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This paper employs three acoustic propagation codes to explore variable-depth liner configurations for the NASA Langley Grazing Flow Impedance Tube (GFIT). The initial study demonstrates that a variable impedance can acceptably be treated as a uniform impedance if the spatial extent over which this variable impedance occurs is less than one-third of a wavelength of the incident sound. A constrained optimization study is used to design a variable-depth liner and to select an optimization metric. It also provides insight regarding how much attenuation can be achieved with variable-depth liners. Another optimization study is used to design a liner with much finer chamber depth resolution for the Mach 0.0 and 0.3 test conditions. Two liners are designed based on spatial rearrangement of chambers from this liner to determine whether the order is critical. Propagation code predictions suggest this is not the case. Both liners are fabricated via additive manufacturing and tested in the GFIT for the Mach 0.0 condition. Predicted and measured attenuations compare favorably across the full frequency range. These results clearly suggest that the chambers can be arranged in any order, thus offering the potential for innovative liner designs to minimize depth and weight.

This effort presents the application of an experimental high frequency and time-resolved global optical flow diagnostics for the characterization of pulsed spray flows. Such flows are encountered during active control of thermoacoustic instabilities, where high-bandwidth fuel modulation is often utilized to disrupt the combustor acoustic and unsteady heat release coupling. The understanding of spray dynamics is thus of paramount importance for these active control methodologies in order to achieve optimum control authority. A novel time-resolved Digital Particle Image Velocimetry (TRDPIV) implementation is employed for the dynamic investigation of the modulated spray. The method can measure both the droplet velocities as well as the droplet size distribution, from the same recorded images. The method provides planar image based droplet sizing using Mie scattering from DPIV measurements, with >5KHz sampling rate. Thus, eliminating complicated experimental approaches based on interferometer or fluorescence-Mie ratio. This paper presents the results of drop size characterization. Data processing is performed using different particle size evaluation schemes. The results are compared with measurements acquired from Phase Doppler Anemometry (PDA), conducted under same the experimental conditions. Experiments are conducted in non-reacting quiescent conditions, using an industrial simplex nozzle. The proportional spray modulation is obtained using a throttle valve-piezoelectric stack actuation system. The measurements for the current DPIV work are obtained under different pulsing amplitudes and frequencies. The results indicate that time-resolved DPIV can be a valuable tool in investigating dynamic response of modulated sprays.

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Three perforate-over-honeycomb liner configurations, one uniform and two with spanwise variable impedance, are evaluated based on tests conducted in the NASA Grazing Flow Impedance Tube (GFIT) with a plane-wave source. Although the GFIT is only 2" wide, spanwise impedance variability clearly affects the measured acoustic pressure field, such that three-dimensional (3D) propagation codes are required to properly predict this acoustic pressure field. Three 3D propagation codes (CHE3D, COMSOL, and CDL) are used to predict the sound pressure level and phase at eighty-seven microphones flush-mounted in the GFIT (distributed along all four walls). The CHE3D and COMSOL codes compare favorably with the measured data, regardless of whether an exit acoustic pressure or anechoic boundary condition is employed. Except for those frequencies where the attenuation is large, the CDL code also provides acceptable estimates of the measured acoustic pressure profile. The CHE3D and COMSOL predictions diverge slightly from the measured data for frequencies away from resonance, where the attenuation is noticeably reduced, particularly when an exit acoustic pressure boundary condition is used. For these conditions, the CDL code actually provides slightly more favorable comparison with the measured data. Overall, the comparisons of predicted and measured data suggest that any of these codes can be used to understand data trends associated with spanwise variable-impedance liners.