Parametric Study of Fan Flow Deflectors for Jet Noise Suppression

This is continuation of experimental work at U.C. Irvine on fan flow deflection (FFD) [1]. The working principle of FFD is reduction of the convective Mach number of turbulent eddies that generate intense downward and sideward sound radiation. In a coaxial separate-flow turbofan engine this is achieved by tilting in the general downward direction, by a few degrees, the bypass (secondary) plume relative to the core (primary) plume. Mean flow surveys show that the misalignment of the two flows causes a thick, low-speed secondary core on the underside of the high-speed primary flow, especially in the region near the end of the primary potential core which contains the strongest noise sources. The secondary core reduces the convective Mach number of the primary eddies, thus hindering their ability to generate sound that travels to the downward acoustic far field [2]. Tilting of the bypass stream is possible by means of fixed or variable vanes installed near the exit of the bypass duct. Figure 1 depicts the general concept. Computational work has shown that the thrust penalty is on the order of 0.05% [3]. This paper describes a parametric study of FFD and some preliminary trends that have emerged.