Aeroacoustic design and broadband noise predictions of a fan stage with serrated outlet guide vanes

Abstract

This work, realized in the framework of the European project TurboNoiseBB, presents an advanced aeroacoustic design methodology of Outlet Guide Vanes (OGVs) with leading edge serrations, including details on their broadband noise and aerodynamic performance. The serrated OGV corresponds to a modified stator from an aero-engine fan stage tested at the AneCom Aerotest's facility (Germany). Sinusoidal leading edge patterns with varying amplitude and wavelength along the span are designed in collaboration with Safran Aircraft Engines. Serrations are adjusted to account for the turbulence characteristics provided by Reynolds-Averaged Navier-Stokes (RANS) calculations. Optimal parameters are found using simple design rules discussed in the paper. Down selection of serrated OGV designs (patent pending) are conducted through a RANS control of aerodynamic performances and in accordance with industrial specifications, ensuring acceptable penalties on the loss coefficient and isentropic efficiency. Broadband noise simulations are performed using a computational aeroacoustic (CAA) code that solves the linearized Euler equations with a synthetic turbulence model. Additionally, the acoustic response of the serrated leading edge airfoils is also estimated using the most relevant analytical formulation in the literature, based on the Wiener-Hopf technique. Numerical predictions at approach conditions are compared to available experimental measurements (on the untreated baseline case) and analytical Amiet-based and Wiener-Hopf solutions, showing a satisfactory agreement in the sound power spectrum in the bypass duct. Finally, the acoustic performances estimated at the design stage are numerically assessed by both CAA and Wiener-Hopf methods, providing around 2.5 dB and 3.5 dB reduction on the overall power level, respectively.

Comparison of Techniques for the Estimation of Flow Parameters of Fan Inflow Turbulence from Noisy Hot-Wire Data

“…Polacsek et al [20] showed for this same data set, that the band-width limitation of the hot-wires could explain this steeper slope. This results in an attenuation of the measured unsteady velocity signals at high frequency.…”

“…This is in agreement with the theoretical relation, given by Hinze [24] (Equations ( 3)-( 74) and ( 3)-( 75)), between the longitudinal and transverse integral scales for HIT, i.e., Λ 2 = Λ 1 /2. Equations ( 12) and ( 13) can be easily checked by setting f = 0 (or correspondingly kn = 0) onto Equations ( 17) and (20), as an example. This will be later introduced in Section 2.4.…”

“…The objective of this study is to identify the most suitable technique for the estimation of turbulence parameters, namely, the turbulence integral length scale and the turbulence intensity (which can be understood as a normalized turbulent kinetic energy (TKE)). Commonly, methods for fan broadband noise prediction are based on analytic models [14][15][16][17], synthetic turbulence methods [18,19], and scale-resolving techniques [6,20,21]. They depend on turbulence parameters, which can include boundary layer thickness, turbulence integral length scale, and flow turbulence kinetic energy of the inlet flow or/and of the flow at the fan interstage section.…”

3D CAA methodology using synthetic turbulence to assess turbulence-cascade interaction noise emission and reduction from serrated airfoils

“…The goal is to study promising passive and active technologies which might be implemented in future innovative stators. ONERA is involved in the design of a successful passive concept based on leading edge serrations [1,2]. A set of these serrated low-noise vanes will be evaluated during the first test campaign of the InnoSTAT project which will take place at Ecole Centrale de Lyon (ECL).…”

“…This equivalence is drawn for both 2D and 3D turbulence structures. This paper follows on previous work at ONERA on CAA with synthetic turbulence based on Fourier modes decomposition [1,2,18]. Turbulence generation processes able to tackle some of the issues arising in simulations of practical configurations are implemented, such as wall and periodic boundary conditions (BC), while keeping the CPU time as low as possible.…”