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Recent studies have leveraged wall-mounted flexible reeds to augment heat transfer efficiency in channel flows. In this study, we demonstrate that tuning the reed's mass distribution can substantially elevate this heat transfer enhancement. Numerical simulations incorporating the fluid–structure–thermal interaction are performed to investigate the impact of mass distribution on the reed dynamics and the associated heat transfer augmentation. The results indicate that the mass distribution of the reed significantly affects its motion mode, which, in turn, critically modulates the heat transfer characteristics. The maximum thermal efficiency factor is obtained when the reed's mass is concentrated at its distal end. Furthermore, the enhancement effect of tuning reed's mass distribution on heat transfer efficiency is closely related to the bending stiffness γ. Within the range of bending stiffness considered in this study (0.02–0.14), the effect of tuning the reed's mass distribution on the thermal efficiency factor exhibits a trend of increase–decrease–increase as the bending stiffness increases. At high bending stiffness, simply tuning the reed's mass distribution can increase the channel heat flux and reduce energy loss, thereby achieving the goal of enhancing the thermal efficiency factor. At γ = 0.14, allocating the reed's mass at its distal end resulted in a notable enhancement, with a thermal efficiency factor surge of 11.1%. [ABSTRACT FROM AUTHOR]

• Oscillatory flows shorten needed channel length but increase focusing time. • Sine waveform yields closer centerline particle focusing. • Higher Re p focus particle closer to channel center while higher Wo closer to walls. • Sine waveform offers superior particle focusing efficiency. • Optimal focusing obtained at Re p =0.075 and Wo=3.5. This study leverages the immersed boundary-lattice Boltzmann method to investigate the inertial focusing dynamics of a small neutrally buoyant particle in oscillatory channel flows driven by two distinct pressure gradient (PG) waveforms. Through an in-depth analysis of particle behavior, this research delineates the effects of PG waveforms, particle Reynolds number (Re p), and Womersley number (Wo) on the inertial focusing processes, and contrasts these findings with conventional Poiseuille flow-induced inertial focusing. Our results reveal that oscillatory flows induce more intricate focusing patterns than those observed in Poiseuille flow. The streamwise PG oscillation alters the particle focusing positions and introduces minor lateral oscillations in the post-focusing phase. The square PG waveform positions particles closer to the channel wall compared to Poiseuille flow, whereas the sine PG waveform positions particles closer to the channel center. Moreover, an increase in Re p nudges the focusing positions towards the channel center, while a higher Wo pushes them towards the walls. Notably, at Wo ≥ 5, the flow's oscillatory effect dominates over its inertial effect (i.e. Re p) on particle focusing. The study also underscores the capability of oscillatory flows to reduce the required channel length for particle focusing, albeit at the cost of increased focusing time. Contrary to Poiseuille flow, where the focusing length shortens with an increased Re p , oscillatory flow demonstrates an inverse correlation, with an increase in Re p extending the focusing length. Furthermore, while a higher Re p is beneficial in Poiseuille flow, it adversely affects the focusing effect in oscillatory flow. The focusing efficiency peaks at Wo = 3.5 for both waveforms, with the sine PG waveform offering superior efficiency due to the lower lift forces generated. Consequently, for optimal focusing in similar conditions, it is recommended to set Re p and Wo to 0.075 and 3.5, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

• Different initial fiber states lead to different fiber dynamic patterns in the downstream • Two kinds of dynamic patterns are observed in the downstream laminar channel, i.e., translation pattern and tumbling pattern • Side placed or deviated fiber in the upstream will migrate to the channel central region in the downstream • Fiber with different initial states settles at different vertical positions in the downstream and the off-centerline distance is dynamic-pattern dependent • The fiber's eventual conveyance speed can be roughly approximated by the local flow velocity The dynamics of an elastic fiber with various initial states in a laminar channel flow is investigated using the immersed boundary-lattice Boltzmann method. Fiber-wall collisions are solved by adding a repulsive force in the model. Our simulation results demonstrate that the initial fiber state closely relates to the stability of the considered conveyance system. Different initial states may lead to different dynamic patterns in the downstream. The fiber is found to go straight forward along a horizontal path when it is horizontally and symmetrically placed at the channel centerline. Breaking this symmetry by varying the fiber's initial vertical position or orientation will induce the instability of the system, which then causes deviations and fluctuations in fiber's conveyance path. No matter how large the deviation occurs in the upstream, the fiber is always found to migrate to the channel central region in the downstream and would eventually settle in a vertical position slightly away from the channel centerline. Moreover, the off-centerline distance that a fiber settles depends on its dynamic pattern rather than a specific initial fiber state. For our system, there are two kinds of dynamic patterns observed in the downstream channel. In the first pattern, the fiber eventually reaches its equilibrium state and is observed to do a translational motion. In the second pattern, no equilibrium state is observable, and the fiber is found to do a periodically tumbling motion. The fiber's eventual conveyance speed depends on the vertical position it eventually settles and can be roughly approximated by the local flow velocity. [Display omitted] [ABSTRACT FROM AUTHOR]

The lateral migration of a neutrally buoyant fiber in a three-dimensional (3D) channel flow bounded by four walls is numerically studied using a generalized immersed boundary-lattice Boltzmann method. The slender fiber is modeled as an elastic Kirchhoff rod to well reproduce its bending and twisting behaviors. Our simulations first examined the lateral migration of a flexible fiber released at six typical positions. The results show that the fiber accumulate at an isosurface of a specific shear rate value regardless of its initial lateral position. Thus, it would be more physical to characterize fiber accumulation behavior by the accumulation shear rate (γ α *) rather than the accumulation position. The fiber basically migrates laterally along the shear rate gradient when there is a discrepancy between the local and the accumulation (target) shear rates. For a fiber of medium flexibility, varying its initial position does not change its γ α * , while it could impact its orbits and tumbling plane. Compared to planar Poiseuille flows, more complex fiber orbits with out-of-plane deformations are observable in channel flows, and the fiber could adjust its tumbling plane to adapt to the surrounding shear rate gradient during its lateral migration. Our simulation results further confirm that the tumbling rate of the fiber increases with the increasing local shear rate. The impact of the effective flexibility of the fiber (Λ eff) on its lateral migration is also explored by considering a broad range of effective flexibility (Λ eff). The results show that the Λ eff relates to the γ α * non-monotonically, and five monotone intervals are captured. The results would enrich our understanding of fiber accumulation behavior in 3D wall-bounded flows and may also shed light on how we can sort fibers/filaments in microfluidic devices. [Display omitted] [ABSTRACT FROM AUTHOR]