Flow and self-noise around bypass transition strips

To analyze the downstream effects of bypass transition strips on a laminar incoming flow, a direct numerical simulation of the fully transient, explicit and compressible Lattice Boltzmann equations is performed. Near wake analysis of a staggered grid of cubic blocks as transition device is compared with a more conventional zigzag strip, to ensure transition to a fully developed conical boundary layer. The staggered grid of blocks is more efficient in stopping the flow and creating large, coherent, flow structures of the size of the blocks, which results in a stronger transition. However, the downstream merging of spanwise created structures is relatively long resulting in higher correlated boundary layers. If the spanwise variation of the zigzag strip is small, the streamwise vortices created merge quicker, resulting in an earlier uncorrelated boundary layer. The self-noise created by zigzag strip is also significantly less than the noise from the staggered grid of blocks and becomes only dominant at high frequency compared to the predicted trailing edge noise.