Blade surface pressure fluctuations and acoustic radiation from an axial fan rotor due to turbulent inflow

The paper is concerned with the measurement of the random pressure fluctuations on the rotating blades of a low pressure axial fan. The force fluctuations due to the pressure fluctuations are the dominant aeroacoustic sources which lead to the radiation of sound into the far field. It is aimed to find dependencies of the surface pressure fluctuations from integral flow parameters and thus to localise the sources of sound. The rotor of the test fan is exposed to various turbulent flow fields, which are achieved by grids in the inflow. The baseline inflow is without grids, but - in order to control the spatial distribution - the duct boundary layer is removed by a suction flow through a porous wall section. The turbulent intensity and the correlation length were measured using a hot wire probe. One blade is instrumented with miniature piezoresistive pressure sensors. They are flush mounted at six positions along the chord of the blade, approximately at midspan on the surface of the blade's pressure and suction side. The fan rotor was tested in an anechoically terminated duct test stand. The sound power is measured downstream of the rotor in the duct. As expected the signatures of the various turbulent inflows on the radiated sound as well as on the blade surface pressures are clearly measurable. Increasing turbulent intensity of the inflow causes higher sound power levels. The fluctuating surface pressures show a dependency not only on the turbulent inflow parameters but also on the chordwise position, the frequency range, and somewhat on the side of the blade (pressure or suction side). The chordwise distribution of the low-frequency components, corresponding to an acoustically compact blade in chordwise direction, is strongly peaked towards the leading edge. Further downstream towards the trailing edge the influence of the incident turbulence seems to vanish. Here the high-frequency components tend to increase, especially at the blade suction side. It is concluded that in this blade region the turbulent boundary layer becomes more and more the driving mechanism for the surface pressure fluctuations, irrespective of the ingested turbulence. Non-dimensionalisation of the fluctuating surface pressures employing (i) the unsteady thin airfoil theory and the parameters of the ingested turbulence, (ii) the so called outer variables (i.e. the boundary layer displacement thickness and the free field velocity of the blade flow) supports this hypothesis.