Combined PIV and PLIF measurements in a polymer drag reduced turbulent boundary layer.

摘要

Much progress has been made in the understanding of the phenomenon of drag reduction by dilute solutions of polymers since its discovery in 1948 but a complete and conclusive explanation of the physics associated with the phenomenon is still lacking. In particular, drag reduction in boundary layers with injection has not been studied extensively to understand the physics that govern the dispersion of the injected polymer in the flow. In the present work, drag reduction due to polymer injection in an evolving turbulent boundary layer is studied using simultaneous Particle Imaging Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). PIV is used to measure the velocity statistics of the flow while PLIF is used to study the distribution and spread of the injected polymer in the boundary layer. The data from the two techniques are combined to calculate turbulent fluxes and to estimate the turbulent Schmidt numbers in polymer drag reduced flows.; Drag reductions up to 70% were achieved in the boundary layer by injecting varying concentrations of the polymer solution. PIV measurements showed that the injected polymer solution suppresses the Reynolds stresses close to the wall. PLIF measurements of the concentration showed that the injected polymer solution tends to remain adjacent to the wall in drag reduced flow. The polymer acts to change the state of turbulence such that the turbulent dispersion in the near-wall region is greatly reduced and, as a result, the injected solution does not rapidly mix into the outer regions of the boundary layer. The turbulent fluxes show, quantitatively, that the streamwise and wall-normal fluxes are suppressed by the polymer and gives rise to a modified state of turbulence in the flow. The rate of mixing of the polymer away from the wall decreases with increasing concentration of the injected polymer solution and increases with distance from the injection slot. The turbulent Schmidt numbers were greater than unity with the highest values estimated around 5 for the highest concentration polymer solutions tested. The results observed in these experiments were compared with available literature and found to be consistent with previous measurements and numerical simulations.