The fan interaction noise is modeled as an annular cascade in terms of the Euler equations linearized about a nonuniform rotational flow. The model accounts for the inflow-fan-duct coupling and the high frequency of the interaction process. An efficient split velocity formulation is developed, making it suitable for broadband calculations. The validity of the inviscid approach for loaded cascades is supported by recent LES simulations of an airfoil in a gust indicating that the interaction

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... ss is dominated by inertia forces. Analytical and numerical analysis of disturbances in rotational flows is developed and exact inflow/outflow boundary conditions are derived, yielding directly the radiated acoustics. The upstream disturbances evolve in rotational flows and as a result the aerodynamic-aeroacoustic response of the annular cascade depends on the initial conditions location. Comparison with a linear cascade model shows a significant change of the blade sectional lift and the acoustic response. The model is extended to broadband noise calculations using the method of multiple scales. Comparison with NASA SDT data shows excellent agreement. Interaction noise is a byproduct of the interaction of nonuniform turbulent flows with bodies such as aircraft wings, turbofan engine blades, wind turbines, etc. Such flows have irregular flow patterns caused by a variety of phenomena such as atmospheric turbulence, inlet distortion, momentum defects due to viscous boundary layers and wakes, secondary flows, installation effects, etc. In the present paper, we mainly focus on the interaction processes in turbofan engines and the resulting noise generation and propagation. This problem is closely connected to the fan unsteady aerodynamics; the radiated sound essentially represents the unsteady far-field. The irregular turbulent flow in the fan is characterized by multiple scales. The organized nonuniformities, such as wakes and secondary flows, are inherently large scale. Their interaction with the rotating fan and guide vanes is usually deterministic and results in tonal noise mostly at multiples of the blade passing frequency. In contrast, the interaction processes of random disturbances associated with turbulence are mainly stochastic, resulting in broadband noise spectra. The unsteady aerodynamics of a fan has the following characteristics: (1) the incoming flow has a strong swirl component and thus is essentially three-dimensional and rotational, (2) the fan geometry is complex and involves strong inter-blade interference, (3) the Mach number is generally transonic, (4) the frequency is high, (5) the aerodynamic interaction process is strongly connected to the duct acoustics, and (6) the fan rotation adds additional complexity to the unsteady aerodynamics and sound scattering. Because of this complexity, current design codes for noise reduction in the industry remain on the whole based on the two-dimensional linear cascade models. These features c