Low Reynolds Number Research Articles

Effect of Corrugations on an Airfoil Performing Plunging at Low Reynolds Number

Effect of Corrugations on an Airfoil Performing Plunging at Low Reynolds Number

Optimizing bioconvective heat transfer with MHD Eyring–Powell nanofluids containing motile microorganisms with viscosity variability and porous media in ciliated microchannels

Purpose This paper aims to optimize bioconvective heat transfer for magnetohydrodynamics Eyring–Powell nanofluids containing motile microorganisms with variable viscosity and porous media in ciliated microchannels. Design/methodology/approach The flow problem is first modeled in the two-dimensional frame and then simplified under low Reynolds number and long wavelength approximations. The numerical method is used to examine the impact of thermal radiation, temperature-dependent viscosity, mixed convection, magnetic fields, Ohmic heating and porous media for velocity, temperature, concentration and motile microorganisms. Graphical results are presented to observe the impact of physical parameters on pressure rise, pressure gradient and streamlines. Findings It is observed that the temperature of nanofluid decreases with higher values of the viscosity parameter. It is absolutely in accordance with the physical expectation as the radiation parameter increases, the heat transfer rate at the boundary decreases. Nanoparticle concentration increases by increasing the values of bioconvection Rayleigh number. The density of motile microorganisms decreases when bioconvection Peclet number is increased. The velocity of the nanofluid decreases with higher value of Darcy number. With increase in the value of bioconvection parameter, the flow of nanofluid is increased. Originality/value The bioconvective peristaltic movement of magnetohydrodynamic nanofluid in ciliated media is proposed. The non-Newtonian behavior of the fluid is described by using an Eyring–Powell fluid model.

Flow of a Viscous Fluid Past a Porous Oblate Spheroid at Small Reynolds Number

This study deals with the uniform motion of an adhesive, incompressible fluid flowing over a porous oblate spheroid at tiny values of the Reynolds number. These types of problems have been considered by dividing fluid flow into three regions, namely, zone I, zone II, and zone III. In the zone I, which is completely filled with viscous fluid, is the region of the porous oblate spheroid, and in this region fluid flow is governed by the equation suggested by Brinkman. The zone II and the zone III, where the clear fluid flows, are the regions outside the porous oblate spheroid. The fluid flow in these two zones has been discussed using the perturbation method given by Proudman and Pearson in which the Stokes stream function is expanded in terms of Reynolds number. This solution is then matched with the Oseen solution, At the interface of zone II and zone I, the matching conditions suggested by Ochoa-Tapia and Whitaker are applied for matching the stream function of the clear fluid region with that of the porous region at the surface of the oblate spheroid. It has been found that the drag on the oblate spheroid reduces with that of the departure from the spherical shape. Similar effects of the drag on the spheroid are obtained when the permeability of the porous medium increases. Also, the drag experienced on the porous oblate spheroid is directly proportional to Reynolds number and the ratio of effective viscosity of the porous medium to the real viscosity of the fluid. The application of a viscous fluid flow past a porous oblate at low Reynolds number is to calculate the friction factor and drag in internal and external flow, hydraulics study, aerofoil design, filtration technology, geothermal energy, and precipitation.

Alfvén waves at low magnetic Reynolds number: transitions between diffusion, dispersive Alfvén waves and nonlinear propagation

Alfvén waves at low magnetic Reynolds number: transitions between diffusion, dispersive Alfvén waves and nonlinear propagation

Computational simulation of the Aerodynamic performance of NACA2412 and NACA25112 in low Reynolds numbers

The present study introduces the basic definition of aerofoil and NACA series aerofoil. Then numerically investigates the two-dimensional aerodynamic characteristics of NACA 2412 and NACA 25112 aerofoils at a Reynolds number of 2. 0106. This study utilises the fluid dynamics simulation tool ANSYS Fluent, which is based on the finite volume method, to acquire the necessary data. The geometric model and grid of aerofoil were built with ANSYS Meshing and simulated with the pressure-based solver. In addition, the experimental environment of this study is the inlet speed of 30m/s and the air pressure of 1ATM. The study based on the Nabier-Strokes control equation completes simulation processing with SSTK-Omega. The aerodynamic characteristics of NACA2412 and NACA25112 were compared, including the pressure, velocity distribution map and the lift resistance coefficient change with the angle of attack. These experimental results are presented in the form of graphs. This experiment can be used for subsequent wind tunnel experiments or for developing a new aerofoil.

