Modeling tonal and broadband interaction noise

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 process 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.