Vorticity transport in turbulent channels under large-scale active drag control (i.e., spanwise opposed jet forcing) is investigated via direct numerical simulation of the Navier–Stokes equation. The skin-friction coefficient is newly expressed by the volume integration average of the mean flow dissipation expressed in terms of the spanwise vorticity (Ωz), and the turbulent transports of the spanwsie vorticity fluctuation () and of the wall-normal vorticity fluctuation (). Three Reynolds number cases (i.e., , and 550) with notable drag reductions (i.e., 18%, 16%, and 15%, respectively) are examined, following the setup in Yao et al. [Phys. Rev. Fluids 2, 062601(R) (2017)]. The transports of vorticity fluctuations dominate the contributions to the frictional drag, consistent with previous results for the passive drag reduction strategy of slip-wall [Yoon et al., Phys. Fluids. 28, 081702 (2016)]. Specifically, the effect of is to increase drag (due to sweeping ), while decreases the drag. A triple decomposition (mean, coherent, and random) reveals the random as the only term adding to drag, while the random and the coherent and transports all decrease the drag. Weighted joint probability distribution function (p.d.f.) of and shows a transition from the first–third quadrant dominance (hence positive-correlated) near the wall to the second–fourth quadrant dominance (hence negative-correlated) away from wall. In contrast, and are negatively correlated in the entire region. The analysis here suggests that the suppression of random spanwise-vorticity transport () is the target for more effective drag reduction under the current method.
Vorticity transports in turbulent channels under large-scale control via spanwise wall jet forcing
Yong Ji, Jie Yao, Fazle Hussain, Xi Chen; Vorticity transports in turbulent channels under large-scale control via spanwise wall jet forcing. Physics of Fluids 1 September 2021; 33 (9): 095112. https://doi.org/10.1063/5.0062937
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