ABSTRACT The wavy channel configurations have gained importance to improve transport phenomena in biological and engineering processes such as biomedical equipment, micro-flows, heat exchangers, and cooling systems, among others. With this in mind, the current research explores the peristaltic flow phenomenon of Ree-Eyring liquid in an inclined wavy channel taking into account the effect of both the magnetic field and nanoparticles transport mechanism using the Buongiorno’s approach. The resulting nonlinear coupled partial differential equations for momentum, temperature, and nanoparticle volume fraction are transformed into nonlinear ordinary differential equations using long wavelength and low Reynolds number assumptions and solved analytically via OHAM technique. In the present investigation, the effect of different parameters on velocity distribution, temperature distribution, and concentration distribution is analyzed, and various other parameters of engineering interest such as skin friction, Nusselt number, Sherwood number, pressure gradient, and volume flux are also studied. It was observed that the presence of magnetic field opposes the movement of fluid by virtue of Lorentz force, and, on the other hand, the phenomenon of thermophoresis helps enhance thermal convection, whereas Brownian motion plays an important role in increasing the movement of nanoparticles. This interplay of different phenomena provides better understanding about the coupled flow mechanisms and its effective control.

This study investigates the aerodynamic characteristics of a swallow-inspired wing based on a NACA 2415 airfoil at Reynolds numbers of 7.5 × 104and 1.25 × 105using experimental force measurements and flow-visualization techniques. The bioinspired wing model was designed according to the top-view geometry of a swallow wing and tested in a low-speed open-suction wind tunnel together with a rectangular wing of identical planform area and airfoil profile to provide a baseline comparison. Aerodynamic measurements were conducted over an angle-of-attack range between -6° and 24° to determine lift, drag and moment coefficients as well as aerodynamic efficiency. In addition to force measurements, smoke-wire and TiO2based flow visualization techniques were used to examine the flow topology around the test models. The results show that the swallow-inspired wing exhibits improved aerodynamic performance compared with the rectangular configuration under both Reynolds number conditions. AtRe= 1.25 × 105, the bioinspired wing achieved a maximum lift coefficient of 0.694 at an angle of attack of 9°, while the minimum drag coefficient was approximately 7.48 × 10-3. The aerodynamic efficiency reached a maximumL/Dratio of 42.9 at an angle of attack of 2°. Similar aerodynamic trends were observed atRe= 7.5 × 104, although slightly reduced lift and aerodynamic efficiency were recorded due to stronger viscous effects at lower Reynolds numbers. The pitching moment coefficient varied approximately linearly with angle of attack, with a slope of -7.8 × 10-3per degree and a zero-lift moment value of -7.47 × 10-3, indicating stable longitudinal aerodynamic behavior. Flow visualization results revealed a gradual transition from attached laminar flow to separated turbulent flow, suggesting smoother stall development and improved flow stability for the bioinspired configuration. These findings highlight the aerodynamic advantages of swallow-inspired wing geometries and their potential to enhance aerodynamic efficiency in low Reynolds number flight.

This paper introduces the concept of ultrasound acoustic self-rotating cylinders, in which the source of self-rotation is due to the acoustic field radiation into the ambient fluidic environment. For a monochromatic radiating cylindrical body, we derive analytical expressions for the acoustic radiation torque and force and show that for particular designs of imposed normal velocity patterns over the cylinder's surface, the wave-solid-fluid interactions lead to the exertion of non-zero torque on the body while maintaining zero net motion. The mathematical modeling of the self-excitation of the body is simplified as a distribution of normal velocities across the cylinder's boundary with specified amplitude, frequency, and phases. We then propose several simple and versatile scenarios of velocity distribution and introduce several cases of desirable distributions as design strategies for self-rotating cylinders, which are validated numerically. Assuming a low Reynolds number condition, the frequency-dependent rotation velocity is estimated as a function of design parameters, and the feasible operating conditions are obtained.

