Three-dimensional characterization of Reynolds shear stress in near-wall coherent structures of polymer drag reduced turbulent boundary layers

Local flow topology of a polymer-laden turbulent boundary layer

“…The observation of a thicker viscous sublayer is consistent with the lower magnitude in the wall-normal Reynolds stress v 2 + and Reynolds shear stress uv + within the buffer layer of the polymer-laden flow compared with water, seen in figure 5(b) of § 3. The 2-D shear-dominant flow and expanded viscous sublayer agrees with the strong attenuation of wall-normal sweep and ejection motions commonly observed in drag-reduced viscoelastic DNS (Pereira et al 2017) and experiments of polymer drag-reduced boundary layers (Shah et al 2021). A suppression of sweep and ejection motions in the polymer-laden flow can also be reasonably implied from the current measurements based on the reduction in uv + shown in figure 5(b), as well as the stronger alignment between large-scale turbulent motions and the streamwise x direction shown in plots of the two-point correlation C uu in figure 6(a).…”

“…Using a modified Q-criterion (Hunt et al 1988;Martins et al 2016), Shah et al (2021 demonstrated that extensional and vortical flow regions weaken as drag was reduced, similar to the findings of the numerical investigation by Pereira et al (2017). Rather than Q-criterion, the current work expands on the experimental findings of Shah et al (2021) by utilizing the Δ-criterion to obtain distributions, mainly j.p.d.f.s, of the different types of topologies within a Newtonian and polymer-laden turbulent boundary later (TBL). Compared with the Q-criterion, use of the Δ-criterion, as well as other deformation tensor invariants, facilitates a more complete depiction of the topology of fine-scale motions within the turbulent drag-reduced flow.…”

“…A suppression of sweep and ejection motions in the polymer-laden flow can also be reasonably implied from the current measurements based on the reduction in uv + shown in figure 5(b), as well as the stronger alignment between large-scale turbulent motions and the streamwise x direction shown in plots of the two-point correlation C uu in figure 6(a). The mechanism by which these sweep and ejection motions are suppressed appears to be an attenuation of the extensional flow motions where sweep and ejection events meet -an observation that was similarly implied in the polymer-laden boundary layer experiments of Shah et al (2021). Conditionally averaged velocity fields for the Newtonian boundary layer, shown in figure 12(a,b), show that the dominant dissipative motion within the buffer layer is an extensional saddle point, located at the interface of a sweep and ejection.…”

“…A limited number of experimental investigations have used 3-D flow measurements to measure the velocity in polymer drag-reduced flows, let alone one that quantifies the topology of fine-scale motions within the flow. Shah, Ghaemi & Yarusevych (2021) performed 3-D tomographic PIV on a polymer drag-reduced boundary layer with heterogeneous wall injection, an Re θ of 520 and DR of 20 %-30 %. Using a modified Q-criterion (Hunt et al 1988;Martins et al 2016), Shah et al (2021 demonstrated that extensional and vortical flow regions weaken as drag was reduced, similar to the findings of the numerical investigation by Pereira et al (2017).…”

“…Shah, Ghaemi & Yarusevych (2021) performed 3-D tomographic PIV on a polymer drag-reduced boundary layer with heterogeneous wall injection, an Re θ of 520 and DR of 20 %-30 %. Using a modified Q-criterion (Hunt et al 1988;Martins et al 2016), Shah et al (2021 demonstrated that extensional and vortical flow regions weaken as drag was reduced, similar to the findings of the numerical investigation by Pereira et al (2017). Rather than Q-criterion, the current work expands on the experimental findings of Shah et al (2021) by utilizing the Δ-criterion to obtain distributions, mainly j.p.d.f.s, of the different types of topologies within a Newtonian and polymer-laden turbulent boundary later (TBL).…”