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As part of a program to enable accurate simulation of in-flight fan tone noise during ground static tests, four devices intended to reduce inflow disturbances and turbulence were tested with a JT15D-1 turbofan engine. These inflow control devices (ICD's) consisted of honeycomb/ r, screen structures mounted over the engine inlet. The ICD's ranged from 1.6 to 4 fan diameters in size, and differed in shape and fabrication method. All the ICD's significantly reduced the BPF tone in the farfield directivity patterns, but the smallest ICD's apparently introduced propagating modes which could be recognized by additional lobes in the patterns. All the ICD's had negligible transmission loss at low fan speeds; at supersonic fan tip speed the smallest ICD's had some measurable loss, but the largest had no loss. The JT15D-1 engine was found to have a tone source which generated a strong propagating mode at fan speeds corresponding to "approach" power and higher. This mode, which was not affected by an ICD, is associated with six structural struts behind the fan, but the exact noise producing mechanism was not identified. Two of the ICD's were tested with miniature dynamic pressure transducers mounted on the fan blades. Data from a typical transducer showed that the unsteady inflow distortion modes (turbulence) were eliminated or significantly reduced when either of the ICD's was installed. However, some steady inflow distortion modes remained; those that could result in propagating acoustic modes are believed to be related to lobes observed in the directivity patterns.

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