This paper presents an overview of recent numerical efforts at JAXA aimed at predicting the aerodynamic performance of high-lift full-aircraft configurations at near-stall conditions. Both Reynolds-Averaged Navier–Stokes (RANS) and hybrid RANS/Large Eddy Simulation (HRLES) methods have been employed in free-air and wind-tunnel environments. The RANS approach demonstrates significant limitations near the maximum lift coefficient (CLmax), with accuracy issues persisting despite the inclusion of wind-tunnel walls, as well as severe challenges in achieving iterative solution convergence. In contrast, HRLES methods show improved agreement with experimental data, maintaining error margins within 5

In 2017, the Japan Aerospace Exploration Agency conducted a flight demonstration using its business-jet research aircraft, Hisho, and achieved a 4.2-4.4 dB reduction in overall sound pressure level at the main-landing-gear component level, as measured from the overhead position. Several noise-reduction devices were installed, among which a porous-plate axle cover proved to be the most effective. Fabricated from a metal sheet with a defined perforation pattern, the porous plate reduces landing-gear noise by shielding geometrically complex downstream components from the high-speed upstream flow. This paper summarizes the design process of the porous-plate axle cover based on wind-tunnel test results. The perforation pattern consists of 2-mm-diameter circular holes on a 3 mm pitch in a 60 deg staggered arrangement. The effects of different porous plates and wire meshes were also evaluated using computational fluid dynamics simulations of simplified geometries, clarifying the noise-reduction mechanisms associated with the porous-plate axle-cover concept. From an acoustic perspective, the porous plate should face the high-speed flow to minimize self-generated noise.

In this study, unsteady-flow simulations are performed around two settings of the leading-edge Krueger flap as well as a conventional slat designed for a natural laminar flow airfoil at JAXA. The objectives are to clarify the flowfield, noise sources, and their characteristics around the Krueger flaps compared to the conventional slat noise, and to show the potential of Krueger flaps in lowering the noise compared to the slat. The simulations are conducted for infinite wing configurations with a Krueger flap. The influence of Krueger track noise is investigated through simulations with and without a simplified Krueger track. Furthermore, a study is conducted to assess how the fidelity of the Krueger flap track affects noise levels. The comparison between the slat and Krueger flaps shows significantly less noise for the Krueger flaps, which are designed to have a smaller recirculation region of the shear layer from the cusp, resulting in a longer distance between the reattachment point and the trailing-edge. When the track is attached, the noise level increases significantly across all frequency ranges. However, the Krueger flap shows lower noise levels compared to the slat. Moreover, it is found that the fidelity of the Krueger flap track geometry affects noise levels by increasing high-frequency noise and decreasing low-frequency noise, but the overall change is not significant when compared to the noise level increase associated with the slat track. This study indicates the Krueger flap has promising potential to lower noise compared to the slat.

JAXA's fixed grid RANS simulations for the Fifth High Lift PredictionWorkshop (HLPW-5) are summarized. Using the TAS code, an unstructured grid RANS solver with the SpalartAllmaras (SA) turbulence model, we investigate prediction accuracies for grid-sensitive study in Case 1, configuration build-up in Case 2, and Reynolds number effects in Case 3. Additional simulations using the SA-noft2-R model with a different control parameter C-rot = 1 or 2 assess turbulence model dependency in high-lift flow predictions. In Case 1, a grid convergence study is performed for Pointwise, ANSYS ICEM-CFD, and Heldenmesh grids. Heldenmesh grids showed superior convergence performance. Flow predictions over high-lift configurations in Case 2 revealed underprediction of lift coefficient due to "pizza-slice" separations behind slat brackets. Simulations on the WBSHV-nb configuration (without slat brackets) aligned better with experimental data, suggesting that slat bracket-induced separations negatively impact prediction accuracy. In Case 3, Reynolds number dependency is examined using two grid refinement levels: F- and M-level Heldenmesh grids. Significant differences are found, with M-level grids showing lower lift and earlier stall due to larger inboard separations. The numerical results show grid resolution's sensitivity in Reynolds number effect predictions, necessitating further grid refinement. In addition, SA-noft2-R model results with C-rot = 1 or 2 revealed substantial inboard flow separations compared to the standard SA model. Larger separations with increasing.rot suggest dependency on turbulence model parameters.

