Measured energy injection, transfer, and dissipation rates in the bulk of dilute polymeric turbulent flow: The concentration and Weissenberg number effects

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We present an experimental study of the effects of polymer additives on the energy cascade in the bulk of turbulent von Kármán swirling flow and its dependences on the polymer concentration (ϕ) and Weissenberg number (Wi). The turbulent flow is viscous, driven by the rotation of a pair of smooth disks. The velocity measurements show that the flow is more coherent in the presence of the polymers. It is found that there is a critical concentration ϕc below which the energy injection, transfer, and dissipation rates are rapidly suppressed to about 20% of the Newtonian case with increasing ϕ (referred to as the rapid suppression regime) and above which the suppression is saturated at that level (referred to as saturated suppression regime). We found that the Wi dependence of the energy cascade resembles that of concentration dependence with one additional regime referred to as no suppression regime when Wi is very small. The sharp transition from the no suppression regime to the rapid suppression regime implies the occurrence of the coil–stretch transition of the polymers. Although the energy dissipation rate is greatly suppressed in the presence of polymers, the functional form of its probability density function is the same as that of the Newtonian case, suggesting that the mechanism governing the energy dissipation is universal for both the Newtonian and dilute polymeric turbulence. Our experiments show for the first time the similarity between the effects of Wi and ϕ in dilute polymeric bulk turbulence.