The Development and Prospect of Micro Air Vehicles

This paper systematically reviews the evolution of Micro Air Vehicle (MAV) aerofoil design, covering the geometric optimization and development from fixed-wing, rotary-wing, and flapping-wing to biomimetic-wing designs. In low Reynolds number environments, there are four main types of fixed-wing aerofoils, considered the classic designs for MAVs and have significantly contributed to the future development of MAV aerofoils. Rotary-wing and flapping-wing MAVs represent the initial application of bionics in aerofoil design. At the same time, the lift mechanisms discovered in insects and birds have since officially introduced MAV aerofoil design into the field of biomimetics. By integrating the locomotion mechanisms of animals such as stingrays and beetles, current research on biomimetic aerofoils has developed deployable and multi-parameter morphing wings Meanwhile, through the fusion of new and traditional technologies, the lift-to-drag ratio and flight efficiency are significantly improved. Finally, the paper summarizes the research methods and future directions of MAV aerofoil design, pointing out that the aerodynamic principles at low Reynolds numbers and the bionic kinematic principles of animals are the current research bottlenecks. These challenges can be addressed through cross-disciplinary research and further experimental validation to optimize MAV aerofoil designs.

Smoke visualization of the effect of freestream turbulence on a laminar separation bubble over an airfoil at low Reynolds numbers

Smoke visualization of the effect of freestream turbulence on a laminar separation bubble over an airfoil at low Reynolds numbers

The influence of trailing edge flaps on the aerodynamic characteristics of S1223 aerofoil

The S1223 aerofoil is a low Reynolds number high lift aerofoil. Analysing the aerodynamic characteristics of this aerofoil can provide a theoretical basis for enhancing flight efficiency in the design of small unmanned aerial vehicles. This study employed the k--SST turbulence model in the fluid simulation software (ANSYS Fluent) to conduct two-dimensional numerical aerodynamic simulations on the original aerofoil and its modified versions, which included flaps (deflected 10 at 70% chord length) and slotted flaps (deflected 10 at 70% chord length with a slot width of 1.5% of the chord). The aerofoil's aerodynamic performance was confirmed by comparing lift and drag coefficients at different angles of attack under Reynolds number 5.0105, as well as by analysing the pressure and velocity gradients in the flow field. The results indicate that both the lower flap and the slotted flap can greatly enhance the lift-to-drag ratio. In contrast, slotted flaps delay boundary layer separation, providing greater lift at high angles of attack while reducing the impact on drag.

Design and Investigation of a Passive-Type Microfluidics Micromixer Integrated with an Archimedes Screw for Enhanced Mixing Performance

In recent years, microfluidics has emerged as an interdisciplinary field, receiving significant attention across various biomedical applications. Achieving a noticeable mixing of biofluids and biochemicals at laminar flow conditions is essential in numerous microfluidics systems. In this research work, a new kind of micromixer design integrated with an Archimedes screw is designed and investigated using numerical simulation and experimental approaches. First, the geometrical parameters such as screw length (l), screw pitch (p) and gap (s) are optimized using the Design of Expert (DoE) approach and the Central Composite Design (CCD) method. The experimental designs generated by DoE are then numerically simulated aiming to determine Mixing Index (MI) and Performance Index (PI). For this purpose, COMSOL Multiphysics with two physics modules—laminar and transport diluted species—is used. The results revealed a significant influence of screw length, screw pitch and gap on mixing performance. The optimal design achieved is then scaled up and fabricated using a 3D additive manufacturing technique. In addition, the optimal micromixer design is numerically and experimentally investigated at diverse Reynolds numbers, ranging from 2 to 16. The findings revealed the optimal geometrical parameters that produce the best result compared to other designs are a screw length of 0.5 mm, screw pitch of 0.23409 mm and a 0.004 mm gap. The obtained values of the mixing index and the performance index are 98.47% and 20.15 Pa−1, respectively. In addition, a higher mixing performance is achieved at the lower Reynolds number of 2, while a lower mixing performance is observed at the higher Reynolds number of 16. This study can be very beneficial for understanding the impact of geometrical parameters and their interaction with mixing performance.

Topologies of Nanoscale Droplets upon Head-On Collision from Large Molecular Dynamics Simulations.

