The Effects of Fluid Elasticity on Grid Turbulence

Experiments on grid turbulence are reported for pure water and dilute Polyox WSR 301 solutions. A novel passive grid, which consists of a square mesh grid with tethered spheres, has been developed to enhance the turbulence properties. In pure water the new grid produces higher turbulence intensities per unit Cp (pressure drop coefficient) than the classic plain grid, and the turbulence Reynolds number Reλ is increased by a factor of roughly two. In polymer solutions turbulence dissipation rates and energy spectra were measured using PIV with a high spatial resolution. The energy spectra reveal a characteristic length scale at which the polymers begin to affect the energy cascade. Above this scale the turbulence is essentially Newtonian, whereas below this scale the energy flux from large to small scales is reduced proportional to the length scale squared. Consequently, the energy in this new self-regulating spectral region scales according to a power-law with an exponent of -3 instead of the -5/3 for Newtonian turbulence, and the excess energy is dissipated by the polymers.