Application of 3D Laser Doppler Anemometry for measurements close to the surface of a high lift airfoil configuration in a large wind tunnel

Kurzfassung

In the development of numerical codes for the prediction of aerodynamic flows there is a strong need to obtain reliable flow field data which is suited to validate turbulence models used in numerical calculations. In the present investigation the flow close to the surface of a high lift configuration has been studied experimentally to obtain data suitable to validate advanced numerical models for complex flow around a high lift airfoil configuration [1]. The investigations have been performed in the closed 3 m x 3 m test section of the DNW-NWB wind tunnel at a constant flow velocity of 50 m/s. The flow velocity has been determined by means of 3D Laser Doppler Anemometry (LDA) using a system developed by DLR Göttingen [2] with a working distance of 3.8 m. The LDA system was operated in a back scatter optical arrangement with a resulting diameter of the probe volume of 0.18 mm. The investigated model is a Swept Constant Chord half (SCCH) model provided by DaimlerChrysler Aerospace with a span of 1.14 m and cord length of 0.45 m. This corresponds to a Reynolds number based on the model chord length of 1.5 x 106. The highly three dimensional flow made it necessary to measure all three velocity components. Due to the swept wing and the measuring position close to the slat and the flap the flow alters the flow direction and all three velocity components contributed significantly to the total flow vector. To achieve knowledge of the influence of the angle of attack two angle of incident a, 10° and 22° respectively, have been investigated. Velocity data as well as turbulent quantities have been successfully acquired close to the surface of the model. It was possible to obtain data up to 0.4 mm close to the surface of the SCCH model. To determine the actual position of the model the DLR position monitoring system has been employed in the DNW-NWB [3]. The system is capable to acquire data with frequencies up to 400 Hz. The present optical set up yields to a spatial resolution better than 0.02 mm. For LDA measurements position data has to be recorded simultaneously with LDA data which is controlled by the instantaneous velocity data rate and not by fixed time intervals. The digital position data is therefore continuously converted into analogous voltage which was recorded by the LDA processor. This permitting storage of latest position data synchronously with LDA data. The experimental results of the position monitoring show a static deviation of the model position under aerodynamic forces but with no visible influence of the model vibration on the flow velocity. Which has been determined by a correlation of flow velocity and model position, which provide no significant correlation between these two properties. For LDA measurements it is necessary to apply high efficient seeding with small tracer particles without disturbing the flow. During these experiments an appropriate seeding technique has been successfully applied on the bases of oil droplets. The flow close to the surface of a high lift configuration airfoil has been studied by means of 3D LDA. The LDA data show clearly the different types of boundary layer flow. In the present investigation it has been found the influence of the wake of the slat and the main wing became more important with increasing angle of incidence. Furthermore it can clearly be seen that the three dimensional mean flow leads to non isotropic turbulence properties and hence appropriate numerical models have to be applied. The obtained results demonstrate that the 3D LDA set up has been successfully adapted to investigate the flow close the surface of a high lift airfoil configuration in the closed test section of the DNW-NWB. The achieved data will be used to validate results obtained by different advanced numerical codes applied by DLR Braunschweig, , TU Berlin and DaimlerChrysler Aerospace.