Tone Generation by Rotor-Downstream Strut Interaction

A JT15D fan stage was acoustically tested in the NASA Lewis anechoic chamber as part of the joint Lewis/Langley Research Center investigation of flight simulation techniques and flight effects using the JT15D engine as a common test vehicle. Suspected rotor-downstream support strut interaction was confirmed through the use of simulated support struts, which were tested at three axial rotor-strut spacings. Tests were also per- formed with the struts removed. Inlet boundary layer suction in conjunction with an inflow control device was also explored. The removal of the boundary layer reduced the fan fundamental tone levels, suggesting that the mounting and mating of such a device to the nacelle requires careful attention. With the same inflow control device installed, good acoustic agreement was shown between the engine on an outdoor test stand and the fan in the anechoic chamber. HE development of effective inflow control devices (ICD's) makes it possible to study noise generation mechanisms, such as rotor-stator interaction, with reduced masking effects of inflow disturbances. Modern turbofan engines are often designed with blade/vane numbers selected to prevent propagation of the fundamental rotor-stator in- teraction tone. However, less consideration has been given to possible rotor interactions with engine support struts. These struts are either located downstream of the stator row or are integrated into the stator as large cross-section vanes.! This paper presents results for a JT15D fan stage which was acoustically tested in the NASA Lewis Research Center anechoic chamber2 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. Blade pressure results showing a strong six per revolution disturbance pointed to these struts as the probable noise source. The interaction between the 28-blade rotor of the JT15D and the six support struts would result in a m-22 acoustic spinning mode having 22 circumferential lobes. 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. Downstream support struts were not required for the JT15D fan installation in the anechoic chamber. Six simulated support struts were fabricated and installed in the test fan stage to simulate the actual engine support strut installation. These simulated struts were located at three axial spacings from the stator trailing edge. Thus, in the present study results were obtained for the spacing effect of downstream support struts as well as for fan stage alone with no downstream struts.