Optimization and control of acoustic liner impedance with bias flow.

摘要

This research involves the investigation of using bias flow to change the impedance and to improve the absorption of an acoustic liner. Bias flow offers a mechanism for in-situ control of liner impedance. It would provide the ability to tune the installed liner impedance to better attain the design value and allow the ability to change the liner impedance during operation in order to optimize the liner performance at different operating conditions.; An experimental investigation was conducted to generate a high quality database, from which the effects of a mean bias flow on the acoustic impedance of lumped-element single-degree-of-freedom liners was determined. Acoustic impedance measurements were made using the standard two-microphone method in the NASA Langley Normal Incidence Tube. Liner resistance was shown to increase and to become less frequency and sound pressure level dependent as the bias flow was increased. The resistance was also consistently lower for a negative bias flow (suction) than for a positive bias flow (blowing) of equal magnitude. The slope of the liner reactance decreased with increased flow.; An optimization study was implemented to determine the improvements in sound absorption of acoustic liners using bias flow in the normal incidence sound environment. Overall, bias flow liners had a higher average absorption over passive liners. Absorption variations were much higher in the passive liner over the targeted frequency range. The bias flow liners also performed better over changing sound pressure level. The non-linearity of the passive liner caused a variation in absorption as the sound pressure level changed whereas the bias flow liner performed linearly.; A preliminary grazing flow impedance model with bias flow was used to investigate the feasibility of controlling the liner impedance to improve liner attenuation rate. The passive liner proved almost equal to the bias flow liner in its variation from the optimum duct wall impedance over a part of the grazing flow Mach number range. However, the bias flow was able to maintain the low level of variation over a much greater range of Mach numbers.