Concentration flux measurements in a polymer drag-reduced turbulent boundary layer

摘要

The drag-reducing action of dilute solutions of long-chain polymers in a flat-plateturbulent boundary layer is studied using particle imaging velocimetry (PIV) andplanar laser induced fluorescence (PLIF). The results are used to characterize andquantify the spatial distribution of the injected polymer solution and the downstreamdevelopment of the DR along the flat plate. The two techniques were used simultan-eously to document and study the spread of the injected polymer solution and theresulting changes in the structure and statistics of the turbulence in the boundary layer.The PLIF images provide a qualitative and quantitative measure of the dispersion ofthe injected polymer solution. The mean and root mean square (r.m.s.) concentrationprofiles obtained using PLIF showed that the polymer greatly suppressed the turbulentdispersion in the near-wall region. The quantitative concentration measurementsacross the boundary layer, combined with simultaneous velocity measurements, areused to obtain concentration flux measurements in the boundary layer and are usedto study the effect of the turbulence on the dispersion of the injected polymer. Thevariation of the fluxes with concentration of the injected polymer solutions and withincreasing downstream distance is also studied and documented. The action of thepolymer is to reduce the streamwise fluxes in the boundary layer, the suppressionincreasing with concentration. Further, the fluxes are also used to estimate theturbulent Schmidt number (Sc_T) for the drag-reduced flow.