Tonal Noise Generation Research Articles

Influence of mean shear on sound produced by turbulent flow over surface slots

An analytical investigation is made of the sound produced by low Mach number turbulent flow over a slot in a thin rigid plane and over an array of parallel slots in the plane. The plane separates the turbulent mean stream from nominally stagnant fluid. The turbulent wall pressures excite unstable oscillations of the mean shear layer in the slots, and linear perturbation theory is used to estimate the influence of these instabilities on sound generation. For both isolated slots and parallel slots, it is shown that the growth of shear layer disturbances across a slot can greatly increase the contribution from the slot trailing edge to the volume flux through the slot, and produce a corresponding increase in the net radiation relative to that predicted by the usual ‘‘diffraction theory,’’ which neglects the presence of the shear layer. Previous analyses of this problem have ignored the influence of mean shear, and their predictions at higher frequencies are typically 20–40 dB smaller than those given here. At very low frequencies, additional vorticity shed from the leading edge of a slot effectively blocks unsteady motion through the slot and predicted sound levels are then very much smaller than those of diffraction theory. An extended slotted surface can support subsonic surface waves that decay with distance into the fluid; these are shown to be unstable within a certain narrow range of frequencies, and may well be associated with the generation of tonal noise by the boundary layer.

An experimental investigation of the sources of propeller noise due to the ingestion of turbulence at low speeds

Noise radiation from a four bladed, 10 in. diameter propeller operating in air at a rotational speed of 3000 RPM and a freestream velocity of 33 ft/s was experimentally analyzed using hot-wire and microphone measurements in an anechoic wind tunnel. Turbulence levels from 0.2 to 5.5% at the propeller location were generated by square-mesh grids upstream of the propeller. Autobicoherence measurements behind the blade trailing edges near the hub and tip showed regions of high phase-coherence between the blade-passage harmonics and the broadband frequencies. Inflow turbulence reduced this coherence. By relating the fluctuation velocities in the propeller wake to the unsteady blade forces, the primary regions of tonal noise generation have been identified as the hub and tip regions, while the midspan has been identified as a region responsible for broadband noise generation. These measurements were complimented by cross-spectra between the propeller wake-flow and the measured sound. The effect of turbulence on the radiated noise level showed an overall increase of 2 dB in the broadband levels for every 1% increase in turbulence. This effect varied for different frequency bands in the acoustic spectrum.

Turbofan noise research at NASA

Results of recent NASA research to reduce aircraft turbofan noise are described. For very high bypass ratio turbofan engines, the dominant source of engine noise is the fan. A primary mechanism of tone noise generation is the rotor blade wakes interacting with downstream stator vanes. Methods of analyzing rotor–stator tone noise generation are described and sample results are given. The analysis includes the unsteady aerodynamic response of the stators to gusts, coupling to duct modes, and finite element calculation of the far-field radiation accounting of for flow in the fan ducts. Wind tunnel tests of model fans and nacelles are described including comparisons between measured and predicted tone directivities. A novel rotating microphone technique is used to measure the acoustic modes in the fan inlet, and the results indicate that a mechanism associated with unsteady fan tip loading can be important in addition to rotor–stator interaction. Finally, concepts for active fan noise control which emphasize control at the source are addressed by an unsteady aerodynamic analysis of compliant stator vanes.

Axial flow fan noise caused by inlet flow distortion

Flow distortions at the inlet of an axial flow fan will cause discrete tone noise generation at shaft rotational frequencies. Controlled experiments have been carried out to check the validity of existing theories to predict these tone levels. The results have shown satisfactory agreement at low frequencies for a concentrated force representation of the blade chordwise load distribution. Both tone and broadband noise predictions have been used to calculate the total 1/3 octave spectrum for the type of fan used in cooling flow applications.