Application of Coherent Structure and Vortex Sound Theories to Jet Noise
Abstract
The basic theory of aeroacoustics, formulated four decades ago, has never been verified directly for jet noise. In the present study, coherent structures (CS) are used to illustrate how, within its strict assumptions, aeroacoustic theory can indeed predict jet noise. Velocity and sound pressure fields were measured in an excited, subsonic (0.08 <=q M <=q 0.35) cold jet (diameter D = 4cm) with low freestream turbulence (u' /U ~ 0.02%) located in a large anechoic chamber. Several instances of stabilized, nearly periodic, pairing were educed. Coherent vorticity <omega > was measured over the region 0 <=q y/D <=q 1.2, 0 <=q x/D <=q 5, while the coherent sound pressure
was measured over polar angles 20^ circ <=q phi <=q 125^circ for the radial range 10 <=q r/D <=q 50. The interactions and sound of three-dimensional vortex rings were simulated using vortex filaments and free-space vortex sound theory (VST), and compared with experiments. The difference between experimental and theoretical sound fields was a dipole field which appears to come from the neglected surface integrals over the nozzle body. The importance of the nozzle body was studied experimentally by changing the geometry and shape of the nozzle body. It was determined that: flow-body interaction produced a significant dipole field, reflection and diffraction were significant for upstream observation angles, and refraction of the sound is small at low M. The sensitivity of the sound field to variations in the vortical structures was investigated using simulations of vortex rings with three-dimensional perturbations and temporal jitter. Incoherence in the vorticity field was highly magnified in the sound field; only in a very clean facility, such as the one built for this work, can the coherent sound be measured well. In addition, the first full-field, time-resolved measurements of CS in an intermittently pairing jet were made, launching studies of subharmonic resonance and feedback in jets. The effect of excitation on the evolution of the first two azimuthal instability modes was measured to observe tilting of CS. Finally, a general theory was proposed which can predict the evolution of noncircular vortex rings such as squares, ellipses and triangles.- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1990
- Bibcode:
- 1990PhDT........37B
- Keywords:
-
- AEROACOUSTICS;
- Engineering: Mechanical; Physics: Fluid and Plasma; Engineering: Aerospace;
- Aeroacoustics;
- Coherence;
- Jet Aircraft Noise;
- Prediction Analysis Techniques;
- Vortex Rings;
- Vortices;
- Gas Jets;
- Pressure Distribution;
- Sound Fields;
- Sound Pressure;
- Subsonic Flow;
- Turbulence;
- Velocity Distribution;
- Vortex Filaments;
- Vorticity;
- Acoustics