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In this paper, a hybrid immersed boundary technique has been developed for simulating turbulent flows past airfoils with moving trailing-edge flaps. Over the main fixed part of the airfoil, the equations are solved using a standard body-fitted finite volume technique, whereas the moving trailing-edge flap is simulated using the immersed boundary method on a curvilinear mesh. An existing in-house-developed flow solver is employed to solve the incompressible Reynolds-Averaged Navier–Stokes equations together with the k-ω turbulence model. To achieve consistent wall boundary conditions at the immersed boundaries the k-ω turbulence model is modified and adapted to the local conditions associated with the immersed boundary method. The obtained results show that the hybrid approach is an efficient and accurate method for solving turbulent flows past airfoils with a trailing-edge flap and that flow control using an adjustable trailing-edge flap is an efficient way to regulate the aerodynamic loading on airfoils.

The flow-acoustic splitting technique for aeroacoustic computations is extended to simulate the propagation of acoustic waves generated by three-dimensional turbulent flows. In the flow part, a subgrid-scale turbulence model (the mixed model) is employed for large-eddy simulations. The obtained instantaneous flow solution is employed as input for the acoustic part. At low Mach numbers, the differences in scales and propagation speed between the flow and the acoustic field are quite large, and hence different meshes and time steps can be used for the two parts. The model is applied to compute flows past a NACA 0015 airfoil at a Mach number of 0.2 and a Reynolds number of 1.6 x 10 5 for different angles of attack. The flow solutions are validated by comparing lift and drag characteristics with experimental data. The comparisons show good agreements between the computed and measured airfoil lift characteristics for angles of attack up to stall. For the acoustic solutions, predicted noise spectra are validated quantitatively against experimental data. A parametrical study of the noise pattern for flows at angles of attack between 4 and 12 deg shows that the noise level is small for angles of attack below 8 deg, increases sharply from 8 to 10 deg, and reaches a maximum at 12 deg.

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.