On noise reduction by flap side edge fences

This paper discusses the effects of fences on side edge cross flows. Experimental data are reviewed to show that fences can modify the cross flow in the side edge region of aircraft flaps in such a way that noise reduction can be achieved. Near field pressure data reveal that fences reduce the characteristic frequency of the flow fluctuations. This leads us to postulate that the fences make the far field noise spectra shift downward in frequency. Because airframe noise spectra typically have a negative slope in the most important mid to high frequency domain, this downward shift in frequency leads to an equivalent reduction of noise levels, when measured by conventional metric such as the effective perceived noise level in aircraft certification. To explain the decrease of the characteristic frequency of the flow fluctuations due to the fences, an analytical model is developed, based on vortex dynamics. The predictions from the model are shown to be in very good agreement with measured data, both for the dominant frequency in the baseline case without the fences and for the trends of the decreasing frequency with increasing fence height.