This paper summarizes the development of an 8% scale main landing gear model based on NASA's design for the high-lift common research model (CRM-HL). It also presents the main landing gear model's initial far-field noise measurement results independently used in a JAXA low-noise wind tunnel. Each key component of the stand-alone MLG model was acoustically evaluated to assess component-level noise characteristics. Computational results are also presented to compare with the experiment and understand the flow fields. These are part of JAXA's ongoing efforts to improve the understanding of landing gear noise, assess the impact of the main landing gear installed on the semi-span high-lift common research model, validate computational fluid dynamics results, and support initiatives aimed at reducing airframe noise in medium-sized passenger airplanes.

Noise reduction design concepts have been applied to a regional jet’s two-wheel main landing gear to reduce airframe noise during the approach. Several concepts were initially developed for the main landing gear of the Japan Aerospace Exploration Agency’s research aircraft, “Hisho,” based on a Cessna Citation Sovereign business jet. The concepts were tailored to the larger regional jet main landing gear after a careful investigation of aircraft noise sources during flight and wind tunnel tests, the flowfields around the main landing gear by computational fluid dynamics simulations, far-field noise measurements with a 10%-scale wind tunnel model, and an evaluation of structural feasibility. The initial design was then improved upon by further investigations with a 20%-scale wind tunnel model and additional computational simulations. New noise reduction concepts and those applied to Hisho were also evaluated and adopted to enhance the overall noise reduction performance. Compared with a baseline configuration, significant noise reduction, 5.5 dB(A) in overall sound pressure level at a position directly below the main landing gear, was achieved in a wind tunnel test with the 20%-scale model.

This study aims to investigate the effectiveness of a porous-plate in reducing low-frequency airframe noise associated with a landing gear bay of a main landing gear. The study compares the noise reduction effects of a wind deflector with a triangle cross-section and a porous plate bent forward, installed upstream of the gear bay, through CFD simulations. Initially, a noise reduction study is conducted solely with the cavity model, excluding the landing gear. The wind deflector works effectively, deflecting the shear layer away from the cavity opening and reducing the low-frequency noise. The porous plate bent forward further decreases the noise level. This is because the flow passes through the holes effectively with the porous plate bent forward, decreasing the velocity gradient of the shear layer. Subsequently, the study evaluates the effectiveness of the noise reduction methods by applying them to a configuration that includes the side brace and struts of landing gear. The porous plate bent forward proves to be more effective in decreasing noise levels in the low-frequency range, without increasing much in the higher-frequency range.

In this study, numerical simulations are performed to investigate the flowfield, the noise sources, and their detailed characteristics around high-lift configurations featuring either a conventional leading-edge slat or a Krueger flap. While the flowfields and potential noise sources are similar for the two types of configurations, the noise directivity patterns exhibit some differences. The influence of the cavity where the Krueger flap is stowed on the lower surface is also examined via numerical simulations with and without the cavity. Results show that the cavity does not have a significant influence on the flowfield around the Krueger flap and the gap region. However, when the cavity is present, additional noise sources are observed around its rear part, although they are not as significant as the noise sources on the lower side of the Krueger flap. The additional noise sources contribute to enhanced noise levels toward the upstream direction.

The Japan Aerospace Exploration Agency achieved a main landing gear noise reduction of 4.0 to 4.8 EPNdB in a flight demonstration with its business jet based research aircraft, "Hisho," in 2017. Several noise reduction devices were installed, with a main landing gear axle cover made from a porous-plate, a metal sheet with a given configuration of holes, being the most effective. This paper summarizes how the shape of the porous-plate axle cover was designed based on wind tunnel test results, and how its perforation configuration, 2 mm-diameter round holes at a 3 mm pitch in a 60 degrees staggered pattern, was selected. The effects of different porous-plates and wire meshes were also evaluated by computational fluid dynamic simulations of simplified test geometries to deepen the understanding of the noise reduction effect of the porous-plate axle cover device.

In this study, unsteady-flow simulations are performed around a leading-edge Krueger flap and a conventional slat designed for a natural laminar flow airfoil at JAXA. The study aims to explore the noise reduction potential of the Krueger flaps by clarifying their flowfield, noise sources, and characteristics. The simulations are performed for infinite wing configurations, and the effect of their track noise is also investigated. The flowfields around the designed Krueger flap are found to be similar to those around the slat. However, the designed Krueger flap shows significantly less noise, particularly in the lower frequency range, at all the directivity angles, achieving a smaller re-circulation region in the cove and locating the reattachment point of shear layer further upstream of the trailing-edge for low noise performance. When the track is attached, the noise level increases significantly across all frequency ranges and directivities. However, the Krueger flap shows lower noise levels compared to the slat. Moreover, it is found that the thickness of the Krueger flap track does not contribute significantly to the noise level when compared to the slat track. This study indicates the Krueger flap has a promising potential to lower noise compared to the slat.

