Sound Generated by Fan Wakes and Validation with Experiments

This paper presents a validation of the rotor-wake/stator interaction code, CAAT-S, with NASA Source Diagnostic Test (SDT) experiments. CAAT-S is a computationally efficient three-dimensional code based on the Euler equations linearized about a nonlinear rotational mean flow. It accounts for the flow swirl, the blade and fan 3D geometry and loading, and yields directly the radiated sound in terms of the duct acoustic modes. Two exit guide vane configurations, three power levels, and three blade passing frequencies (BPF) are considered. The power levels presented are taken at approach, cutback, and takeoff conditions for (a) the SDT low vane count (LVC) stator and (b) the SDT low noise (LN) stator. The mean flow and wake profiles are taken from the SDT experiments and exhibit strong radial dependencies. The reduced frequencies, based on the blade tip and speed of sound, associated with SDT data range from 15 for the 1BPF at approach power to 72 for 3 BPF at takeoff power. This produces a large number of propagating acoustic modes and makes the prediction of the vane forces and the radiated sound power a real computational challenge. Improved nonreflecting boundary conditions are developed extending the capability of CAAT-S to resolve the large number of high frequency modes, and making it suitable for turbofan engine noise prediction. CAAT-S calculated sound power level (PWL) underpredicts data by about 3 dB for both LVC and LN configurations. However, CAAT-S trend predictions are within the scatter of the experimental data. These results validate CAAT-S for moderate to high subsonic Mach numbers and over a broad range of blade passing frequencies.