Methodology recommended to evaluate aeroacoustic related problems is provided, and approaches to their solutions are suggested without extensive tables, nomographs, and derivations. Orientation is toward flight vehicles and emphasis is on underlying physical concepts. Theoretical, experimental, and applied aspects are covered, including the main formulations and comparisons of theory and experiment. The topics covered include: propeller and propfan noise, rotor noise, turbomachinery noise, jet noise classical theory and experiments, noise from turbulent shear flows, jet noise generated by large-scale coherent motion, airframe noise, propulsive lift noise, combustion and core noise, and sonic booms.

Aeroacoustics of Flight Vehicles: Theory and Practice. Volume 1: Noise Sources

Simulation of inflight fan noise and flight effects was discussed. The status of the overall program on the flight effects of fan noise was reviewed, and flight to static noise comparisons with the JT15D engine were displayed.

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Flight effects of fan noise

Inflow control device (ICD's) of various shapes and sizes have been used to simulate inflight fan tone noise during ground static tests. A small, simple inexpensive ICD design was optimized from previous design and fabrication techniques. This compact two-fan-diameter ICD exhibits satisfactory acoustic performance characteristics without causing noise attenuation or redirection. In addition, it generates no important new noise sources. Design and construction details of the compact ICD are discussed and acoustic performance test results are presented.

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A compact inflow control device for simulating flight fan noise

The several phenomena involved in theoretical prediction of the far-field sound radiation attenuation from an acoustically lined duct were studied. These include absorption by the suppressor, termination reflections, and far-field radiation. Extensive parametric studies show that the suppressor absorption performance can be correlated with mode cut-off ratio or angle of propagation. The other phenomena can be shown to depend explicitly upon mode cut-off ratio. A complete system can thus be generated which can be used to evaluate aircraft sound suppressors and which can be related to the sound source through the cut-off ratio-acoustic power distribution. Although the method is most fully developed for inlet suppressors, several aft radiated noise phenomena are also discussed. This simplified suppressor design and evaluation method is summarized, the recent improvements in the technique are presented, and areas where further refinement is necessary are discussed. Noise suppressor data from engine experiments are compared with the theoretical calculations.

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A theoretical approach to sound propagation and radiation for ducts with suppressors

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.

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Tone Generation by Rotor-Downstream Strut Interaction

The paper describes the tests of four devices intended to reduce inflow disturbances and turbulences using a JT15D-1 turbofan engine. The tests were made to simulate the in-flight fan tone noise; the inflow control devices (ICD's) consisted of honeycomb/screen structures mounted over the engine inlet. The ICD's ranged from 1.6 to 4 fan diameters in size, and were made with several fabrication methods. All the ICD's significantly reduced the BPF tone in the far-field directivity patterns, but the smallest ICD's introduced propagating modes which could be recognized by additional lobes in the patterns. The JT15D-1 engine had a tone source which generated a strong propagating mode at fan speeds corresponding to 'approach' power and higher. Data from a typical transducer showed that the unsteady inflow distortion modes were eliminated or reduced when either of the ICD's was installed.

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

A highly noise suppressed TF 34 engine was used to investigate the noise of several powered lift configurations involving upper surface blown (USB) flaps. The configuration variables were nozzle type (i.e. slot and circular with deflector), flap chord length, and flap angle. The results of velocity surveys at both the nozzle exit and the flap trailing edge are also presented and used for correlation of the noise data. Configurations using a long flap design were 4 db quieter than a short flap typical of current trends in USB flap design. The lower noise for the long flap is attributed primarily to the greater velocity decay of the jet at the flap trailing edge. The full-scale data revealed substantially more quadrupole noise in the region near the deflected jet than observed in previous sub-scale tests.

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L. Homyak

Papers

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

Full-scale upper-surface-blown flap noise