This manuscript describes unstructured mesh generation and related techniques using the Mixed-Element Grid Generator in Three Dimensions (MEGG3D) software developed by the Japan Aerospace Exploration Agency. MEGG3D uses lossless data compression through open-source libraries, ensuring smoother file input and output. Unstructured volume meshes are created from stereolithography files exported from computer-aided design software. The quality of elements at concave corners is improved using the suppressed marching direction method. Volume meshes can be partially modified to accommodate shape changes, such as adding vortex generators, without requiring the entire mesh to be regenerated. This feature allows for efficient evaluation of the effects of the changes. Additionally, MEGG3D provides postprocessing tools for calculating wall distances, refining meshes, and interpolating solution data.

Airframe noise is one of the main noise sources of the latest passenger aircraft during the approach phase. To demonstrate noise reduction technology based on unsteady Computational Fluid Dynamics (CFD) and wind tunnel tests, JAXA conducted flight tests using the JAXA Experimental Aircraft "Hisho" in 2016 and 2017 in cooperation with industries in Japan. The noise data obtained from flyover noise source measurements successfully showed consistent characteristics with the noise reduction designs for the flaps and the main landing gear. The noise measurement data from the flight test 2017 were used to validate the wind tunnel tests and CFD used in the design phase. Subsequently, the research activities proceeded towards the research and development targeting regional jet aircraft, aiming for practical application in passenger aircraft. This paper overviews these research activities toward developing airframe noise reduction technology for passenger aircraft.

This work summarizes the contribution of the Japan Aerospace Exploration Agency (JAXA) to the 7th AIAA CFD Drag Prediction Workshop (DPW7). This workshop aims at assessing the capabilities of state-of-the-art computational fluid dynamics (CFD) solvers at off-design conditions on industry relevant geometries, such as the NASA Common Research Model (CRM) wing-body configuration, for which experimental data are available from different wind tunnel facilities. Using the committee-provided 6-member family grids, Reynolds-averaged Navier-Stokes (RANS) calculations have been performed with two of JAXA’s in-house solvers, namely TAS-code and FaSTAR, for the grid convergence (case 1), angle of attack sweep (case 2) and Reynolds number sweep (case 3) studies. For the simulations at target lift-coefficient of 0.58 in case 1, the flow is fully attached on the aircraft and good grid convergence is obtained. The grid sensitivity at high angles of attack, for which large separations occur, remains to be determined. The angle of attack sweep study in case 2 shows that the quadratic constitutive relation (QCR) needs to be applied to the one-equation Spalart-Allmaras model with rotation correction (SA-R) to avoid overpredicting the side-of-body separation. The aerodynamic coefficients differ with the experimental results and need to be arbitrarily shifted to at least match the measurements at low angles of attack. This shift is intended to group all numerical and experimental uncertainties and its necessity is currently under discussion within the workshop. In case 3, when Reynolds number and dynamic pressure are changed, the correct drag trends are captured. When TAS-code and FaSTAR used the same turbulence model, remarkable agreement was achieved, and these results are consistent with those of the other DPW7 participants. The differences with the reference experiments and the difficulties in understanding their origin suggest that a more detailed characterization of the experimental setup is required in order to promote CFD progress in adequately capturing the flow physics during off-design phases of the flight envelope.

View Video Presentation: https://doi.org/10.2514/6.2023-4174.vid In this study, CFD simulations were conducted to investigate the noise reduction effects of solid- and porous-plates, as well as their combination and the side-edge effect of the porous-plates, when installed on the side braces of a main landing gear. Initially, a noise reduction study was conducted using a simplified infinite-long side brace-only model. It was shown that the distant porous-plate with both side-edges bent forward was highly effective in reducing the noise by reducing pressure fluctuation observed in the wake generated from the flat side-edges. Additionally, the use of porous side-edges bent forward attached directly to the side brace was also shown to be effective in reducing noise, which has the advantage of being easier to install on the side brace. Subsequently, the effectiveness of these two promising noise reduction methods was evaluated through their application to a three-dimensional geometry of the side brace. It was confirmed that both plate devices with the porous side-edges bent forward effectively reduce the noise level.