Aerodynamic Noise

Abstract

AbstractAerodynamic noise refers to the noise generated by unsteady flow. It is also called aeroacoustics or flow noise. The noise sources can be connected with a turbulent flow, as in jet or boundary layer noise, or with the relative motion with respect to the observer of a steady flow, as in propeller or rotorcraft noise. Aerodynamic noise can contain both broadband and tonal components. To describe the noise generated by a turbulent flow, acoustic analogies have been developed that replace the turbulent flow by a distribution of equivalent sources. Turbulent flow noise can also be predicted using direct numerical simulations of the unsteady flow and the noise it generates. This chapter provides an overview of both propulsive and non‐propulsive system noise. The former systems include jets, fans, and propellers. The latter is generally referred to as airframe noise, and includes the noise generated by high lift devices, such as flaps and slats, and landing gear noise. Noise generated by the turbulent boundary layer and trailing edge noise also falls into the category of airframe noise. Though the sources of aerodynamic noise are generally nonlinear, the propagation of the sound they generate is usually a linear phenomenon. The exceptions include, multiple pure tone (buzz‐saw) noise in turbomachinery, the propagation of noise from very high speed jets, such as those found in military fighter aircraft, and sonic boom.