A Testbed for large scale and high Reynolds number Airframe Noise Research

Abstract

Airframe noise testing in open jet wind tunnels is and will be an indispensable means to (i) identify airframe noise sources and develop noise reduction technologies and (ii) to provide validation data in order to support the development of numerical acoustic methods like e.g. CAA codes. Usually such wind tunnel experiments were conducted in small research facilities like DLR's AWB (nozzle size 1.2 m x 0.8 m) using 2-dimensional wind tunnel models featuring a retracted chord length of about 300 to 400 mm. In very rare cases full aircraft models of 1:7.5 to 1:11 scale were tested in the Large Low-Speed Facility of DNW (DNW-LLF), which provides a nozzle size of 8 x 6 m2 and a maximum flow speed of 78 m/s. Even though these full aircraft models provide almost realistic 3-dimensional flow conditions the mean retracted chord length is pretty similar to the 2D test cases. Therefore most airframe noise tests are restricted to a Reynolds number regime between Re=1.0*106 and Re=2.0*106. This finally means that the aerodynamic conditions during most airframe noise wind tunnel experiments do not comply with realistic operating conditions. In order to overcome these deficiencies DLR, Airbus and EADS-IW decided to design a new large scale test bed for airframe noise testing. This plan could be realized in the mainframe of the German national founded research project FTEG (Flight Physics Technologies for Green Aircraft).The finally designed high lift system features a retracted chord length of 1200 mm and a wing span of 7200 mm. It was tested for representative approach conditions and Reynolds numbers up to 5.0*106. By means of this wind tunnel model high Reynolds number slat noise data were acquired that later serve for the validation of the DLR CAA code PIANO. It reveals the slat noise spectra did not contain any relevant tonal slat noise components that are well known from small scale experiments. Dedicated Reynolds number variations gave evidence that (i) low Reynolds number tests on high lift airfoils do not completely represent the broadband sound radiation typical for respective full scale components and that (ii) the tonal components are related to more than one generation mechanism. The new large scale F15LS high lift system successfully proved its ability to enable the assessment of noise reduction technology and the acquisition of validation data for numerical methods under almost realistic flow conditions.