Airfoil Noise and Unsteady Flow Measurements for the Validation of CFD/CAA Methods

Abstract

In the framework of airframe noise, computational aeroacoustic methods give results which have to be compared with experimental measurements. Such a validation process requires to perform aerodynamic measurements in high quality flows, and acoustic measurements in an quiet and anechoic environment. The French Aerospace Lab (ONERA) launched the EXAVAC experimental programme in order to constitute a complete aeroacoustic database. Measurements were carried out in two ONERA’s test facilities : the F2 aerodynamic wind tunnel and the CEPRA19 acoustic wind tunnel. The test model is a NACA0012 airfoil with a blunted trailing edge. This paper describes the experimental programme and its principal results. The growing interest for aerodynamic noise generated by civil aircrafts in approach, primarly generated by high-lift systems and landing gears, strongly stimulates the development of Computational Fluid Dynamics (CFD) methods and Computational AeroAcoustic (CAA) methods. Whereas CFD methods aim at reproducing unsteady flows around airfoils, CAA methods are intended to simulate the acoustic radiation of aeroacoustic sources through mean flow, which is non-uniform near airfoils and uniform in the far field. Today, these numerical methods give results which have to be compared with experimental measurements. Therefore, the need for fundamental experiments intended to a validation process of CAA methods is critical, both for aerodynamicists and acousticians. Such a validation process requires aerodynamic measurements in high quality flows, and acoustic measurements in an quiet and anechoic environment. The best way would be to carry out all measurements in the same test facility, but such installations are very rare and inevitably lead to compromises. Indeed, aerodynamic wind tunnels are often noisy and incompatible with any acoustic measurement, whereas anechoic wind tunnels are seldom convenient for local flow measurements using traditional probes (pitot tubes, hot wires, etc.) or laser velocimetry (particule image velocimetry, laser doppler velocimetry, etc.). Another approach consists in successively performing aerodynamic measurements and acoustic measurements in two different test facilities. Of course, that supposes to check flow reproducibility between installations, and thus to carry out a minimum of aerodynamic measurements in the acoustic test facility to correctly align the model with respect to the flow. If this aspect is fulfilled, the approach makes it possible to choose test facilities whose equipments and characteristics are perfectly adapted to each type of measurements. EXAVAC programme (Experimental Aerodynamic and Acoustic programme for the Validation of CFD/CAA methods applied to airfoil noise prediction) aims to constitute a complete database on an academic airfoil in terms of unsteady aerodynamics and acoustics. The test model is a NACA0012 airfoil with a chord length of 0.5 m, a span length of 1.4 m, and a blunted trailing edge with a thickness of 2.5 mm. Measurements were carried out in two ONERA’s test facilities : the F2 aerodynamic wind tunnel and the CEPRA19 acoustic wind tunnel. This paper describes the experimental programme and its principal results.