Radiated Noise Due to Boundary-Layer Transition.
PENNSYLVANIA STATE UNIV UNIVERSITY PARK APPLIED RESEARCH LAB
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A theory is presented for the noise radiated by incompressible boundary-layer transition that occurs on an infinite, rigid flat plate. It is hypothesized that it is the intermittency of the boundary-layer flow within the transition zone that is dominant in noise production. Using Lighthills analogy, it is shown that dipole, quadrupole, and octupole sources are generated. The dipole sources are attributable to the shear stress fluctuations that occur in transitional flow while the others are due to fluctuating Reynolds stresses and their images. Under the assumption that dipole sources are more efficient than quadrupoles or octupoles when the Mach number is very small, the power spectrum of the radiated noise due to the dipole contribution is derived. The spectral level rises at 6 dboctave, peaks at a frequency corresponding to the time it takes for a turbulence burst to move through the transition region, and then drops off again at 6 dBoctave. The radiation efficiency is analyzed and found to be quite low it is only 20 percent of that for a fully-developed turbulent boundary layer flow. Author
- Fluid Mechanics