Effects of Flow Profile on Educed Acoustic Liner Impedance

Abstract

This paper presents results of an investigation of the effects of shear flow profile on impedance eduction processes employed at NASA Langley. Uniform and 1-D shear-flow propagation models are used to educe the acoustic impedance of three test liners based on aeroacoustic data acquired in the Langley Grazing Flow Impedance Tube, at source levels of 130, 140 and 150 dB, and at centerline Mach numbers of 0.0, 0.3 and 0.5. A ceramic tubular, calibration liner is used to evaluate the propagation models, as this liner is expected to be insensitive to SPL, grazing flow Mach number, and flow profile effects. The propagation models are then used to investigate the effects of shear flow profile on acoustic impedances educed for two conventional perforate-over-honeycomb liners. Results achieved with the uniform-flow models follow expected trends, but those educed with the 1-D shear-flow model do not, even for the calibration liner. However, when the flow profile used with the shear-flow model is varied to increase the Mach number gradient near the wall, results computed with the shear-flow model are well matched to those achieved with the uniform-flow model. This indicates the effects of flow profile on educed acoustic liner impedance are small, but more detailed investigations of the flow field throughout the duct are needed to better understand these effects.

Investigation of straightforward impedance eduction in the presence of shear flow

“…Usually the flow measurement devices used in the flow duct facilities do not allow to satisfactorily resolve the near-wall boundary layer profile, such as the Pitot probe in the NASA grazing incidence tube (GIT). So, according to Jones et al [12], the possible effects of the boundary layer on the impedance eduction results cause the greatest concern in the existing work. In fact, the current shear-flow methods for impedance eduction are mainly slip shear-flow method, which also depends on the Ingard-Myers' boundary condition at the duct wall similarly as the uniform-flow methods.…”

“…For the iterative methods, it is problematic to determine the acoustic boundary condition at the duct exit when shear flow with increasing velocity gradient is considered. Therefore, more recently, researchers have modified the infinite waveguide method to include the shear flow effect by means of the Pridmore-Brown equation [12,19].…”

Evaluation of Wall Boundary Conditions for Impedance Eduction Using a Dual-Source Method

“…A full description of the GFIT is given in a previous paper. 19 A sketch of the LaRC GFIT and its instrumentation is given in Fig. 1 and the rectangular flow duct (computational domain within the GFIT for which the analysis is performed) is illustrated in Fig.…”

Comparison of Liner Impedance Derived From the Results of Measurements at Two Different Setups Using a Small Number of Microphones

“…Анализ зарубежных публикаций [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] показывает, что, несмотря на проведенные многочисленные экспериментальные и теоретические исследования, достаточно надежной методики по извлечению импе-данса по результатам измерений звукового поля еще не создано. Все предлагаемые методы определения импеданса находятся в стадии тес-тирования.…”