Laminar Channel Flow Research Articles

Vertical concentration distribution of fine settling particles in a pulsatile laminar open channel flow

Vertical concentration distribution of fine settling particles in a pulsatile laminar open channel flow

CFD Modeling of Film Condensation from a Steam-Air Mixture in Vertical Channels

Steam condensation in the presence of a noncondensable gas is of vital importance for passive cooling containment systems. The noncondensable gas causes a significant reduction in the condensation rate and heat transfer across the containment, which is important for postulated loss-of-coolant accidents in a nuclear reactor. In this work, computational fluid dynamics models of condensation and the adjacent single-phase steam-air mixture flow are developed for laminar and turbulent flow in vertical channels by two distinct wall condensation modeling approaches using the commercial code STAR-CCM+. The first is the fluid film model available in STAR-CCM+, which solves liquid layer governing equations with connections to the adjacent gas mixture flow. The second is a user-defined wall condensation model that neglects the fluid film and instead accounts for mass, momentum, and heat transfer via user-defined volumetric sink terms adjacent to the cold wall. The condensation models are assessed by first comparing the calculated results with the numerical solution of laminar flow, solved using a complete two-phase model that solves parabolic equations based on conservation of mass, momentum, energy, and species for each phase. Next, the results of a two-dimensional analysis are compared with COPAIN experiments and existing numerical solutions from three-dimensional analyses. The comparisons include new, detailed results that have not been reported in previous analyses of a COPAIN case. These new results include local field profiles of velocity, temperature, and air mass fraction, and local mass flux.

Evolution of concentration distribution and removal of a solute in magnetohydrodynamics channel flow: effects of buoyancy-driven force and induced magnetic field

The dispersion of reactive solutes in various flow geometries has significant implications for ecological and environmental applications. Researchers such as Poddar et al . (Poddar et al . 2021 Proc. R. Soc. A 477 , 20200830 (doi: 10.1098/rspa.2020.0830 ) and Dhar et al . (Dhar et al . 2021 Phys. Fluids. 33 , 083609) have extensively studied solute transport in magnetohydrodynamics (MHD) channel flow, focusing on the applied magnetic field. However, the impact of the induced magnetic field on solute dispersion between two parallel plates remains poorly understood. This article presents an analytical investigation using Mei’s multi-scale homogenization approach to examine the effects of the Hartmann number, Grashof number and absorption parameter on the multi-dimensional concentration distribution and removal efficiency of a reactive solute in MHD laminar channel flow. The findings provide insights into concentration behaviour, such as the decrease in the Taylor transport coefficient with an increase in the induced magnetic field. These insights have practical implications for improving sewage and wastewater treatment by effectively separating desired particles from contaminants, contributing to advancements in environmental purification techniques.

Equilibrium distributions under advection–diffusion in laminar channel flow with partially absorbing boundaries

Equilibrium distributions under advection–diffusion in laminar channel flow with partially absorbing boundaries

Breaking the symmetry of a wavy channel alters the route to chaotic flow.

We numerically explore the two-dimensional, incompressible, isothermal flow through a wavy channel, with a focus on how the channel geometry affects the routes to chaos at Reynolds numbers between 150 and 1000. We find that (i) the period-doubling route arises in a symmetric channel, (ii) the Ruelle-Takens-Newhouse route arises in an asymmetric channel, and (iii) the type-II intermittency route arises in both asymmetric and semiwavy channels. We also find that the flow through the semiwavy channel evolves from a quasiperiodic torus to an unstable invariant set (chaotic saddle), before eventually settling on a period-1 limit-cycle attractor. This study reveals that laminar channel flow at elevated Reynolds numbers can exhibit a variety of nonlinear dynamics. Specifically, it highlights how breaking the symmetry of a wavy channel can not only influence the critical Reynolds number at which chaos emerges, but also diversify the types of bifurcation encountered en route to chaos itself.

