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.
In order to examine mechanical interactions between erythrocytes and a blood vessel surface, the frictional characteristics between erythrocytes and plates in plasma have been measured by an inclined centrifuge microscope. The frictional characteristics have been properly reproduced by a numerical simulation of a rigid erythrocyte model assuming a flat bottom surface. However, validity of the assumption has not been confirmed. The purpose of this fundamental study, therefore, was to clarify the behavior of a two-dimensional circular capsule subjected to inclined centrifugal force near a plate in a fluid. An unsteady simulation was performed for various values of the angles of the inclined centrifugal force and membrane elasticity. In equilibrium states, a lubrication domain with high pressure and a large shear stress is formed between the capsule and the base plate, and the bottom surface of the capsule becomes flat with a positive attack angle. The gap distance and translational and rotational velocities increase with decreasing membrane elasticity or increasing centrifugal force angle. The attack angle increases with increasing membrane elasticity or centrifugal force angle. The results in this study qualitatively justified the assumption of the former numerical study that erythrocytes in an inclined centrifuge microscope have a flat bottom surface and its result that they have a positive attack angle in equilibrium state.
This study examines experimentally the vortex-induced vibration (VIV) of a mechanical system with two eigenmodes. A previous experiment setup was refined to enable the experiment, and was placed in a circulating water channel to submerge a movable circular cylinder (cylinder A). This cylinder was subjected to fluid flow while being supported elastically, whereupon VIV occurred. A second movable cylinder (cylinder B) above the water was connected to cylinder A and supported elastically. The displacements of those two cylinders were measured. The underlying hypothesis was that the vortex shedding frequency would become locked to one or other of the two eigenfrequencies, and that which eigenmode the vortex shedding frequency became locked to would depend on the reduced velocity. To test this hypothesis, the experimental setup was refined to withstand the high flow speeds required to allow the vortex shedding frequency to become locked to either eigenfrequency. From the results obtained, the amplitude ratio (the ratio of the amplitude of cylinder B to that of cylinder A) and the frequencies of the two cylinders were determined. It was found that those amplitude ratio and frequencies were close in two ranges of reduced velocity to those calculated theoretically by solving the eigenvalue problem of the 2DOF system. This demonstrated that, depending on the reduced velocity, the vortex shedding frequency could become locked to either the first or second eigenmode to produce a definite vortex-induced vibration.
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