On the effects of a separation bubble on fan noise

Turbomachinery Noise Review

“…This is turn may trigger additional LSB on the stator vanes around mid-chord [85]. All these vortical flow features are potential candidates for additional rotor and stator noise sources, as suggested by the recent analysis of Kholodov & Moreau [79] and Lewis [84], similarly to what was found on the CD airfoil at all simulated Mach numbers [76,78,99] and on the small-span ECL5 CD blade row [90]. For instance, all three cases showed some high-frequency contribution for the LSB or LEV with quasi-tones in the case of the radial-slice cascade caused by the highly coherent rollers formed in the small-span; this noise radiation is clearly seen in the dilatation field or in a modal analysis filtered at high frequencies.…”

“…The classical rotor usually presents a strong LEV that spirals toward the blade tip and strongly interacts with the tip leakage vortices that possibly get totally dislocated. The LEV can be seen as the three-dimensional generalization of the LSB that was previously observed at similar loading on the isolated CD airfoil [78,[97][98][99], and more recently on the small-span CD blade row, which corresponds to 80% blade span of the ECL5 configuration [90]). The tip leakage vortices and the wakes then impinge on the stator vanes without immediate transition to turbulence at the vane leading edge.…”

“…On the ECL5 turbofan case, Al Am and co-workers have considered three different configurations using the same high-order unstructured LES approach with AVBP [90,91]. They have first simulated a radial-slice sector centered around 80% of the actual blade span, as was done previously by de Laborderie et al on the CME2 compressor [32,92]; then they have computed a full-span sector as shown above on the other turbofans; and finally, they have achieved the first aero-acoustic prediction of a full-annulus turbofan.…”

“…For the radial slice and the full-span sector, the OGV has again been rescaled to yield a 1 rotor blade-2 stator vanes configuration (2π/16 periodicity). The radial-slice sector has been simulated at four different mass flow rates corresponding to four fan blade angles-of-attack, to illustrate how the flow topology and noise generation evolve around the approach condition [90]. Three competing noise mechanisms have been observed: a first one coming from the LSB also found near the rotor leading edge in this ECL5 configuration, a second one at the rotor trailing-edge and a final one on the stator leading-edge caused by the rotor wake interaction.…”

Direct noise predictions of a 360° full fan stage using LES

“…Good agreement was obtained between direct noise predictions from a periodic fan/OGV stage and analytical models [12]. To further reduce the computation cost, some studies use simplified configurations of the periodic sector, such as linear cascades [21] or radial slices [22].…”