Effects of porous-wall acceleration on laminar flows in semi-porous channels with a rectangular cross section

This paper aims to examine the effects of wall acceleration on the steady laminar flow of an incompressible Newtonian fluid along a semi-porous channel with a rectangular cross section. The lower wall of the channel is impermeable and at rest, while the upper wall is porous, and experiences longitudinal acceleration so that the flow is steady despite the accelerated movement of the wall. The fluid is injected or suctioned through the porous wall at uniform and constant velocity, orthogonally to the wall. The positive acceleration ratio defined as the ratio of longitudinal and transverse velocities, and the Reynolds number constructed by adding these velocities are the governing parameters. The similarity-solutions method, which is applied for the vorticity stream-function equation, gives numerical solutions using the shooting technique. Results indicate that the flow under study exists in the case of injection if the acceleration ratio does not exceed the value of 1, but in the case of suction, the flow remains unchanged if the acceleration ratio varies above the value of 9.5. For increasing acceleration ratios, the absolute value of transverse velocity decreases, the transverse pressure gradient remains unchanged at a specific fluid layer, the absolute value of longitudinal velocity increases near the porous wall and decreases near the impermeable wall, and in the case of injection, the algebraic wall shear stress decreases for the impermeable wall and increases for the porous wall. In the case of suction, there is a specific fluid layer at which the longitudinal velocity cancels by changing sign.

Green's Function for Laminar Flow in Channels With Porous Walls in the Presence of a Transverse Magnetic Field

Abstract Despite the significant and ongoing interest in Green's functions from scientists, engineers, and mathematicians, the area remains underdeveloped with respect to understanding problems from laminar fluid flow and magnetohydrodynamics (MHD) in porous media. The purpose of this paper is to partially address this gap by constructing a new and explicit representation of the Green's function for a boundary value problem that is derived from laminar flow in channels with porous walls in the presence of a transverse magnetic field. We discuss some interesting consequences of our constructed Green's function, including: the establishment of an equivalent integral equation; and the generation of new information regarding solutions to our boundary value problem. We discover that, for any given transverse magnetic field, our laminar flow problem has a unique solution in a particular location provided the Reynolds number is sufficiently small, and that the solution may be approximated by Picard iterations.

Enhancing heat transfer in laminar channel flow by tuning the mass distribution of a flexible reed

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%.

Forced Convection Heat Transfer Characteristics of Developed Laminar Flow in the Octagonal Channels of Octo-Square Asymmetric Particulate Filters

Forced Convection Heat Transfer Characteristics of Developed Laminar Flow in the Octagonal Channels of Octo-Square Asymmetric Particulate Filters

Fast macroscopic forcing method

The macroscopic forcing method (MFM) of Mani and Park [1] and similar methods for obtaining turbulence closure operators, such as the Green's function-based approach of Hamba [2], recover reduced solution operators from repeated direct numerical simulations (DNS). MFM has already been used to successfully quantify Reynolds-averaged Navier–Stokes (RANS)-like operators for homogeneous isotropic turbulence and turbulent channel flows. Standard algorithms for MFM force each coarse-scale degree of freedom (i.e., degree of freedom in the RANS space) and conduct a corresponding fine-scale simulation (i.e., DNS), which is expensive. We combine this method with an approach recently proposed by Schäfer and Owhadi [3] to recover elliptic integral operators from a polylogarithmic number of matrix–vector products. The resulting Fast MFM introduced in this work applies sparse reconstruction to expose local features in the closure operator and reconstructs this coarse-grained differential operator in only a few matrix–vector products and correspondingly, a few MFM simulations. For flows with significant nonlocality, the algorithm first “peels” long-range effects with dense matrix–vector products to expose a more local operator. We demonstrate the algorithm's performance for the eddy diffusivity and eddy viscosity operators, which correspond to the unclosed parts of the ensemble-averaged transport equations, excluding the analytically known, closed parts of such equations. However, the algorithm can also be applied to the full operators. We focus on scalar transport in a laminar channel flow and momentum transport in a turbulent channel flow. For these problems, we recover eddy diffusivity- and eddy viscosity-like operators, respectively, at 1% of the cost of computing the exact operator via a brute-force approach for the laminar channel flow problem and 13% for the turbulent one. Applying these operators to compute the averaged fields of interest has visually indistinguishable behavior from the exact solution. Our results show that a similar number of simulations are required to reconstruct the operators to the same accuracy under grid refinement. Thus, the accuracy corresponds to the physics of the problem, not the numerics, so long as the grid is sufficiently refined. We glean that for problems in which the RANS space is reducible to one dimension, eddy diffusivity, and eddy viscosity operators can be reconstructed with reasonable accuracy using only a few simulations, regardless of simulation resolution or degrees of freedom.

