Superwicking Functionality of Femtosecond Laser Textured Aluminum at High Temperatures

An advanced superwicking aluminum material based on a microgroove surface structure textured with both laser-induced periodic surface structures and fine microholes was produced by direct femtosecond laser nano/microstructuring technology. The created material demonstrates excellent wicking performance in a temperature range of 23 to 120 °C. The experiments on wicking dynamics show a record-high velocity of water spreading that achieves about 450 mm/s at 23 °C and 320 mm/s at 120 °C when the spreading water undergoes intensive boiling. The lifetime of classic Washburn capillary flow dynamics shortens as the temperature increases up to 80 °C. The effects of evaporation and boiling on water spreading become significant above 80 °C, resulting in vanishing of Washburn’s dynamics. Both the inertial and visco-inertial flow regimes are insignificantly affected by evaporation at temperatures below the boiling point of water. The boiling effect on the inertial regime is small at 120 °C; however, its effect on the visco-inertial regime is essential. The created material with effective wicking performance under water boiling conditions can find applications in Maisotsenko cycle (M-cycle) high-temperature heat/mass exchangers for enhancing power generation efficiency that is an important factor in reducing CO2 emissions and mitigation of the global climate change.

Transient Evolution of Rheological Properties of Dense Granular Inertial Flow Under Plane Shear

Exploration on the transient evolution of the rheological properties of dense granular inertial flow is essential for revealing how the balance is established between the boundary drive strength and the internal shear strength. In this paper, discrete element method simulations are performed to study the transient flow characteristics of a dense granular system under plane shear in the inertial regime. To this end, we quantitatively analyze the changes in the system’s flow state, interfacial friction coefficient, internal friction coefficient, and microstructure. Simulation results show that the evolution of the horizontal flow experiences three typical stages, namely transmission, adjustment, and stabilization. Meanwhile, the shear dilatancy caused by the vertical movement of particles, gradually loosens the filling state, weakens the spatial geometric constraint and the system’s tangential load-bearing capacity, thereby decreasing the interfacial friction coefficient and reducing the boundary drive strength. On the other hand, the variations in the anisotropies of both contact orientation and contact forces, increase the internal friction coefficient and improve the internal shear strength. Therefore, the evolution of flow state from initially static to finally stable reduces the boundary drive strength while enhances the internal shear strength, and eventually a balance between them is achieved. Distinguished from the micromechanical behaviors, under different shear velocities the internal shear strength always mainly originates from the anisotropies in contact orientation and in normal contact force. Moreover, the contribution of the anisotropy in contact orientation becomes more predominant with the increase of shear velocity.

Early turbulence and pulsatile flows enhance diodicity of Tesla’s macrofluidic valve

Microfluidics has enabled a revolution in the manipulation of small volumes of fluids. Controlling flows at larger scales and faster rates, or macrofluidics, has broad applications but involves the unique complexities of inertial flow physics. We show how such effects are exploited in a device proposed by Nikola Tesla that acts as a diode or valve whose asymmetric internal geometry leads to direction-dependent fluidic resistance. Systematic tests for steady forcing conditions reveal that diodicity turns on abruptly at Reynolds number $${\rm{Re}}\approx 200$$ Re ≈ 200 and is accompanied by nonlinear pressure-flux scaling and flow instabilities, suggesting a laminar-to-turbulent transition that is triggered at unusually low $${\rm{Re}}$$ Re . To assess performance for unsteady forcing, we devise a circuit that functions as an AC-to-DC converter, rectifier, or pump in which diodes transform imposed oscillations into directed flow. Our results confirm Tesla’s conjecture that diodic performance is boosted for pulsatile flows. The connections between diodicity, early turbulence and pulsatility uncovered here can inform applications in fluidic mixing and pumping.

Inertial Flow of Viscoelastic Second-Grade Fluid in a Ciliated Channel under a Magnetic Field and Darcy’s Resistance

This paper proposes a mathematical analysis of the inertial flow of an MHD second-grade non-Newtonian fluid in a ciliated channel. The two-dimensional flow is modelled under the effect of inertial forces, magnetic field and Darcy’s resistance, which make the system of partial differential equations highly non-linear. To solve the complex system of partial differential equations, the Homotopy Perturbation Method (HPM) is preferred. The HPM solutions for the velocity profile, stream function and pressure gradient are obtained using the software MATHEMATICA. The significances of the Reynolds number (due to inertial forces), Hartmann number (due to magnetic field), porosity parameter (due to Darcy’s resistance) and fluid parameters (related to the second-grade fluid) on the pressure gradient, stream function and velocity profile are discussed in detail. The pertinent parameters show that the horizontal velocity decays in the presence of a magnetic field, whereas it rises under the effect of inertial forces, Darcy’s resistance and fluid viscosity in the centre of the channel. This research indicates that, for the ciliary flow of a second-grade fluid, a favourable pressure gradient (negative pressure gradient) in the horizontal direction increases when applying a magnetic field, whereas it decreases due to the porous medium. This mathematical model can be helpful to observe ciliary activity under magnetic resonance imaging, when ciliary activity is abnormal.

Analogue tuning of particle focusing in elasto-inertial flow

We report a unique tuneable analogue trend in particle focusing in the laminar and weak viscoelastic regime of elasto-inertial flows. We observe experimentally that particles in circular cross-section microchannels can be tuned to any focusing bandwidths that lie between the “Segre-Silberberg annulus” and the centre of a circular microcapillary. We use direct numerical simulations to investigate this phenomenon and to understand how minute amounts of elasticity affect the focussing of particles at increasing flow rates. An Immersed Boundary Method is used to account for the presence of the particles and a FENE-P model is used to simulate the presence of polymers in a Non-Newtonian fluid. The numerical simulations study the dynamics and stability of finite size particles and are further used to analyse the particle behaviour at Reynolds numbers higher than what is allowed by the experimental setup. In particular, we are able to report the entire migration trajectories of the particles as they reach their final focussing positions and extend our predictions to other geometries such as the square cross section. We believe complex effects originate due to a combination of inertia and elasticity in the weakly viscoelastic regime, where neither inertia nor elasticity are able to mask each other’s effect completely, leading to a number of intermediate focusing positions. The present study provides a fundamental new understanding of particle focusing in weakly elastic and strongly inertial flows, whose findings can be exploited for potentially multiple microfluidics-based biological sorting applications.

Study on Flow Behavior of Parallel Lumped-Model under Constant Flow for Bronchial Tree

Redistribution of flow in the bronchial tree is an important factor that enhances gas exchange in the lungs, especially, in diseased lungs. The bifurcated bronchial tree is like an electric network in series and parallel. A lumped-model of parallel system for constant flow rate is solved analytically to demonstrate the intrinsic characteristics and the dynamic behavior of the system. Inertial and capacitive time constants are calculated for 19th generation airways of human lung to control the solution. The investigation revealed that (i) higher inertial force takes more time to maximize the inertial flow to steady state and more time to minimize the resistive flow to steady state and (ii) the compliant effect is negligible for a relatively higher inertial time constant, on the same experimental conditions. GANIT J. Bangladesh Math. Soc. 40.2 (2020) 86–94