Computing Aerodynamically Generated Noise

In contrast to computational aerodynamics, which has advanced to a fairly mature state, computational aeroacoustics (CAA) has only recently emerged as a separate area of study. Following a discussion of the classical field of aeroacoustics as introduced by Lighthill, the paper provides an overview and analysis of the problems associated with utilizing standard computational aerodynamics procedures for acoustic computations. Numerical aspects of computing sound-wave propagation are considered, including assessments of several schemes for spatial and temporal differencing. Issues of particular concern in computing aerodynamically generated noise, such as implementing surface and radiation boundary conditions and algorithms for predicting nonlinear steepening and shocks, are discussed. In addition, the paper briefly reviews alternatives to the conventional finite-difference schemes, such as boundary-element and spectral methods and the uncommon lattice-gas method. MODERN AEROACOUSTICS Sir James Lighthill (1993b) has stated that the aeronautical community has recently entered into the second Golden Age of Aeroacoustics. The first of these eras, as defined by Lighthill, focused on the problems of jet noise and jet engine noise, and lasted from the late 1940s until the mid 1970s. By that time, high-bypass-ratio turbine engines had become the standard for all new transport aircraft, and the initially excessive noise produced by turbojets had been 161 0066-4189/97/0115-0161$08.00 November 28, 1996 16:13 Annual Reviews FM29WELL.TE AR23-06