Fan-OGV Broadband Noise Prediction Using a Cascade Response

The present study aims at improving and assessing a previously presented analytical model for the broadband noise produced by the impingement of turbulence on a blade row, and to extend it to the case of a stator downstream of a rotor. The noise prediction model is a strip-theory approach based on a previously published formulation of the threedimensional unsteady blade loading for a rectilinear cascade. The issue is to account for the main three-dimensional effects due to the annular geometry, the swirling mean flow and the radial variation of the excitation. That is why a strip theory in an annular duct coupled with a three-dimensional cascade response have been chosen. To further improve some of the identified limitations of this approach, a correction of the unsteady blade loading formulation, previously developed in a tonal configuration, is then introduced in the broadband noise model to account for the real annular dispersion relation. Secondly, the influence of the radial wave number and the crucial need for a consistant definition of the radial correlation length of the unsteady blade loading are discussed. These aspects are first studied using the reference three-dimensional linearized Euler computations in the configuration of a single cascade previously published, showing the crucial need of the unsteady blade loading correction in moderate to high frequencies and of a correct definition of the radial correlation length, which can be chosen equal to the radial correlation length of the turbulence in moderate to high frequencies. Finally, the frequency domains of validity of several formulations, based on different levels of assumptions, are studied. Threedimensional gusts enable to avoid overpredictions obtained with two-dimensional response functions in low to moderate frequencies, whereas at high frequencies, a two-dimensional response might be used. Finally, the model is applied to the case of the wake-interaction broadband noise, introducing both a background turbulence and a wake turbulence in a procedure similar to other works and is compared with the experimental results of the 22-inch source diagnostic test (SDT) fan rig of the NASA Glenn Research Center.