| This thesis tackles two distinct issues in wall turbulence: (1) Contributions of large and very-large-scale motions to the turbulent kinetic energy and shear stress at laboratory Reynolds numbers in canonical channel and zero-pressure-gradient boundary layers. (2) The nature of core turbulence in fully transpired channel flows with an imposed side-wall injection length scale.; Inside canonical channel flows and zero-pressure-gradient boundary layers (ZPGBLs), a significant portion of the streamwise kinetic energy and shear stress is carried by motions with length scales larger than 2h , where h, is the outer scale of the flow. This behavior is universal and observed in channel, ZPGBL, and pipe flows despite the additional azimuthal symmetries for the growth of structures in pipes. A bimodal distribution of spectra inside the log-layer is interpreted to be the result of a dual effect of hairpin vortex packets and the alignment of packets one behind another.; The smaller scales uniformly retard the mean flow throughout while the larger scales retard the flow only in the wake region. In the log-layer, the larger scales begin increasingly to accelerate the flow as the shear stress peak is approached. These trends point to the existence of an equilibrium at the shear-stress peak, where the forces due to large-scale fluctuations that accelerate the flow are balanced by the forces due to small-scale fluctuations that retard the flow.; Particle-image velocimetry measurements inside a fully transpired channel flow apparatus with side-wall injection are used to determine the role of side-wall injection length scale on core turbulence. Smaller pore sizes create laminar flow near the head end of the channel while the larger pore sizes induce transition to turbulence at the head end itself. Flow visualization studies are used to identify persistent axial and spanwise vortices in the flow. The vortices swept by the mean velocity affect the injection velocity streamlines that emerge out of the pores creating injection-velocity fluctuations, that are amplified by the mean strain field to create Reynolds shear stress downstream. Perturbations of the Taylor-Culick solution are numerically computed to demonstrate the amplification of axial and azimuthal vorticity. |
