Logarithmic structures in the far-field stage of over-tripped boundary layer turbulence

This study reports on over-tripped boundary layer (OT-BL) turbulence induced by a family of leading-edge wall-mounted tripwires. The friction Reynolds number Reτ is more than twice that of the unperturbed canonical turbulent boundary layer. Despite differences in perturbations, all OT-BLs establish a new logarithmic region in the downstream far field.In far field, both the mean velocity profile and the logarithmic region diagnostic function indicate a significant wall-normal extension of the logarithmic region in OT-BLs. The construction of the logarithmic region is a consequence of the wall-driven mechanism. The extent of the wall-driven region shows weak dependence on the tripwire-induced perturbations. To reveal the structural framework in the logarithmic region, the OT-BLs are reduced into uniform momentum eddies and intra-layer shear interfaces. Uniform momentum eddies show wall-attached behavior in the logarithmic region similar to the canonical situation. The structural framework in the new logarithmic region is proven to be consistent with Prandtl mixing length theory. This demonstrates the upward construction of the structural mixing length framework in the logarithmic region compared to the unperturbed case. The pre-multiplied velocity spectra show the emergence of larger-scale inertial structures in the extended logarithmic region of the OT-BL. The larger-scale inertial motions result in a more scale separation in the logarithmic region and lead to the emergence of an inertial subrange. The interaction between high- and low-speed inertial motions exhibits a compression/stretching mechanism within the inertial subrange and production range. The inertial-driven compression/stretching mechanism is primarily dominated by large-scale motions and weakens as scale decreases.