Mach number flows as a diffraction problem for turbulent flow quadrupole sources by the rigid body [18,35]. To illustrate the idea, let us first consider the simple problem of a lateral quadrupole sound source near a circular cylinder. We restrict the problem for the present purpose to the consideration of a two-dimensional case without a mean flow; the cylinder is assumed rigid

Landing gear model

As described in the Introduction, the previous studies with the truncated cylinders were mainly performed either for isolated cylinders or for the main leg of small-scale landing gears. In the present tests, the truncated-cylinder geometry was applied not to the main leg, but to the strut (see Fig. 5) located in the wake of the main leg, so that the strut is changeable and can have either circular or truncated-cylinder cross-section (Fig. 6). The diameter of the circular strut is 70 mm; when it

Experimental facility

To investigate the truncated-cylinder geometry for landing gear noise reduction, the tests with large-scale landing gear models were performed in aeroacoustic wind tunnel FL-17 of CARDC (Mianyang, China) [39], which is one of the largest in the world, shown in Fig. 8. FL-17 is a closed-circuit open-jet wind tunnel; its circuit length is 368 m. In the test hall, the nozzle has an exit area of 5.5 m (width) × 4 m (height).

With the 12.5 MW power supply a maximum wind speed of U = 100 m s−1 is

Experimental results

To ensure the quality of the obtained results, it is expedient to check that facility background noise (i.e. noise in the absence of the model) is low enough to allow the useful signal to be measured. To this end, the background noise in specific frequency bands should be ≥ 3 dB lower than the noise due to the landing gear model.

As an example, comparison of the facility background noise with the noise of the landing gear with circular and truncated-cylinder struts is provided in Fig. 15 for

Conclusions

The paper considers a novel method for landing gear noise reduction; namely, using truncated-cylinder elements of landing gears instead of circular-cylinder parts for a full-scale model. Although previous small-scale experimental studies have shown that the method does indeed lead to noise reduction, there still have been concerns regarding its applicability to full-scale landing gears with larger Reynolds numbers and with the presence of other elements of landing gears. The simple analytical

CRediT authorship contribution statement

Victor Kopiev: Conceptualization, Methodology, Validation, Investigation, Supervision, Funding acquisition. Ivan Belyaev: Formal analysis, Validation, Investigation, Writing – review & editing. Mikhail Zaytsev: Conceptualization, Validation, Investigation. Kun Zhao: Methodology, Investigation, Resources, Visualization, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgments

The Russian authors gratefully acknowledge the financial support provided by the Ministry of Science and Higher Education of the Russian Federation (grant no. 075-11-2020-023) within the program for the creation and development of the World-Class Research Center “Supersonic” for 2020-2025. The Chinese author was supported by National Key R&D Program of China (grant no. 2017YFE0123300).

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