The binary collision of nanoscale droplets is studied with molecular dynamics simulation for droplets consisting of up to 2 × 107 molecules interacting via a truncated and shifted form of the Lennard-Jones potential. Considering head-on collisions of droplets with a temperature near the triple point that occur in a saturated vapor of the same fluid, this work explores a range of collision topologies. Four droplet sizes, with a radius ranging from 30 to 120 molecule diameters, are simulated with a varying initial relative collision velocity, covering 36 cases in total. Due to the relatively large size of the droplets, this study aims to resolve the differences in the collision behavior between droplets on the micro- and on the macroscale. By analyzing various metrics of the impact, four distinct collision regimes are found: coalescence, stable collision, holes and shattering. Coalescence, observed at low Weber and Reynolds numbers, is the formation of a stable droplet without significant deformations of the merging objects. Stable collisions, characterized by the formation of one stable droplet with notable deformations during collision, occur within a Weber number range between 10 and 505. The holes regime is only observed for droplet radii greater than 30 molecule diameters and a Weber number between 505 to 750, while collision cases surpassing this Weber number fall into the shattering regime, resulting in the breakup into satellite structures.

Particle Sedimentation in a Fluid at Low Reynolds Number: A Generalization of Hindered Settling Described by a Two‐Phase Continuum Model

AbstractParticle‐fluid separation by settling is a ubiquitous process in Earth and Planetary Sciences. The settling velocity of particles is controlled by a balance between buoyancy and drag forces. Since the seminal work of George Gabriel Stokes, parameterizations for the reduction of particle velocities caused by viscous dissipation due to mutual interactions have been described by a non‐linear mapping between particle volume fraction and separation velocity. We argue that these parameterizations neglect important physical behavior at high particle volume fractions (>80% of the maximum packing) and are only appropriate when considering suspensions where the particle volume fraction does not evolve in space or time. We introduce a more general model that accounts for the energy dissipation caused by changes in local particle volume fraction, which introduces a new term similar to the compaction term at higher particle volume fraction. This term depends on a consolidation/compaction viscosity that measures the resistance to changes in solid volume fraction. We derive closure equations for this compaction viscosity under dilute and concentrated particle volume fraction limits. Numerical simulations show that the extended hindered settling model predicts two significant differences compared to traditional hindered settling. First, while the steepening of particle volume fraction fronts remains, a dynamic instability is also generated at the front. Second, the rate of growth and structure of a cumulate layer growing above a no‐flux boundary is affected by the compaction‐like term and predicts the trapping of a higher volume fraction of interstitial melt in a correspondingly thicker cumulate layer.

The in-depth analysis and validation of momentum dispersion modeling based on direct numerical simulation

Based on the pore scale prevalence hypothesis (PSPH) in porous media flows, the PSPH model accounts for the effects of momentum dispersion and has been preliminary validated and applied by Rao and Jin [Possibility for survival of macroscopic turbulence in porous media with high porosity, J. Fluid Mech. 937, A17 (2022)]. This paper conducts further in-depth analysis of the PSPH model and validates it across various geometries of homogeneous porous media based on direct numerical simulation (DNS). A detailed analysis of DNS results indicates that the Darcy and Forchheimer terms dominate the flow field with low and high Reynolds numbers, respectively. The Darcy–Forchheimer term effectively approximates the total drag, which consists of both pressure drag and viscous drag throughout the flow field. The contribution of the momentum dispersion term is significant only within the first representative elementary volume adjacent to the boundary, especially at high Reynolds numbers. These findings verify the necessity of Reynolds number-dependent effective viscosity in the PSPH model. Results of the PSPH model are compared with DNS results for isotropic and anisotropic porous media across various geometries. The remarkable consistency between the PSPH model and DNS across a wide range of Reynolds numbers and porosities strongly indicates the potential power of the PSPH model in related industrial applications.

