Abstract
Although the turbulent intensity is suppressed in the drag-reducing channel flow by viscoelastic additives, the mean velocity distribution in the channel flow is symmetrical and tends to be similar to the laminar flow. In the study of near-wall modulation of the drag-reducing flow with an injected ultrathin water layer, an asymmetrical mean velocity distribution was found. To further investigate this phenomenon and the underlying cause, an experiment was carried out with the water injected from a porous channel wall at a small velocity (~10−4 m/s) into the drag-reducing flow of surfactant solution. The instantaneous concentration and flow fields were measured by using planar laser-induced fluorescence (PLIF) and particle imaging velocimetry (PIV) techniques, respectively. Moreover, analyses on turbulent statistical characteristics and spatial distribution of viscoelastic structures were carried out on the basis of comparison among various flow cases. The results showed that the injected ultrathin water layer under present experimental conditions affected the anisotropy of the drag-reducing flow. The characteristics, such as turbulence intensity, showed the zonal feature in the wall-normal direction. The Reynolds shear stress was enhanced in the near-wall region, and the viscoelastic structure was modified severely due to the redistributed stress. These results may provide experimental supports for the near-wall modulation of turbulence and the exploration of the drag-reducing mechanism by viscoelastic additives.
Highlights
The drag reduction effect caused by viscoelastic polymer or surfactant aqueous solution as additives has been successfully applied to many practical engineering problems
The typical visual injected water layer obtained by the planar laser-induced fluorescence (PLIF) technique is shown in Figure 11a to illustrate the near-wall modification in the channel
The ultrathin water applied to the viscoelastic velocity drag-reducing flow of surfactant solutions to layer study isthe emerged asymmetrical distribution in cetyltrimenthyl ammonium chloride (CTAC)
Summary
The drag reduction effect caused by viscoelastic polymer or surfactant aqueous solution as additives has been successfully applied to many practical engineering problems. It plays a significant role in energy conservation and emission reduction [1,2,3]. It is generally shown that the distribution of the mean streamwise velocity in the drag-reducing channel flow is changed when compared with the conventional wall turbulence. The overall trend is similar to the laminar flow, and the velocity gradient is more consistent from the wall to the core region in the channel center
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