Comparison of several inflow control devices for flight simulation of fan tone noise using a JT15D-1 engine

Abstract

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.

Tone generation by rotor-downstream strut interaction

“…3 and discussed in the Introduction. Other circumferential modes, such as m=\6 and 10 are possible, due to rotor interaction with higher harmonics of strut-induced distortion.…”

“…This mode was shown to exist in the inlet duct of a JT15D engine when the results from two pressure sensors located in the duct so as to allow spining mode identification by signal phase relationship were used. 3 However, it was not possible to alter the support struts in the engine to establish the behavior of this apparent noise source.…”

“…3 ' 7 In addition, the anechoic chamber installation had provisions for inlet boundary layer suction. The fan stage could be run with either a hard inlet duct surface or with a porous metal section in the outer duct wall, which was connected to a suction system to allow removal of about 10% of the inlet airflow.…”

“…This paper presents results for a JT15D fan stage which was acoustically tested in the NASA Lewis Research Center anechoic chamber 2 as part of a joint NASA Lewis/Langley investigation of flight simulation techniques and flight effects using the JT15D-1 engine as a common test vehicle. 3 " 7 The engines used in these studies were instrumented with blade and vane pressure transducers to assist in isolating noise generation mechanisms. Although the primary goal of this study was to evaluate inflow control techniques, the results revealed that for the JT15D-1 engine in particular speed ranges the fundamental tone was controlled by the presence of six engine support struts located downstream of the stator.…”

Turbomachinery noise studies of the AiResearch QCGAT engine with inflow control

“…3 showed that propagating modes at BPFF could be producedby rotor interaction with distortions from ICU ribs and from engine struts aft of the fan in the bypass flow duct. On the basis of those results and as illustrated in Fig.…”