Turbulence modification by large-scale organized electrohydrodynamic flows

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Abstract
The interactions of flows generated by ionic discharges with wall turbulence are not only of interest for turbulence control, but also for devices of industrial importance, such as wire-plate electrostatic precipitators (ESPs). Under conditions of uniform discharge, in wire-plate ESPs, arrays of regular, spanwise vortices are found in the absence of a through-flow. These arise from ionic discharges from the spanwise wires placed between the grounded plates on each side. The interactions of such electrohydrodynamic (EHD) flows with a turbulent through-flow are still poorly understood. Direct numerical simulation (DNS) is an attractive method for investigating such problems since the details of the interactions can be unraveled, and the results are directly applicable to industrial-scale systems because their Reynolds numbers are typically quite low. In this study, pseudospectral channel flow simulations were performed with the electrohydrodynamic effects being modeled by a spatially varying body-force term in the equations of fluid motion. The interactions between EHD flows and wall structures were elucidated by examining the instantaneous structure of the flow field. Results indicate that the mean flow, the EHD flows, and the turbulence field undergo significant modifications caused by mutual interaction. First, it is found that EHD flows reduce drag, allowing larger flow rates for a given pressure drop. Second, the EHD flows themselves appear weakened by the presence of the through-flow, particularly in the central region of the channel. The EHD flows affect the turbulence field by both increasing dissipation and turbulence production, the overall turbulence level being determined by the balance between the increased dissipation and production. Even though high EHD flow intensities may increase streamwise and wall-normal turbulence intensities, the Reynolds stress is reduced, consistent with the observed reduction in drag. From a mechanistic viewpoint, there are indications that EHD flows of the type investigated here reduce drag by decreasing the relative importance of the positive Reynolds stress contributions, i.e., second (ejections) and fourth (sweeps) quadrant events, compared to the negative Reynolds stress contributions, i.e., first and third quadrant events.