ON THE ROLE OF THE OUTER REGION IN THE TURBULENT-BOUNDARY-LAYER BURSTING PROCESS

The dynamics and interaction of turbulent-boundary-layer eddy structures was experimentally emulated. Counter-rotating streamwise vortices and low-speed streaks emulating turbulent-boundary-layer wall eddies were generated by a Gortler instability mechanism. Large-scale motions associated with the outer region of turbulent boundary layer were emulated with -omega(z) spanwise vortical eddies shed by a periodic non-sinusoidal oscillation of an airfoil. The scales of the resulting eddy structures were comparable to a moderate-Reynolds-number turbulent boundary layer. Results show that the emulated wall-eddy breakdown was triggered by streamwise acceleration associated with the outer region of turbulent boundary layer. This breakdown involved violent mixing between low-speed fluid from the wall eddy and accelerated fluid associated with the outer structure. Although wall eddies can break down autonomously, the presence of and interaction with outer-region - omega(z) eddies hastened their breakdown. Increasing the -omega(z) eddy strength resulted in further hastening of the breakdown. Conversely, + omega(z), eddies were found to delay wall-eddy breakdown locally, with further delays resulting from stronger +omega(z) eddies. This suggests that the outer region of turbulent boundary layers plays a role in the bursting process.