The objective of this paper is to expose the computational aeroacoustics (CAA) community to the current unresolved issues in modeling and predicting fan noise The paper includes a description of the sources of fan noise and a discussion of the current status of the fan noise prediction methods and their shortcomings The discussion is focused on the issues and includes sufficient details to help outline the scope of the fan noise problem and define the level of fidelity required for meaningful CAA simulations

Fan Noise: A Challenge to CAA

An analytical study was performed which predicted the noise generated from the interaction of the wake of an inlet probe with a rotor. Absolute sound pressure level (SPL) predictions of the blade-passage frequency (BPF) were based on engine and probe geometry for the Boeing 18-inch fan rig. The interaction between the fan rotor and the wake of an inlet probe was shown to produce noise at the BPF which may introduce significant noise during approach conditions when the rotor/stator interaction is cut off. Predictions from this study showed rotor/probe-wake interaction noise to be greater than lOdB above the rotor/stator interaction noise in some regions. The radiated noise was found to have a well-structured, highly three-dimensional character. For an azimuthal probe location of 0°, peak far field SPL's were concentrated in the region bounded by the engine axis and a sideline angle of 50° and azimuthal angles from 270° to 360° as measured in the direction of rotor rotation. Background The continuing decrease in the dominant noise sources from modern turbofan engines continues to expose the once-insignificant noise sources as contributors to the overall noise levels. One of these noise-generating mechanisms is the interaction of the fan blades with the wake from an inlet probe. These probes vary in size, shape, and location for each engine. The purpose of this study was to determine the relative level of this noise source compared to the dominant noise sources such as the fan wake/stator interaction. Although work has been performed in related areas, only the work by Kobayashi, and Kobayashi and Groeneweg simulate the distortion profiles similar to those which would be generated by an inlet probe. These studies analytically modeled the rotor as a threedimensional cascade and calculated the acoustic field from the distortion-produced dipole distributions on the blades for a single rotor configuration. The sound power calculations were compared to experimental results and were reported to have good agreement. The effect of the probe was examined by relating the sound power to the wake width and the insertion depth of the probe. The conclusions from this study showed the generated sound power to be dominated by the fan-tip region. Other investigations of the effects of the interaction of the rotor with the wakes of upstream rods" illustrated the need for azimuthal measurements when multiple spinning modes exist. Similar investigations related to this phenomenon deal with inflow distortions generated by some object upstream, (e.g., a strut, a pylon) or nonsymmetric inlets". Emphasis is placed on using caution when comparing sideline-measured noise from these sources with uniformly-directed sources. Inlet Instrumentation Monitoring the flow conditions entering an engine is performed with EEC probes (electronic engine control), which measure pressure and temperature. Typical probes being used on Boeing airplanes today are shown in Figure 1. For the most part, single probes are mounted between the 11:00 and 1:00 positions. Exceptions to this general rule occur when multiple probes are used. Figure 1 Typical Inlet Probe Configurations.

Noise predictions for rotor/inlet-probe wake interactions

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Noise predictions for rotor/inlet-probe wake interactions