Acoustic emissions from unsteady transitional boundary layer flow structures: theoretical model refinements

There have been significant efforts over the last twelve years to model the wall pressures resulting from flow structures within the laminar-to-turbulent boundary layer transition zone. Models for the directly radiated noise from these structures have depended upon these models for wall pressure fluctuations. Recent measurements of the space-time statistics for these pressures allow further development of prediction codes. This paper describes the incorporation of a semi-empirically derived space-time correlation function for the intermittency indicator (Josserand and Lauchle 1990 and with modification as suggested by Chase 1989) into a refined theoretical model for the directly radiated noise. This model is based upon the Liepmann analogy for fluctuating displacement thickness (Lauchle 1981, 1989). A similar model exists consisting of a Lighthill acoustic analogy using a two-fluids model (Lagier and Sornette 1986 and Audet et al. 1989a,b). Comparisons are made of directly radiated noise from transition flow structures such as turbulent spots as predicted by these two models and their variants. An additional model for irradiation by vortex structures within the transition zone is qualitatively reviewed (Kambe 1986 and Hardin 1991). The role of pressure gradient in axisymmetric body flows and flat plate flows is discussed generally through presentation of a sensitivity analysis on affected flow parameters.