On the 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. Under the assumption of low Mach number flow, the power spectral density per unit spanwise width of transition for the radiated acoustic pressure is derived for the dipole contribution which is acoustically more efficient than the other sources by a factor inversely proportional to the Mach number squared. The spectral shape corresponds to one obtained by passing white noise through a realizable bandpass filter. The low-frequency cutoff scales with the burst frequency associated with turbulent spot formation, and the high-frequency cutoff scales inversely with the time required for the wall shear stress to change from a laminar value to a turbulent value at a given point of local laminar flow breakdown. As one example, the theory is used to predict the noise radiated by an under-water buoyant body for which there are experimental data.
- Fluid Mechanics