Toward tuning flow charecteristics in microchannel by nanotechnology and electrokinetic: Numerical simulation of heterogenous electroosmotic flow

Nanotechnology in recent years helps researcehers to design flow rate and profile in microsized channel via making surface charge heterogenous. This paper numerically simulates the electroosmotic flow inside a microchannel with non-uniform Zeta potential (ZP). The problem is a two-dimensional, incompressible, steady, and laminar channel flow between two parallel plates. The numerical results show that the flow rate changes compared to a constant ZP by ascending, descending, and parabolic modifications of the wall ZP at the mid-length of the microchannel. Results show that flow volume rate increases by microchannel width from 4 mm3/s to 6.5 mm3/s in best condition.

Effect of transient inhalation on powder evacuation and dispersion in a typical dual air inlet dry powder inhaler

An optically accessible research dry powder inhaler (DPI) was used to investigate the impact of initial acceleration during inhalation on the powder evacuation and drug dispersion processes. The inhaler featured a swirling chamber connected with two inlets, closely resembling commercial devices such as the Aerolizer® and Osmohaler®. The dimensions of the inhaler were kept similar to the commercial devices, mimicking their flow situations. High-speed microscopic imaging and an in-house developed image processing technique were used to extract relevant statistics. The current literature on DPI flow dynamics provides valuable insights into the correlation between inspiratory flow rate, peak inspiratory pressure, and inhaler performance. However, it lacks data correlating powder evacuation characteristics and deagglomeration mechanisms for transient inhalation conditions inside the DPIs. This paper is an attempt to fill this gap by investigating the evacuation pattern and deagglomeration mechanism for three different initial accelerations qualitatively representing asthma severity and steady-state inhalation conditions representing laminar, transitional, and turbulent channel flow, using both pure drug and carrier particles. The study's key findings revealed that powder evacuation progressed in two stages. The first stage was slow, with a slope ranging from 0.25 to 0.27. Also, the evacuation rate was independent of the initial inhalation acceleration. The second stage was rapid, with a slope ranging from 0.87 to 1.23, which led to faster and complete powder evacuation. The slow stage consumed approximately ∼60 % of the total evacuation time to evacuate only ∼20% of the loaded drug, while the rapid stage evacuated ∼80% of the powder in the remaining time. The data collected from these experiments advances the understanding of the DPI flow dynamics and provides high-quality data for validating computational models used in the analysis of such inhaler systems.

Flow transitions during laminar stratified two-phase flow in mini channels

Flow transitions during laminar stratified two-phase flow in mini channels

Laminar drag reduction in surfactant-contaminated superhydrophobic channels

Although superhydrophobic surfaces (SHSs) show promise for drag reduction applications, their performance can be compromised by traces of surfactant, which generate Marangoni stresses that increase drag. This question is addressed for soluble surfactant in a three-dimensional laminar channel flow, with periodic SHSs made of long finite-length longitudinal grooves located on both walls. We assume that bulk diffusion is sufficiently strong for cross-channel concentration gradients to be small. Exploiting long-wave theory and accounting for the difference between the rapid transverse and slower longitudinal Marangoni flows, we derive a one-dimensional model for surfactant transport from the full three-dimensional transport equations. Our one-dimensional model allows us to predict the drag reduction and surfactant distribution across the parameter space. The system exhibits multiple regimes, involving competition between Marangoni effects, bulk and interfacial diffusion, bulk and interfacial advection, shear dispersion and surfactant exchange between the bulk and the interface. We map out asymptotic regions in the high-dimensional parameter space, and derive explicit closed-form approximations of the drag reduction, without any fitting or empirical parameters. The physics underpinning the drag reduction effect and the negative effect of surfactant is discussed through analysis of the velocity field and surfactant concentrations, which show both uniform and non-uniform stress distributions. Our theoretical predictions of the drag reduction compare well with results from the literature solving numerically the full three-dimensional transport problem. Our atlas of maps provides a comprehensive analytical guide for designing surfactant-contaminated channels with SHSs, to maximise the drag reduction in applications.

Towards energy harvesting through flow-induced snap-through oscillations

Recently, energy harvesting through periodic snapping, namely snap-through, has gained significant attention for energy harvesting applications. In this study, the snapping dynamics of a buckled sheet with two ends clamped were numerically investigated to explore its energy harvesting characteristics in a Poiseuille channel flow. It is found that the elastic sheet comes into either a static equilibrium or snap-through oscillation state. The oscillation state can be initiated more readily by buckling the sheet to a length ratio in the vicinity of ΔL*=0.3 and/or by raising the Reynolds number. Additionally, the effects of three governing parameters, including the length ratio, the bending stiffness of the sheet, and the Reynolds number, on the energy harvesting characteristics were also examined for the oscillation cases. The finding shows that, in a post-equilibrium state, increasing the length ratio and bending stiffness could enhance the total energy for harvesting, primarily by raising the elastic potential energy. The most effective portion for energy collection always lies in the aft half of the sheet. Moreover, transitions from an equilibrium state to a snap-through oscillation state increase both the elastic potential and kinetic energies. Our numerical results gain deeper insights into the dynamics of a pre-compressed elastic sheet and its interaction with a laminar channel flow. The results may provide some guidance on optimizing relevant energy harvesting systems.

