Low Speed Aeroacoustic Facility Research Articles

Best Practices for Accurate Measurement of Pure Jet Noise

This paper focuses on the best practices that would ensure the vital requirement of accurate measurements of clean jet noise spectra at all frequencies. Issues that are important for jet aeroacoustic tests and the critical role of good data in the development of jet noise technology are examined. Problems with both the jet rig and the instrumentation system could severely compromise the data quality, thereby rendering the measured data unsuitable for scientific and technical applications. The limitations of the instrumentation system are discussed, and a set of recommendations that would lead to clean measurements at the higher frequencies of interest in model tests is provided. Methods for verifying the quality of data, from both single and dual-stream jets, are described in depth. Some of the existing beliefs and approaches for checking data quality are shown to be without merit. It is more difficult to measure pure jet noise from very large nozzles especially at low jet velocities, with a given compact rig. The objectives of an aeroacoustic test should be understood clearly and proper choice of nozzle configurations made to avoid acquiring data corrupted by extraneous noise. This is especially so for typical dual-stream geometries, where the secondary jet generates the high-frequency noise. The consequences of contaminated data in the development of noise reduction technology and theories are highlighted. The results from a major effort for improving the quality of aeroacoustic data acquired at the Boeing Low Speed Aeroacoustic Facility (LSAF) are reported: significant improvements have been made to the flow quality and acoustics. The high quality of noise measurements, as a result of the refurbishment, is established through good spectral agreement with data obtained with a blow-down jet, for a wide range of nozzle conditions. Much of the existing data have been found to be corrupted; it is incumbent upon the experimental community to produce unambiguous data that would advance the state-of-the-art in jet noise technology.

Aeroacoustics of hot jets

A systematic study has been undertaken to quantify the effect of jet temperature on the noise radiated by subsonic jets. Nozzles of different diameters were tested to uncover the effects of Reynolds number. All the tests were carried out at Boeing's Low Speed Aeroacoustic Facility, with simultaneous measurement of thrust and noise. It is concluded that the change in spectral shape at high jet temperatures, normally attributed to the contribution from dipoles, is due to Reynolds number effects and not dipoles. This effect has not been identified before. A critical value of the Reynolds number that would need to be maintained to avoid the effects associated with low Reynolds number has been estimated to be 400 000. It is well-known that large-scale structures are the dominant generators of noise in the peak radiation direction for high-speed jets. Experimental evidence is presented that shows the spectral shape at angles close to the jet axis from unheated low subsonic jets to be the same as from heated supersonic jets. A possible mechanism for the observed trend is proposed. When a subsonic jet is heated with the Mach number held constant, there is a broadening of the angular sector in which peak radiation occurs. Furthermore, there is a broadening of the spectral peak. Similar trends have been observed at supersonic Mach numbers. The spectral shapes in the forward quadrant and in the near-normal angles from unheated and heated subsonic jets also conform to the universal shape obtained from supersonic jet data. Just as for unheated jets, the peak frequency at angles close to the jet axis is independent of jet velocity as long as the acoustic Mach number is less than unity. The extensive database generated in the current test programme is intended to provide test cases with high-quality data that could be used for the evaluation of theoretical/semi-theoretical jet noise prediction methodologies.

Effect of Nozzle Internal Contour on Jet Aeroacoustics

A frequent problem that confronts designers of jet rigs and nozzles used for aeroacoustic testing is the unknown effect of the state of the flow upstream of the nozzle exit on aeroacoustic performance. Though the effect of the inflow conditions on flow development has been studied extensively, there is limited information on their effect on noise. Flow distortion, both in pressure and temperature, thickness and state of the boundary layer, quality of the measurement system and anechoic chamber, and analysis procedure could all play a role in the observed variations in data from different experiments. In order to understand the effect of the thickness of the boundary layer upstream of the nozzle convergent (entrance) section and in the nozzle itself, a computational and experimental investigation has been carried out. Three nozzles of identical exit diameter have been designed and tested in the Boeing Low Speed Aeroacoustic Facility (LSAF). These three nozzles have a shallow conic section, a short cubic contraction and an ASME flow path (contraction followed by a constant section), respectively. Simultaneous aerodynamic and acoustic measurements have been made with a six-component force balance and far field microphone arrays for a wide range of nozzle operating conditions. The numerical simulations and the measurements of nozzle coefficients indicate that the initial boundary layers are vastly different, with the cubic producing a thin boundary layer and the conic a much thicker one. Surprisingly, the noise levels from these two nozzles are virtually identical especially at the higher frequencies. An important practical conclusion drawn from this observation of insensitivity to nozzle exit conditions is that it should be possible to compare noise spectra from different facilities even if measurements of inflow conditions are not available. The ASME nozzle generates higher levels of noise, the magnitude of which is more pronounced at elevated jet temperatures. The characteristics and the stability of the boundary layers inside the nozzles are examined to glean some insights to the observed differences in noise.

Jet Aeroacoustic Testing: Issues and Implications

Issues that are important for jet aeroacoustic tests and the critical role of good data in the development of jet noise technology are reviewed and discussed. A major effort for improving the quality of aeroacoustic data acquired at the Boeing Low Speed Aeroacoustic Facility has been carried out. This extensive undertaking targeted all aspects of model-scale testing and acquisition of good quality data and covered issues of flow quality, nozzle performance, and acoustics. Significant improvements have been made in all of the named categories. Simultaneous measurement of nozzle aerodynamic performance and noise is important, especially for the development of noise suppression devices. The capabilities of a jet rig incorporated with a six-component force balance are described. It is clearly demonstrated that the measured thrust with the current rig is in excellent agreement with that obtained using a dedicated force balance over a wide range of nozzle pressure ratios. Results of the efforts at rig refurbishment, carried out over the last few years, are presented. The high quality of noise measurements is established through good spectral agreement with data obtained with a blowdown jet, for a wide range of nozzle conditions. An extensive study of available jet noise data from various jet noise facilities has been completed. Implications of contaminated data from most tests and the obligations of the experimental community for the advancement of jet noise technology are discussed.