Measured effects of polymer additives on turbulent-velocity fluctuations at various length scales

Turbulent-drag reduction in a dilute polymer solution has been studied using the technique of photon-correlation homodyne spectroscopy to measure velocity differences in a concentric cylinder cell, in which the inner cylinder rotates. A large anisotropic suppression of turbulent-velocity differences is found in the bulk region of the turbulent fluid. The suppression effect occurs at various length scales up to {}1 mm, which is far beyond the Kolmogorov dissipation length ld ({}0.04 mm). The large-scale velocity fluctuations are suppressed, but their statistical properties over varying length scales remain unchanged. The small-scale fluctuations, on the other hand, are damped out much more strongly, resulting in a different functional form for the velocity-density function. The latter observation is consistent with the notion that the polymer-turbulence interaction causes a truncation of the turbulent-energy cascade at small scales. The measurements are also in agreement with laser Doppler velocimetry results that in turbulent polymer solutions the lifetime of large-scale vortices is increased, and the high-frequency velocity fluctuations (small-scale motions) are reduced.