Characteristics of Phase Transitions in Dry Aligning Active Matter

Abstract Active matter is a non-equilibrium condensed system consisting of self-propelled particles capable of converting stored or ambient energy into collective motion. Typical active matter systems include cytoskeleton biopolymers, swimming bacteria, artificial swimmers, and animal herds. In contrast to wet active matter, dry active matter is an active system characterized by the absence of significant hydrodynamic interactions and conserved momentum. In dry active matter, the role of surrounding fluids is providing viscous friction at low Reynolds numbers and can be neglected at high Reynolds numbers. This review offers a comprehensive overview of recent experimental, computational, and theoretical advances in understanding phase transitions and critical phenomena in dry aligning active matter, including polar particles, self-propelled rods, active nematics, and their chiral counterparts. Various ways of determining phase transition points as well as non-equilibrium phenomena, such as collective motion, cluster formation, and creation and annihilation of topological defects are reviewed.

Enhancing aerodynamic efficiency: shape modification of airfoils

Small wind turbines (SWTs) are essential for harnessing renewable energy, particularly in developing regions where energy demands are significant. They provide a sustainable solution to meet the critical energy needs of these areas. However, the low Reynolds number (Re) airflow characteristics can pose challenges, making optimising airfoil shapes essential for enhancing aerodynamic performance and energy capture. This study offers a thorough analysis of six different airfoil profiles within a Re range of 50k to 500k. Modifications to the thickness-to-camber ratio (t/c) were strategically targeted to improve aerodynamic efficiency and structural viability. Using QBLADE software, we evaluated these design alterations to optimise the lift coefficient ( C L ), drag coefficient ( C D ), stall angle of attack ( α stall ) and lift-to-drag coefficient ratio (L/D). The analysis revealed significant improvements in the maximum power coefficient ( C p . max ), with the modified BW-3 airfoil achieving a C p . max of 0.441 at a Re of 500k – a remarkable 40.5% increase over baseline values. This consistent enhancement across various Re emphasises the essential impact of aerodynamic design in enhancing energy capture and reinforcing the feasibility of SWT applications across diverse operating conditions, along with material savings that lead to lighter wind turbine (WT) blades.

Unified view of elastic and elasto-inertial turbulence in channel flows at low and moderate Reynolds numbers

The chaotic flow of elastic fluids at low Reynolds number (Re) is typically distinguished into elasto-inertial and elastic turbulence (EIT/ET). However, the clear separation among these two turbulent regimes in parallel flows with a gradual Re decrease remains elusive. Here, spanning Re over four orders of magnitude, we investigate the statistical and structural nature of the flows computed by fully resolved 3D numerical simulations, revealing that as inertia becomes subdominant all flows share similar dynamical properties. Supported by the results, we thus propose a unified view of fully developed ET and EIT in parallel flows. Published by the American Physical Society 2024

Numerical investigation of Martian blades using various Airfoils

Due to the low Reynolds number compressible regime of the Martian atmosphere, the airfoil utilized to design the blade is of significant importance. The current study aims at evaluating the 2D performance of NACA 5605, Camber 4603 and DEP 4535-01 airfoils and the hovering performance of the blades built with these three airfoils using numerical analysis. The result has revealed that the type of airfoil for the blade design depends on the pitch angle at which the Figure of Merit reaches its peak. Prior to that pitch angle, which is 8deg, DEP 4535-01 airfoil is beneficial otherwise NACA 5605 is desirable. At the pitch angle of 8deg, the Figure of Merit of DEP 4535-01, NACA 5605 and Camber 4603 blades are 0.73, 0.71 and 0.70 respectively. Finally, the structural analysis is conducted on DEP 4535-01 blade to ensure the thickness distribution. The maximum stress and deformation are 393.15MPa and 485.54mm respectively and the minimum factory of safety is 1.12, which indicates the need of internal structures for the blade.

A MINI REVIEW ON THE EFFECTS OF SURFACE ROUGHNESS IN MICROFIUIDICS CHANNELS

The effects of surface roughness at low Reynolds numbers are more pronounced and critical in microchannels due to the relative size of roughness to channel dimensions. Surface roughness in microfluidic channels originates from the machining process during fabrication. This review examines how surface roughness, resulting from various manufacturing processes, influences the performance of microfluidic devices. Different patterns of surface roughness generated through techniques such as photolithography, etching, precision machining, and 3D printing are highlighted. These techniques yield distinct surface characteristics that affect critical microchannel properties, including fluid flow, pressure drop, and stress distribution. In addition to that, specific fabrication methods can minimize surface roughness, enhancing the performance of microchannels for applications in diagnostics, lab-on-a-chip systems, and small-scale heat exchangers are addressed. The review provides insights into selecting optimal fabrication techniques to achieve desired performance characteristics in microfluidic devices.

