The Impact of Three Dimensional Flow Anisotropy and Transients on Turbulence Ingestion Noise in Open Rotors

The effect of flow anisotropy and three dimensional separation on the turbulent structure and radiated turbulence ingestion noise of a rotor in two experimental configurations is studied. The first consists of a non-axisymmetric boundary layer wake ingested by a rotor mounted at the aft of a body of revolution inclined at 5 degree angle of attack. In the second configuration a transient disturbance is generated by an upstream wing body junction pitching from zero to 20 degree angle of attack . This disturbance is convected downstream and ingested into a rotor immersed in a wall boundary layer. In both cases flow velocimetry at the rotor inflow is done and the far field sound is measured. The flow velocimetry in the wake of the inclined body of revolution shows evidence of three dimensional separation and vortex rollup between the lee and body sides. A boundary layer embedded shear layer develops as the turbulent kinetic energy is pulled off the wall by the flow separation and is visible in the port side velocimetry. The turbulent structure of this shear layer and the boundary layer on the lee of the body is visualized using compact eddy structure representation and the modes on the port side are shown to be stretched versions of similar modes seen in an equilibrium, zero pressure gradient boundary layer. The effect these structures had on the radiated sound served to both increase blade to blade correlation and the overall broadband levels of the sound. Measurements of the sound using an acoustic array showed directivity effects that resulted from the location of the embedded shear layer and rollup vortices. Although the vortices likely have some effect on the spectra, most of the noise is dominated by the turbulence ingestion of the embedded shear layer.

For the second experimental configuration the transient motion was documented through repeated measurements of the flow field and sound, and an ensemble average of the measurements taken. Overall the flow was unsteady, particularly in the outer region of the boundary layer. The sound radiated was shown to be tonal during the first half of the interaction, where the flow is dominated by a deterministic mean flow change, and attributed to a form of periodic unsteady loading. During the latter half of the disturbance the broadband and overall sound levels increased significantly and are associated with the interaction of the rotor with flow separation over the wing body junction when it reached a critical, 16 degree angle of attack.