Effect of viscoelasticity on the soft-wall transition and turbulence in a microchannel

The modification of soft-wall turbulence in a microchannel due to small amounts of polymer dissolved in water is experimentally studied. The microchannels arc of rectangular cross-section with height similar to 160 mu m, width similar to 1.5 mm and length similar to 3 cm, with three walls made of hard polydimethylsiloxane (PDMS) gel, and one wall made of soft PDMS gel with an elasticity modulus of similar to 18 kPa. Solutions of polyacrylamide of molecular weight 5 x 10(6) and mass fraction up to 50 ppm, and of molecular weight 4 x 10(4) and mass fraction up to 1500 ppm, are used in the experiments. In all cases, the solutions arc in the dilute limit below the critical overlap concentration, and the solution viscosity does not exceed that of water by more than 10 %. Two distinct types of flow modifications are observed below and above a threshold mass fraction for the polymer, w(t) which is similar to 1 ppm and 500 ppm for the solutions of polyacrylamide with molecular weights 5 x 10(6) and 4 x 10(4), respectively. At or below w(t), there is no change in the transition Reynolds number, but there is significant turbulence attenuation, by up to a factor of 2 in the root-mean-square velocities and a factor of 4 in the Reynolds stress. When the polymer concentration increases beyond w(t), there is a decrease in the transition Reynolds number and in the intensity of the turbulent fluctuations. The lowest transition Reynolds number is similar to 35 for the solution of polyacrylamide with molecular weight 5 x 10(6) and mass fraction Si) ppm (in contrast to 260-290 for pure water). The fluctuating velocities in the streamwise and cross-stream directions are lower by a factor of 5, and the Reynolds stress is lower by a factor of 10, in comparison to pure water.