Contribution of turbulence motions to turbulent heat transfer in polymer drag-reduced flows

We examined turbulent heat transfer in a polymer drag-reduced flow by analyzing the DNS database for viscoelastic channel flows (Re_τ = 125 and Pr = 5) of a FENE-P fluid. The viscoelastic flows with drag reduction rates of 15%, 34%, and 52% were examined as well as the Newtonian flow. The turbulence structures associated with the turbulent heat flux were investigated through the three-dimensional joint probability density function (JPDF) of temperature fluctuations (θ') and streamwise and wall-normal velocity fluctuations (uˆ and vˆ). The distribution of the JPDF revealed a higher correlation between u′ and θ′ in drag-reduced flow than in Newtonian flow, which was shown to be mainly attributed to the enhanced low-speed streaks. The weighted integral of the JPDF also showed that the contribution of the turbulent motion to the Reynolds shear stress is approximately the same as that to the turbulent heat flux in both Newtonian and drag-reduced flows.