Statistics and spectral analysis of turbulent duct flows with flexible and rigid solutions

We present an experimental investigation of turbulent drag reduction with flexible and rigid polymer solutions in a 2:1 aspect ratio rectangular duct. The flexible polymer is partially hydrolysed polyacrylamide (HPAM) and the rigid polymer is xanthan gum (XG). The experiments are carried out at low drag reduction (%DR < 40), high drag reduction (%DR > 40) and maximum drag reduction (where the velocity profile U+ roughly matches Virk's asymptote). We compare velocity profiles, streamwise and wall-normal Reynolds stresses and power spectra of streamwise velocity fluctuations measured by Laser Doppler Anemometry (LDA). Our results show that the effects of both XG and HPAM polymers on turbulence are similar, provided that the Reynolds numbers and %DR are also similar. At high levels of %DR, the power spectral densities of streamwise velocity fluctuations of both XG and HPAM flows show a power-law decay near -3 instead of -5/3 in the inertial range. A slope of the power spectra of -3 was recently interpreted as evidence of elasto-inertial turbulence (EIT) in polymer jets. At relatively low concentrations, we observe that flexible polymer solutions are more effective at reducing drag, while XG only reaches MDR at very high concentrations. Thus, we hypothesize that the formation of polymer aggregates with higher concentrations contributes to increase viscoelasticity, and thus %DR, with XG and other rigid polymer solutions.