Abstract
ABSTRACTMassively unsteady separated flow past a four-wheel rudimentary landing gear is computed based on three different numerical approaches. The methods include Delayed Detached Eddy Simulation (DDES) and the Unsteady Reynolds-Averaged Navier–Stokes (URANS) method based on the two-equation Shear Stress Transport (SST) model as well as Large Eddy Simulation (LES) based on the dynamic model. Using computational fluid dynamics software ANSYS® CFX®, surface features of both the mean and unsteady flows are studied and compared with experimental data, such as time-averaged pressure, surface flow patterns, sound pressure level and so on, while flow field characteristics like instantaneous vorticity and turbulent kinetic energy are obtained to assess the quality of different numerical methods. The accuracy of DDES in predicting the landing gear flow is assessed both aerodynamically and acoustically from an engineering point of view. As expected, URANS can predict the attached flow near the wall well, but fails to obtain reasonable fluctuations in detached regions. Owing to poor near-wall grid resolution, LES predicts some non-physical separation in the area where the flow was originally attached, which adds superfluous turbulence fluctuations. The results of DDES have the advantages of both, and are in good agreement with the experimental results, which characterize the unsteady properties of the flow better. The feasibility of the CFX-DDES method is demonstrated in predicting the unsteady flow for landing gears with moderate grid scales. What's more, because of its numerical robustness and low dissipation, DDES also obtains better near-field noise distribution which shows the potential in noise prediction for engineering applications. Additionally, a detailed analysis aiming at exploring the troublesome mechanisms of noise source generation is also exhibited using DDES. It shows the possibility that strongly unsteady interactions between vortices and landing gear structures may contribute to noise generation.
Highlights
Landing gears are one of the most important contributors to modern airframe noise
Krajnović et al (2008) presented a comparative analysis of simulations for the Rudimentary Landing Gear (RLG) model between the Partially-Averaged Navier–Stokes (PANS) and Large Eddy Simulation (LES)-Smagorinsky method, and the results showed that PANS had a clear advantage over LES with moderate grid resolution
It is observed that the agreement with measurements between both grids on the averaged pressure is reasonably good, both of them somewhat underpredict the pressure at 0–60◦ of the rear wheel
Summary
Landing gears are one of the most important contributors to modern airframe noise. During the landing process, the noise generated by landing gears can reach 25% of the total aircraft noise when the flaps are closed (Monclar, 2003), whereas for some widebody jets such as the Boeing 777 and Airbus 340, the landing gear-induced noise can play a dominant role (Chow, Mau, & Remy, 2002). The landing gear noise consists of broadband noise generated by unsteady vortex shedding, interactions of turbulent wakes between components, shear-layer breakdown and cavity noise from gear wheels. Neri, and Bennett (2016) generally group these techniques into four categories based on how they function, including component enhancement, component smoothing, flow enhancement and flow deflection. Some of these techniques are still at a low technology readiness level due to highly unsteady flow around landing gears, making it difficult to study the noise by experimental means(Dobrzynski, 2010)
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