Druckwellendämpfung an einem Tragflügelprofil mittels Hinterkantenbürsten im Transschall

  • Pressure wave damping on an airfoil with trailing edge brushes in transonic flow

Hammer, Ulf Bastian Helmut; Olivier, Herbert (Thesis advisor); Heufer, Karl Alexander (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

In transonic flow around a supercritical airfoil, unsteady upstream moving pressure waves occur close to the trailing edge. These pressure waves lead to relatively strong pressure fluctuations along the airfoil. The influence of the pressure waves can be observed in particular on the suction side of the airfoil as a partly periodic pressure oscillation. From both aeroacoustic and aerodynamic point of view, the pressure waves represent a comparatively strong disturbance. A damping of the pressure wave amplitudes is therefore of particular scientific and practical interest.In order to determine the influence of the brushes on the flow around a supercritical airfoil (BAC 3-11), comprehensive experimental investigations in transonic flow are conducted. For these investigations, an existing transonic shock tube, which causes disturbing shock/model interactions with the wind tunnel model, is modified to a Ludwieg tube. The aim of this modification is to improve the flow and measurement quality as well as to extend the measurement time. The wind tunnel model is equipped with trailing edge brushes in order to influence the pressure wave generation as well as the wave propagation. The brushes are made of individual bristle fibres and are glued side by side on the lower surface (pressure side) of the airfoil. As part of a parameter study, different brush lengths and bristle diameters are investigated. The brush material is maintained constantly. The experiments are conducted in the transonic flow regime with Mach numbers between 0.7 and 0.74 under an angle of attack of 0° and a Reynolds number of 1 million based on the airfoil depth c (c = 80 mm). The flow behaviour along the suction side is of particular interest. For pressure measurements, the wind tunnel model is equipped with pressure sensors along the suction and pressure side. The determination of the pressure amplitudes for the reference case ("clean airfoil") is carried out with the help of Fourier analyses. In order to determine the maximum pressure amplitude, a method is developed that dynamically adapts the measurement window until the Fourier analysis has found a maximum pressure amplitude (DMDS method). For the study of the considered brush configurations, the pressure amplitudes as well as other comparative variables (standard deviation and signal energy) are also determined and set in relation to the reference values. High speed Schlieren technique is used to visualize the flow field around the airfoil. These images are also used for comparison between the measurements with brushes and the reference measurements.The influence of the brushes is quantitatively presented by a specific damping factor. The damping factor is determined for the maximum pressure amplitudes and for the signal energies and thus describes the degree of damping of these variables in relation to the reference values ("clean airfoil"). A correlation between the damping factors of the brushes and the brush eigenfrequency is established in order to obtain the most efficient brush. This correlation should be used to design the brush parameters on the basis of the brush eigenfrequency.

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