Aeroacoustic modeling of turbulent airfoil flows

A numerical algorithm for acoustic noise generation is extended to handle turbulent flows. The approach involves two steps comprising a viscous incompressible flow part and an inviscid acoustic part. In the turbulent case, the flow is further split into a Reynolds-averaged component and a component corresponding to the turbulent small-scale fluctuations. The latter is modeled by an eddy-viscosity-based turbulence model and appears as a source term in the acoustic equations. The for mulation is applied to acoustic noise generated by the flow past a NACA 0015 airfoil at an incidence of 20 deg. First, acoustic noise generated by laminar flow is considered as a validation of the acoustic model. The results are compared to solutions obtained using the Lighthill acoustic analogy (linearized wave equations) (Lighthill, M. J., On Sound Generated Aerodynamically. I: General Theory, Proceedings of the Royal Society of London, Series A: Mathematical and Physical Sciences, Vol. 211, 1952, pp. 564-587). The comparisons show that noise levels and frequency content are in good agreement. Next, the acoustic model is applied on a turbulent flow in which the small-scale turbulence is modeled by a Reynolds-averaged turbulence model [Baldwin-Barth one equation model (Baldwin, B. S., and Barth, T. J., A One-Equation Turbulence Transport Model for High Reynolds Number Wall-Bounded Flows, NASA TM 102847, 1990)]. The computations show that the generated acoustic field is dominated by the Strouhal frequency and its harmonics. The acoustic noise level for the turbulent flow is of the same order as for the laminar flow. Because of the turbulence model used in the flow solver, only one frequency and its higher harmonics are seen. For capturing more frequencies, one should combine the acoustic model with large eddy simulation or direct Navier-Stokes simulation.