Q value colored by Mach number
Shia-Hui Peng, FOI/Chalmers
|Start of project:||August 2010|
|End of project:||August 2012|
Nowadays, aviation is becoming more and more important for our everyday lives. The increase in popularity, however, also results in some drawbacks. As airports are often close to highly populated urban areas, people are exposed to significant noise levels, radiated from airplanes during approach and take-off. While during take-off the engine is the main source of noise, the so called airframe noise becomes more important during approach. Airframe noise is the noise radiated by the airplane due to the interaction of structural parts and the airflow. The two main sources for airframe noise are the landing gear and the high-lift devices. High-lift devices are deployed on the airfoil in order to increase the lift force, while the plane is decreasing in speed. These airfoil extensions often include a leading edge slat and one or multiple trailing edge flaps. Besides the noise exposure, high-lift configurations are also interesting to investigate from a technical point of view. It is important to determine the maximum lift force, CLmax, and at which angle of attack it occurs. Increasing the angle of attack to angles beyond this maximum, leads to stall, which describes a sudden reduction in lift force.
The heavy noise exposure has lead to strict regulations for airplanes, especially during night time. Only aircraft that match these regulations are allowed to takeoff or land. This is why industry has a great interest in reducing the noise levels radiated by their aircraft. Traditionally, the approach to analyzing aeroacoustics, has been an experimental one. Of course, these tests can be performed only rather late in the development process, because a prototype or at least model is necessary. Deficiencies discovered that late in the development process can only be fixed in a re-active manner, which is usually quite costly and does not lead to the optimal solution.
Increasing computer power opens up the possibility to investigate the turbulent flow around high-lift configurations numerically with the help of Computational Fluid Dynamics (CFD). This allows not only to study the flow itself, but also to predict the aeroacoustics of the high-lift configuration. Being able to use virtual models for investigating the aeroacoustics, makes it possible to shift the aeroacoustic analysis to an earlier point in the development process, so it becomes much easier to eliminate discovered deficiencies.
There are different approaches to use CFD for aeroacoustics, where the most desirable would be to use Large Eddy Simulation (LES). All scales that are lager than a certain filter width (usually the grid size) are calculated and only the smallest scales are modeled (sub-grid scales). However, even with the rapid increase in computational power, such a simulation is out of reach for decades. A computationally inexpensive approach would be to use Reynolds-Averaged Navier Stokes (RANS). Noise is an unsteady phenomenon and therefore RANS cannot be used to predict it. Even unsteady RANS (URANS) will not suffice, since all turbulence properties are modeled, which does not provide the accuracy needed. An approach to combine the advantages of both, LES and RANS, has been undertaken by Spalart et al. in 1997 and is known as Detached Eddy Simulation (DES). The idea is to use RANS to predict the boundary layers and to use LES in the rest of the computational domain. DES and its recent improvements could already successfully be applied to predict aeroacoustics.
"Aerodynamic and Aeroacoustic Analysis of a Multi-Element Airfoil using Hybrid RANS/LES Modeling Approaches", thesis of Lic. of Engng, Division of Fluid Dynamics, Dept. of Applied Mechanics, Chalmers University of Technology, Göteborg, Sweden, 2012.
View PDF file of thesis
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