Pressure-velocity phase averaged analysis of fan wakes for different blade shapes

The aim of this work is to study the correlation mechanisms underlying the aerodynamic field generated by fans of different shape and the perceived noise. Two different geometries were selected in order to see the influence of the blade shape on both the fluctuating pressure and the velocity field. Both fans have a diameter of 38 cm and were connected to the same electric motor to keep the same driving conditions. The fluid dynamic field was first characterized by using a classical Particle Image Velocimetry technique. Smoke particle were used to seed the surrounding air maintained at rest. The images were recorded for the three rotational speeds of the fan with the maximum frequency allowable by the PIV system, corresponding to 10Hz and for a duration that guarantees the statistics convergence. To study the spatial evolution of the released structures, two sets of images for each speed were acquired at axial distances 0 R and 0.5 R, being R the radius of the fan. Instantaneous and mean velocity and vorticity fields were obtained tracking the movement of the blade tip vortex. From the information obtained by the characterization of the fluid dynamic field, a new set-up including 3 microphones was built. The microphones were positioned in strategic axial positions in order to cope with the supposed position of the coherent structures. The fluctuating pressure and the velocity fields were simultaneously acquired at the same conditions of the preliminary test campaign. A home-made algorithm was used to retrieve the angular position of the blade and phase the simultaneous acquisitions. The velocity fields were finally cross-conditioned with the pressure fluctuations events selected with that algorithm, finalizing the objectives of the work.