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Paper No.16-00653 © 2017 The Japan Society of Mechanical Engineers [DOI: 10.1299/jfst.2017jfst0013] Abstract The influence of the length of polymer aggregation on the turbulent drag reduction effect is investigated through numerical simulation. Polymer aggregation is modeled using a bead-spring chain model, which is a discrete element model. Simulations are carried out for different total natural lengths of the model at a friction Reynolds number of 180, and the numerical results for different spring constants by Fujimura et al. (2016) are analyzed. In addition, the time scale of the model, which corresponds to relaxation time, is investigated using oscillating Couette flow. Relaxation time increases as the total natural length increases and the spring constant decreases, and the drag reduction rate in turbulent channel flow increases with relaxation time. In the present study, it is determined that relaxation time is correlated with the length of the elongated model in turbulent channel flow. The relation between the drag reduction rate and the length of the elongated model can be expressed by a logarithmic function. According to the relational expression, it is expected that the drag reduction effect occurs when the length of the elongated model is longer than the diameter of vortical structures. In the visualization of turbulent flow field, it can be observed that longer models exhibit strong energy dissipation through interaction with the fluid, and suppress velocity fluctuations.

Influence of length of polymer aggregation on turbulent friction drag reduction effect