When calculating broadband fan noise caused by rotor-stator wake interaction analytically, information about the airflow, particularly about the turbulence in the rotor wakes, is necessary. During the pre-design phase, two-dimensional streamline methods are commonly used. These provide only general flow quantities like mean-flow velocities or total-pressure losses. Turbulent parameters such as turbulent kinetic energy and turbulent integral length scale need to be deduced from these quantities. There are several models mentioned in the literature which correlate the wake size with the wake turbulence. But they usually comprise calibration factors that need to be assessed empirically by numerical simulations or measurements. The contribution of the paper is to present an updated semi-empirical model for rotor-wake turbulence quantities, derived on the basis of an extensive comparison of the model with measurements and numerical simulations on four different turbofan stages. A recalibration of the empirical factors improved the noise prediction by 8 dB, reaching an accuracy of 2 dB. In addition, it is shown, that the endwall flow is responsible for large variance in the noise prediction, and may have a contribution of up to 2 dB to the overall sound power.
The acoustic interaction of fan-rotor wakes with the downstream stator vanes is considered as an important noise source of an aircraft engine. The turbulence induced by the rotor generates a stochastic acoustic source that appears as broadband noise in the acoustic spectrum. During the preliminary design phase of an engine, established meanline and throughflow solvers usually do not resolve turbulence and associated unsteady flow parameters. But such solvers provide rotor pressure losses that can be used to estimate the mean and turbulent rotor wakes. A crucial step is the deduction of turbulence parameters from the mean wakes. A semi-empirical model for rotor-wake turbulence estimation is presented in this paper. The meanline method and the throughflow solver are compared to three-dimensional computational flow simulations investigating the capabilities of the different solvers to provide flow data for broadband wake interaction noise prediction. The methods are applied to a representative modern fan stage at a comprehensive number of operating points, comprising several speed lines from surge to choking conditions. Microphone measurements are consulted to assess the noise predictions. The evaluation confirms the applicability of the meanline and throughflow method in combination with the turbulence model for broadband noise estimation during the preliminary design phase. The underestimated turbulence in the tip region of the fan is found to be negligible even during off-design conditions.
The acoustic interaction of fan-rotor wakes with the downstream stator vanes is considered as an important noise source of an aircraft engine. The turbulence induced by the rotor generates a stochastic acoustic source that appears as broadband noise in the acoustic spectrum. During the preliminary design phase of an engine, established meanline and throughflow solvers usually do not resolve turbulence and associated unsteady flow parameters. But such solvers provide rotor pressure losses that can be used to estimate the mean and turbulent rotor wakes. A crucial step is the deduction of turbulence parameters from the mean wakes. A semi-empirical model for rotor-wake turbulence estimation is presented in this paper. The meanline method and the throughflow solver are compared to three-dimensional computational flow simulations investigating the capabilities of the different solvers to provide flow data for broadband wake interaction noise prediction. The methods are applied to a representative modern fan stage at a comprehensive number of operating points, comprising several speed lines from surge to choking conditions. Microphone measurements are consulted to assess the noise predictions. The evaluation confirms the applicability of the meanline and throughflow method in combination with the turbulence model for broadband noise estimation during the preliminary design phase. The underestimated turbulence in the tip region of the fan is found to be negligible even during off-design conditions.
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