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.
A pulsatile turbulent flow within an S-shaped double bend pipe is experimentally and numerically studied to characterize the flow field in conditions resembling an automotive engine environment. Particle image velocimetry (PIV) measurements were carried out to measure streamwise and secondary flow velocities. The flows are accelerated around the inner side walls of both bends. The secondary flow, after passing through the second bend, is directed toward the inner side in the core of the cross section, and, as a result, Lyne-type vortices, which are not consistent with the second bend curvature, are formed. A numerical simulation is performed under the same condition as the experiments with computational fluid dynamics software. The numerical simulation gives qualitative results in comparison with the experimental data though there is some deviation, and shows the cause of the Lyne-type vortex formation in the second bend. After passing through the first bend, the high-speed region appearing around the inner side shifts in accordance with the Dean-type secondary flow formed in the first bend, and thus the non-uniform flow enters the second bend. In the second bend, the low-velocity region in which the centrifugal force is not strong enough to direct the flow toward the outer side, appears in the core of the cross section. Details of the Lyne-type vortex formation are discussed by considering the driving forces of the secondary flow.
July 31, 2017 Due to the end of the Yahoo!JAPAN OpenID service, My J-STAGE will end the support of the following sign-in services with OpenID on August 26, 2017: -Sign-in with Yahoo!JAPAN ID -Sign-in with livedoor ID * After that, please sign-in with My J-STAGE ID.
July 03, 2017 There had been a service stop from Jul 2‚ 2017‚ 8:06 to Jul 2‚ 2017‚ 19:12(JST) (Jul 1‚ 2017‚ 23:06 to Jul 2‚ 2017‚ 10:12(UTC)) . The service has been back to normal.We apologize for any inconvenience this may cause you.
May 18, 2016 We have released “J-STAGE BETA site”.
May 01, 2015 Please note the "spoofing mail" that pretends to be J-STAGE.