Impact of three-dimensional turbulence modelling on rotor/stator interaction broadband noise

Abstract

An accurate prediction of Fan/Outlet-Guide-Vane (OGV) interaction broadband noise (BBN) is fundamental to correctly characterise the noise footprint of high bypass ratio turbofan engines. A three-dimensional synthetic turbulence BBN prediction methodology, which accounts for cascade effects and the airfoil geometry using a frequency-domain Linearised Navier–Stokes Solver, is presented in this work. The methodology computes the BBN generated by an infinite three-dimensional (3D) linear cascade using a spectral representation in the spanwise direction. The 3D problem is reconstructed analytically using two-dimensional (2D) simulations, including additional source terms coming from the spanwise spatial derivatives, leading to a high computational efficiency compared to a standard 3D approach. The methodology has been applied to assess the impact of oblique gusts on a Fan/OGV and a low-pressure turbine airfoil, and the results are compared to a 2D methodology and a previously proposed quasi-3D (q3D) formulation. Differences of more than 10 dB arise when comparing the 3D results with a purely 2D approach caused by the over-estimation of the turbulent kinetic energy that can produce cut-on acoustic modes. The validity of a q3D correction to mitigate this 2D effect has been assessed, demonstrating differences smaller than 2 dB concerning the 3D formulation, and therefore, its use can be justified for rapid predictions. The 3D methodology removes the unphysical impact of the 2D cascade resonances that appear in the q3D approach. The differences between the 3D and q3D approaches have been assessed in detail using an azimuthal noise decomposition. Even though the acoustic response can be remarkably affected by changes in the spanwise wavenumber, the noise azimuthal decomposition and frequency spectra exhibit a similar shape because the acoustic response is relatively constant within a significant fraction of the spanwise wavenumbers that enable sound radiation.