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Purpose The purpose of this paper is to investigate airfoil’s tonal noise reduction mechanism when deploying surface irregularities, such as surface waviness by means of spatial stability analyses. Design/methodology/approach Flow field calculations over smooth and wavy-surface NACA 0012 airfoils at 2° angle of attack and at Reynolds number of 200,000 are performed using the large eddy simulation (LES) approach. Three geometrical configurations are considered: a smooth NACA 0012 airfoil, wavy surface on the suction side (SS) and wavy surface on the pressure side (PS). The spatial stability analyses using the LES-generated flow fields are conducted and validated against the Orr-Sommerfeld stability analysis for the smooth airfoil configuration. Findings The spatial stability analyses show that inclusion of the wavy-type modification on the SS of the airfoil does not lead to altering of the acoustic feedback loop mechanism, with respect to the mechanism observed for the smooth airfoil configuration. In contrast, applying the surface modifications to the airfoil PS leads to a significant reduction of the amplification range of disturbances in the vicinity of the trailing edge for the frequency of the acoustic feedback loop mechanism. Practical implications The spatial analyses using, for example, LES-generated flow fields can be widely used to determine acoustic sources and associated distributions of amplifications for a wide range of applications in the aeroacoustics. Originality/value The spatial stability analysis approach based on flow fields computed a priori using the LES method has been introduced, validated and used to determine behaviour of the acoustic feedback loop when accurate reconstruction of geometry effects is required.

A method is proposed for integrating a source prediction obtained from a Computational Fluid Dynamics (CFD) model for the fan stage of a turbofan engine with a Computational Aero-Acoustics (CAA) propagation code to predict tonal noise radiation in the far field. The Reynolds-Averaged Navier-Stokes equations are used to model the generation of the tones. Their propagation through the intake is simulated by applying the Discontinuous Galerkin Method to solve the linearized Euler equations in the time domain. The CFD and the CAA solutions are matched in a region where both solutions overlap and where non-linear effects, important close to the fan, can be considered to be less significant. An equivalent modal source on a notional source plane behind the fan is used to duplicate the sound field in this matching region and is then to drive a fully three-dimensional CAA radiation model for a near-field acoustic solution. The far-field sound pressure is obtained by applying the Ffowcs Williams-Hawkings formulation on a porous surface within the CAA domain. The accuracy and efficiency of this approach are investigated and results obtained are compared to measured data from a fan rig.

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