Fan Test: Instantenous flow measurements
The unsteady flow field up- and downstream of the fan of a representative fan stage was measured by DLR using hot-wire anemometry. With this measuring technique, unsteady flow structures (e.g. the wake of each individual rotor blade) can be measured. The measurements were performed inside Europe’s largest anechoic chamber, the Universal Fan Facility for Acoustics (UFFA), operated by AneCom AeroTest (ACAT) in Wildau.
The objective is to establish an experimental database for a state of the art fan by performing rig tests, which then enables the detailed characterization of the fan broadband noise generation mechanisms and constitutes a basis to validate prediction tools. The periodic changes of the flow velocity and the characteristics of the flow turbulence directly affect the efficiency and the flow acoustics of a turbofan engine.
Hot-wire anemometry provides insight into the flow structures of the rotor blade wakes. The working principle of the technique is based on the cooling effect of a flow on a heated thin wire. The electrical current which is needed to provide a constant temperature of the wire is measured precisely. Since this electrical current is directly related to the flow velocities, unsteady flow structures can be detected with a high temporal resolution. These flow structures consist mainly of the wake of each individual rotor blade.
The focus of the hot-wire measurements was on the flow field in the interstage section between the fan and the outlet guiding vanes (OGV). Two different fan configurations were measured: the fan with a short OGV gap, i.e. the distance between fan and OGV and a second variant with a larger gap. The measurements were performed at different axial positions up- and downstream of the fan and were taken at selected operating points.
Data gathered at the upstream position of the rotor provide information on the inflow conditions, e.g. boundary layer and turbulence statistics. Another three measurement planes were placed in the long OGV gap configuration downstream of the rotor. The post processed hot wire data give insight into the flow field consisting of instantaneous velocities and associated turbulence intensity downstream of the fan, being responsible for the generation of turbo machine noise.
The animation shows the fan with the downstream flow field, where the turbulent intensity of the flow is visualized. The blue colour represents low turbulence while the red colour stands for highly turbulent areas of the flow. It is visible, that the turbulent intensities are not as evenly distributed as excepted – this might be caused by minor deviations of the rotor geometry from a perfect even rotor shape.
Fan Test: Microphone measurements
One objective of the analysis of broadband noise is to determine the total sound pressure level of the up- and downstream propagating waves as well as the level of the turbulent pressure fluctuations. Using an axial line microphone array wall-flush mounted to the flow duct, DLR enabled the separation of these components by the wavenumber-frequency spectrum into three areas (see figure below): The hydrodynamic components, the upstream propagating acoustic waves and the downstream propagating acoustic waves. The boundaries between the areas are determined by the axial Mach number. Hence, they are related to the mean flow speed and the temperature.
In TurboNoiseBB an axial line microphone array was applied in the bypass duct of the UFFA test rig and the three wave components were separated by performing the wavenumber decomposition technique in order to determine the sound field radiated downstream from the fan stage. An exemplary result, which is shown in the figure below, exhibits the distribution of the hydrodynamic and acoustic pressure fluctuations. At low frequencies up to approx. 2 kHz the hydrodynamic pressure fluctuations have high amplitudes due to convected turbulence. The acoustic components are dominated by the downstream propagating waves that are radiated from the fan stage upstream of the measurement section. Reflections occurring at the rig installations such as the throttle are very small as indicated by the detected upstream propagating waves. The parabolic shapes, which appear in the acoustic wavenumber domain, are caused by the resonances inside the duct, where duct modes turn “cut-on”. The pressure fluctuations that occur at highly negative wavenumbers in the frequency range around 4000 Hz are the result of aliasing, where the spacing of neighboring microphones is too large in comparison to the wavelength of the convected turbulence.