Noise Prediction of a NACA 0015 Airfoil with Gurney Flap

Abstract

Air traffic noise, especially during the aircraft take-off and landing, has been universally acknowledged as a nuisance, for which reason, the International Civil Aviation Organization(ICAO) has ratified articles and established specifications aiming at the attenuation of air traffic noise, such as the Annex 16. The implementation of large bypass ratio turbo-fan engines on civil aircrafts and the application of designs such as engine nacelle and chevron nozzle have shifted the attention of noise control to the deployment of high-lift devices during take-off and landing. Source control is proved to be effective in the reduction of aeronautical noise. Traditional aeroacoustic assessments rely mainly on microphone (array) measurements and instantaneous flow fields obtained from CFD. However, the former fails in providing the mechanism of sound generation and the latter becomes prohibitive in the operating Reynolds number range of real aircrafts. Previous investigations have validated the causality between the unsteady transverse velocity in the wake of the high-lift devices and the far-field pressure fluctuation. This thesis is a continuation of the previous study, focusing on investigating the feasibility of noise prediction of high-lift devices based on experimental data obtained by time-resolved PIV . A model of NACA 0015 airfoil with Gurney flap of the height of 6% the chord length has been investigated by means of 2D time-resolved PIV in combination with simultaneous microphone measurements. The Gurney flap is a simplified model of the high-lift devices with more complex structure in real application. The experimental parameters were selected such that the source of sound can be treated as compact and consequentially guaranteed the applicability of both the distributed and the lumped formulation of the Curle's aeroacoustic analogy. In the first step, the time evolving pressure field was deduced from the velocity field measured by 2D TR-PIV, by the Poisson-based solver for quasi-2D incompressible flows. Pressure on the surfaces of the airfoil then constitutes the source terms of the distributed formulation of Curle's analogy and the corresponding far-field noise was evaluated. In the second step, the aerodynamic loads of the airfoil were evaluated from the velocity field by means of momentum balance, which were then used in the lumped formulation for noise prediction. The spanwise coherence length evaluated from the cross-plane flow visualizations was applied for the correction of 3D effect. Both formulations of the Curle's analogy yield noise evaluation of fair agreement with the simultaneous microphone measurements. All the calculated far-field noise power spectra reproduce the peak at K\'{a}rm\'{a}n vortex shedding frequency, which agrees well with the microphone measurements. The pressure fluctuations yielded by distributed formulation of the Curle's analogy are somehow damped due to the Gaussian smoothing applied to the velocity fields in the process of pressure reconstruction for the elimination of spurious spatial derivatives. Such damping effect can be circumvented by the lumped formulation, since the pressure is only evaluated on control volume boundaries in the locating in the far-wake, where 3D motions are weaker than in the region in direct proximity of the airfoil trailing edge. The high frequency components in the computed acoustic spectra are the most affected by the experimental and numerical errors. The coherence length correction reduce the over-estimation of the predicted noise levels with respect to the microphone measured ones. However, there still exists room for improvement in the procedure of coherence length correction. Concludingly, this thesis proved TR-PIV an effective approach in the noise prediction of airfoil with Gurney flap and stepped out the first step in PIV-based aeroacousitic investigations of the high-lift devices. Application of the approach on high-lift devices with more complex configurations would be a prospective continuation of the current study.