Impact of magnetorheological fluid composition on their behaviour in gradient pinch mode

Magnetorheological (MR) fluids can be utilized in one of the fundamental operating modes of which the gradient pinch mode has been the least explored. In this unique mode non-uniform magnetic field distributions are taken advantage of to develop a so-called Venturi-like contraction in MR fluids. By adequately directing magnetic flux the material can be made solidified in the regions near the flow channel wall, thus creating a passage in the middle of the channel for the fluid to pass through. This leads to unique variations of the slope in the pressure-flow rate characteristics. It can be stated that the effect of the MR fluid composition on the behaviour of the MR fluid in gradient pinch mode has not been thoroughly investigated yet. In this study, the behaviour of MR fluids was assessed with a dedicated pinch mode MR valve that provided a valuable insight into the contraction mechanism using fluorescence microscopy. Briefly, seven MR fluid samples were prepared with different particle concentrations (10%, 22% and 32 vol%) and mean particle sizes (2, 4.5 and 8.2 μm). It was found that the MR fluid sample with the larger particle size exhibits a significantly larger slope change observed in the pressure-flow rate characteristics. Increasing the particle size from 2 μm (base) to 8 μm resulted in the slope increase by a factor of 2.6 compared to the base sample. Increasing the particle concentration has a negligible effect on the pinch mode effect. Finally, these results were analysed using the modified Wuest equation, which is commonly used for characterizing sharp-edged orifices in low Reynolds number flow regimes. The simple equation was determined to describe the behaviour of MR fluids in gradient pinch mode with adequate accuracy.

Pulsation Analysis of Hose Pumps with Different Roller Counts Based on Two-Way FSI

Xinjiang, as a major agricultural region, offers extensive application potential for hose pumps, given their excellent performance as fertilization devices. Analyzing the pulsation characteristics of hose pumps during operation is valuable for reducing noise and extending pump service life. To investigate the pulsation characteristics and unsteady flow of hose pumps with different roller numbers, this study adopts a bidirectional fluid-structure interaction (FSI) method and utilizes ANSYS 19.0 commercial finite element software to analyze the outlet and inlet pressure pulsations, outlet flow velocity pulsations, and the distribution of the flow field at the intermediate plane for both two-roller and three-roller pumps transporting high and low Reynolds number fluids under the same working conditions. It was observed that the three-roller pump exhibited higher outlet pressure compared with the inlet and that the pulsation intensity was lower in the three-roller pump than in the two-roller pump under the same conditions, with an analysis provided on the reasons for this phenomenon. This study offers theoretical support for the selection and further optimization of hose pump designs. To further reduce the negative effects of pulsations, it is recommended to increase the number of rollers in the design while also considering shape optimization of the pump casing or using feedback control systems to adjust and reduce pulsation intensity.

Entropy Generation and Impacts of Activation Energy with Electroosmosis of Couple Stresses on the Peristaltic Transport of Bingham Blood Nanofluid

AbstractIn this theoretical paper, an analysis is undertaken to explore the peristaltic transition of a non-Newtonian Bingham nanofluid within a non-uniform microchannel oriented horizontally. This inquiry investigates the entropy generation arising from the flow of magnetohydrodynamic (MHD) and the accompanying heat transport. This theoretical investigation addresses the behavior of an electrically conductive fluid influenced by electroosmotic flow, incorporating the effects of couple stresses and Darcy law with a heat generation scheme. To bolster the robustness of the study, an activation energy term is incorporated into the nanoparticle concentration using both a modified Arrhenius model and a Buongiorno-type approach. The assumptions of long wavelengths and low Reynolds numbers are applied to change the complex equations that describe fluid motion into ordinary ones. The homotopy perturbation mechanism is utilized to solve the derived neutralized equations. The findings reveal that the critical velocity escalates with an augmentation in both the electroosmotic parameter and the regularization parameter. Moreover, the elevation of the heat absorption parameter and thermophoresis contributes to the augmentation of the temperature profile. Additionally, it is noted that an augmentation in the activation energy parameter has a positive impact on the concentration approach. This consideration recognizes broad applicability in both clinical and industrial settings. This research is beneficial in micro-fabrication mechanisms, reservoir engineering, and the chemical industry, where electro-osmotic energy and mass exchanges play a crucial role.