Isothermal velocity and turbulence measurements downstream of a model multilobed turbofan mixer

An isothermal experimental investigation of the three dimensional flow field downstream of a model multilobed turbofan forced mixer is presented. LDA measurements of the three mean velocities and corresponding turbulence intensities were obtained in the downstream duct where the turbine (primary) and fan (secondary) streams emerging from the lobes mix together. The flow development in the near field was quantified by measuring the cross plane velocities. These were found to consist of large radial flows, of order 15% of the mean axial velocities at the lobe inlet, with changing sign depending on location. The cross-plane flow is consistent with a large scale axial vortex pair (per lobe) which persists throughout the downstream duct and enhances mixing in this region. Turbulence generation and anisotropy of the turbulence structure were evident mainly in the shear layers formed as the fan and turbine streams emanated from the lobe trailing edge. Spatial uniformity in the mean and turbulent fields was measured as little as five heights downstream of the mixer exit, indicating the rapid mixing achievable in these systems.