For predicting acoustic waves emitted from a rocket plume, the overexpansion effects on Mach 3.0 supersonic jet acoustics are investigated using an implicit large eddy simulation. A Mach 2.0 supersonic free jet is computed for code validation, and the results show qualitatively good agreement with the experiments. Then, computations of three different jets (design Mach numbers 3.0, 3.5, and 4.0 with fully expanded jet Mach number 3.0) are conducted, and nondimensionalizations based on design parameters and fully expanded parameters are discussed. Acoustic far-field spectra show that nondimensionalization based on fully expanded parameters works well for the high-Mach-number overexpanded jets, as it does for the low-Mach-number underexpanded jets that were investigated in previous studies. This nondimensionalization improves the accuracy of prediction of the acoustic waves emitted from rocket plumes because one parameter, the design Mach number, can be neglected for acoustic far fields. In addition, actual overexpansion effects after nondimensionalization are discussed. A comparison of the near flowfields and acoustic fields shows that Mach wave sources move upstream because of the existence of Mach disks, which enhances shear-layer mixing. Meanwhile, the overexpanded jet, which possesses only a shock cell without Mach disks, exhibits the same Mach wave generation characteristics as an ideally expanded jet.

Overexpansion Effects on Characteristics of Mach Waves from a Supersonic Cold Jet

Acoustics radiated from a M=1.8 ideally-expanded jet impinging on a 45 degree-inclined flat plate is investigated numerically with the help of the experimental work. Validation of computational method and grid convergence study are conducted firstly. Result of the free jet is satisfactory, while the present study overestimates the noise level of the impinging jet by 5 dB in OASPL. Based on the numerical technique validated here, mechanism of acoustics is analyzed. In addition to the well-known free-jet noise sources such as the Mach wave and the fine-scale turbulent mixing noise, additional two noise sources are found; 1) interaction between the shock waves and the vortex of the shear layer, 2) the Mach wave radiated from the jet flowing on the inclined plate. The former is similar to the shockassociated noise, and the OASPL plot at far-filed shows omni-directional feature. These two waves are also observed in the present experiment.

Numerical Study on Acoustic Radiation from a Supersonic Jet Impinging to an Inclined Plate

The objective of the present study is to actively control the flame oscillation in the experimental lean premixed methane burner by the phase-controlled sound waves. The authors propose the methodology of generating the phase-controlled sound waves by directly sensing the preferential frequency and generating the sensor-triggered sound waves with fixed optimum phase difference with the flame oscillation. The real-time controller is implemented in the present study, and the combustion noise reduction is achieved. The reduction is 20dB at peak frequency and 13dB in the OASPL.

Active control of flame oscillation in lean premixed burners by phase- controlled sound waves

In spite of many experimental studies of low-power applied-field magnetoplasmadynamic (AFMPD) thrusters, thrust efficiencies of the past thrusters are very low. Hence, drastic improvement in thrust performance is required for AF-MPD thrusters to compete against other types of electric propulsion in a moderate power regime around 10 kW. For the optimization of AF-MPD thrusters, a numerical code for the flowfield simulation is now under development. A preliminary result shows that the code can deal with a complicated mixture of the induced and applied magnetic fields, which will lead to a combination of the self-field, swirl, Hall, as well as electrothermal accelerations.

Numerical Study of Low-Power MPD Arcjet

PEM Fuel cell system was designed and constructed to use as a power source of unmanned aerial vehicles (UAV) in the present study. Sodium borohydride was selected as a hydrogen source and was decomposed by catalytic hydrolysis reaction. Fuel cell system consists of a fuel cell stack, a hydrogen generation system (HGS), and power management system (PMS). HGS was composed of a catalytic reactor, micropump, fuel cartridge, and separator. Hybrid power system between lithium-polymer battery and fuel cell was developed. The fuel cell system was integrated and packaged into a blended wing-body UAV. Energy density of the total system was 1,000 W?hr/kg and high endurance more than 5hours was accomplished in the ground tests.

Fuel Cell Powered UAV with NaBH4 as a Hydrogen Source

For the purpose of improving accuracy in jet noise prediction and investigating its generation mechanism, high subsonic jets were computed by using compressible Large Eddy Simulation (LES), wherein the inflow forcing or disturbance added in the inflow shear layer was incorporated. The far-field Sound Pressure Levels (SPL) as well as the flow field resulted in good agreement with available experimental data by applying only the high azimuthal modes among the inflow forcing parameters. We found that this result was due to an important role of the inflow forcing upon breaking down the axiymmetric vortices that caused high amplitude velocity and pressure fluctuations. In order to examine generation mechanism of the dominant noise component, wavelet transformation was introduced to reveal the presence of a well-organized structure of pressure fluctuations that originated mainly from vortex motions near the end of the jet potential core. This structure took a train of alternately positive and negative wavelet-transformed pressure regions along the jet distance, spreading towards the downstream with advection and propagation. It was concluded that this structure and its dynamic motion are the reason why a high subsonic jet produces the dominant noise with a particular downstream directivity.

Yuya Fukuda

Papers

Large Eddy Simulation of a High Subsonic Jet and Noise Generation