Aeroacoustic modeling of turbulent airfoil flows

A numerical algorithm for acoustic noise generation is extended to handle t urbulent Bows, The approach involves two steps comprising a viscous incompr essible how part and an inviscid acoustic part. In the turbulent case, the flow is further split into a Reynolds-averaged component and a component co rresponding to the turbulent small-scale fluctuations. The latter is modele d by an eddy-viscosity-based turbulence model and appears as a source term in the acoustic equations. The formulation is applied to acoustic noise gen erated by the Bow past a NACA 0015 airfoil at an incidence of 20 deg, First , acoustic noise generated by laminar Bow is considered as a validation of the acoustic model. The results are compared to solutions obtained using th e 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, 563-587). The comparisons show that noise levels and freque ncy content are in good agreement. Next, the acoustic model is applied on a turbulent Bow in which the small-scale turbulence is modeled by a Reynolds -averaged turbulence model [Baldwin-Barth one equation model (Baldwin, B, S ., and Earth, T J., "A One-Equation Turbulence Transport Model for High Rey nolds Number Wall-Bounded Flows," NASA TM 102847, 1990)]. The computations show that the generated acoustic field is dominated by the Strouhal frequen cy and its harmonics. The acoustic noise level for the turbulent flow is of the same order as for the laminar Bow 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.