Number Of Struts Research Articles

Jet propulsion for quiet aircraft

A novel method of jet noise reduction has been subjected to bench and wind tunnel tests. This patented method uses very small nozzles arranged on struts protuding from the upper surface of an aircraft wing. Bench tests included various arrangements of nozzles on each strut; tests with heated air; tests of noise power and directivity versus number of struts, strut spacing, and location of struts on a simulated wing; and tests of discharge coefficients and velocity coefficients. Wind tunnel tests included induced aerodynamic effects of upper-surface blowing, effects of tunnel flow on discharge and velocity coefficients of the nozzles, acoustic effects of tunnel flow on the jet noise, and effects of jet flow on aerodynamic noise. A full-scale test of aerodynamic noise was made using dummy struts on a sailplane. Preliminary design of a quiet research test vehicle was prepared and its aural detectability was predicted (on the basis of the test data) to be significantly superior to other approaches. Predicted noise levels for 1500-ft flyover were shown to be less than 25 dB (1/3-octave-band SPL) and were dominated by aerodynamic noise of the aircraft wing. [This work was supported by USAFFDL and USAFAPL.]

Noise Generated by Low Pressure Axial Fans

Noise generated by a fan consists of the rotating noise and the turbulent noise. These two kinds of noise have entirely different characters with respect to decay in an axisymmetric pipe. In this paper the generation and decay of these two kinds of noise are experimentally examined by changing various factors, such as the suction pipe geometry, the location and the thickness of struts, and the combination of the number of impeller blades and the number of struts. The rate of decay of the rotating noise is predictable and in most cases the noise quickly decreases in proportion to the distance from the rotor, providing that the number of impeller blades and the number of struts are properly chosen. Therefore, the noise observeda way from a low pressure axial flow fan is mostly the turbulent noise.

Effects of Stator Pitch and Removal on Axial-Flow Compressor-Noise Generation

This paper presents the results of a series of tests to determine the characteristics of compressor noise emitted from a particular gas turbine. The machine tested was a General Electric Frame 5 industrial gas turbine, which is in the 18 000 hp class. Of primary interest in the test series were the changes in intensity of discrete-frequency, compressor-blade-passage noise caused by varying the pitch of the inlet-guide vanes and by removing the vanes. Removal of the vanes resulted in substantial increases in harmonic noise levels, which would seem to be contrary to current theory. This apparent incongruity is explained, however. The perturbation of the flow field into the first rotor stage that had originally been caused by the stator blading was now caused by the bearing-support struts, which, although farther upstream, were larger in cross section. The number of struts allowed greater propagation of the spinning-duct modes. Removal of the vanes resulted in increased power consumption of the compressor, also, as the rotor blading was not modified to operate without stators.