Combined Radiation and Convection in Developing Flow in a Parallel Plate Channel With Real Gas Behavior: The Case of Gas Cooling

Abstract The effect of real gas volumetric radiation on the thermal development in laminar parallel plate channel flow of H2O and/or CO2 in the case of gas cooling has been investigated numerically. The nongray radiation effects of the gas have been treated using a global spectral approach, the Spectral Line Weighted-sum-of-gray-gases model. The results reveal that gas radiation results in significantly higher total heat transfer to the cooled channel wall, with an attendant more rapid drop in gas mean temperature. Gas radiation is seen to increase the local convective and total (radiative plus convective) Nusselt number for increasing radiating species mole fraction for both H2O and CO2 and for increasing gas inlet temperature. The influence of gas radiation on the thermal development is less pronounced for CO2 than for H2O. An apparent thermally fully developed condition may exist for this combined convection-radiation problem with real gases in the gas cooling scenario, and radiation has the effect of significantly extending the thermally developing region. Combined hydrodynamic and thermal development yields higher heat transfer than the thermally developing condition. Smaller channel wall spacing results in lower radiative heat transfer and the aforementioned radiation effects are diminished. Local convective and radiative flux and thermal entry length also increase with elevated gas total pressure.

Universal properties of non-Hermitian viscoelastic channel flows

An addition of long-chain, flexible polymers strongly affects laminar and turbulent Newtonian flows. In laminar inertia-less viscoelastic channel flow, the supercritical elastic instability of non-normal eigenmodes of non-Hermitian equations at finite-size perturbations leads to chaotic flow. Then three chaotic flow regimes: transition, elastic turbulence (ET), and drag reduction (DR), accompanied by elastic waves, are observed and characterized. Here we show that independently of external perturbation strength and structure, chaotic flows above the instability onset in transition, ET, and DR flow regimes reveal similar scaling of flow properties, universal scaling of elastic wave speed with Weissenberg number, Wi, defined the degree of polymer stretching, and the coherent structure of velocity fluctuations, self-organized into cycling self-sustained process, synchronized by elastic waves. These properties persist over the entire channel length above the instability threshold. It means that only an absolute instability exists in inertia-less viscoelastic channel flow, whereas a convective instability, is absent. This unexpected discovery is in sharp contrast with Newtonian flows, where both convective and absolute instabilities are always present in open flows. It occurs due to differences in nonlinear terms in an elastic stress equation, where except for the advective term, two key terms describing polymer stretching along the channel length are present.

Calculation of the development length of spanwise rotating three-dimensional laminar channel flow

Calculation of the development length of spanwise rotating three-dimensional laminar channel flow

A Semi-Analytical Method for Channel and Pipe Flows for the Linear Phan-Thien-Tanner Fluid Model with a Solvent Contribution

This work presents a semi-analytical method for laminar steady-state channel and pipe flows of viscoelastic fluids using the Linear Phan-Thien-Tanner (LPTT) constitutive equation, with solvent viscosity contribution. For the semi-analytical method validation, it compares its results and two analytical solutions: the Oldroyd-B model and the simplified LPTT model (without solvent viscosity contribution). The results adopted different values of the dimensionless parameters, showing their influence on the viscoelastic fluid flow. The results include the distribution of the streamwise velocity component and the extra-stress tensor components in the wall-normal direction. In order to investigate the proposed semi-analytical method, different solutions were obtained, both for channel and pipe flows, considering different values of Reynolds number, solvent viscosity contribution in the homogeneous mixture, elongational parameter, shear parameter, and Weissenberg number. The results show that the proposed semi-analytical method can find a laminar solution using the non-Newtonian LPTT model with solvent viscosity contribution and verify the effect of the parameters in the resulting flow field.

Three-dimensional simulation of lateral migration of fiber in a laminar channel flow

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 (γα*) 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 γα*, 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 (Λeff) on its lateral migration is also explored by considering a broad range of effective flexibility (Λeff). The results show that the Λeff relates to the γα* 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.