• Comparison of flapping and rotating wings in compressible flows is presented. • Flexibility of flapping wing is considered. • Lift coefficient, lift-to-drag ratio and power factor are discussed. • Wing-wing interaction is discussed. Recently, flapping wings have been identified as a potential solution for future Martian unmanned aerial vehicles, due to their performance in low-Reynolds number and moderately compressible environments. This necessitates a comparison study between flapping wings, and proven rotary wing design for the unique, low-density Martian atmosphere. Here, such a comparison of equivalent flapping and rotating wing configurations is considered through numerical simulations, with an emphasis on lift generation and efficiency. In general, flapping wings generated sufficiently higher lift than their rotating counterparts at low Reynolds numbers, while rotating wings in general outperformed flapping wings in terms of the lift-to-drag ratio and power factor. However, there are exceptions to this: weakly flexible flapping wings at low aspect ratio ( Λ = 2 ) outperformed rotating wings in terms of lift generation and efficiency at Ma ≤ 1. Multi-rotating wing configurations are also considered to determine if wing-wing interactions have any improvement in the performance, finding that the average lift coefficient remains similar to the single rotating wing cases, despite significant changes in the temporal space. Overall, this study identifies that flapping wings have the ability to outperform their rotating counterparts in the Martian environment from an aerodynamic perspective.

Three-dimensional wake dynamics of a square cylinder in oscillatory flows at low Reynolds numbers

Gust response alleviation via wingtip bending freely with fluid-structure interaction approach based on dynamic modal rotation method

• A rigorous two-stage DNS framework is developed to provide high-quality reference data for shock-turbulence-flame interactions in supersonic reacting shear layers using a moderately complex hydrocarbon fuel. • The flow is shown to be remarkably robust to variations in mean scalar dissipation rate, with reaction rates governed by localized regions of high scalar dissipation, implying accurate modelling is possible at lower grid resolutions than traditionally expected. This study examines critical aspects of Direct Numerical Simulations (DNS) of ethylene-air combustion for hypersonic propulsion applications. A combustion mechanism was selected based on a comparative analysis of multiple models, balancing chemical fidelity and computational cost. Detailed configurations of two DNS cases are presented, focusing on high-speed reacting turbulent shear layers including interactions with an oblique shock wave ( M s = 1.3 , inflow angle 5 ∘ ). A lower and standard Reynolds number case are employed to ensure mesh convergence while maintaining computational feasibility. Although most statistical quantities exhibit strong convergence trends, full convergence was not achieved for the scalar dissipation rate. This study investigates the underlying physical mechanisms responsible for this behaviour and demonstrates their limited impact on all other key variables. These results advance the understanding of turbulence-combustion interactions and will be used to guide new developments in numerical modelling for next-generation air-breathing hypersonic propulsion systems.

Modulation and classification of wake transitions through vortex evolution phases in undulated cylinders at low Reynolds number – ERRATUM

Purpose – The purpose of this study, which is inspired by earlier research is to investigate the impact of suction and injection on the peristalsis pumping of a Jeffrey nanofluid in a vertical channel. Design/methodology/approach - The governing equations are developed and analytically solved in this study under the conditions of a low Reynolds number and long wavelength. Expressions are produced for the following quantities: velocity profile, pressure rise per wavelength, temperature distribution, and nanoparticles volume fraction. To visually examine the influence of all physical variables on concentration fields, temperature distribution, velocity, pressure rates, frictional force, streamline, and pressure gradient, we utilize the Wolfram MATHEMATICA tool. Findings - It is explored how the temperature of the Jeffrey nanofluid and the volume fraction of nanoparticles are affected by thermophoresis parameters and Brownian motion parameters. It is intended to emphasize the importance of suction and injection on the peristalsis moving of a Jeffrey nanofluid in a vertical layer. This mathematical model can be used effectively to transport cervical cancer in the tiny blood channels of the cervix. It may be necessary to make considerable revisions to address the issue of internal fluid motion in a non-pregnant uterus brought on by myometrial contractions that resemble peristaltic fluid movement and can occur in both symmetrical and asymmetric directions. Originality- This is the first study, to the best of the author’s knowledge no research has been done into how suction and injection affect the peristalsis pumping of a Jeffrey nanofluid in a vertical channel.

Abstract The paper is devoted to a numerical study of the elastic turbulence effect that occurs at low Reynolds numbers and high Weissenberg numbers in the presence of polymers in the flow. A physical model of the flow of a polymer solution in a weakly compressible flow is constructed, and a hybrid numerical technique for approximation this model is developed. Based on this technique, we present a numerical analysis of polymer flow in a square computational domain with solid boundaries under the action of an external periodic force (Kolmogorov flow). The spectra of velocity, vorticity, and stretching of polymers are constructed.

Active nematics are fluids in which the components have nematic symmetry and are driven out of equilibrium due to the microscopic generation of an active stress. When the active stress is high, it drives flows in the nematic and can lead to the proliferation of topological defects, a state we refer to as defect chaos. Using numerical simulations of active nematics at low Reynolds number, we observe energy transfer from long to short length scales during defect chaos. We demonstrate that this energy transfer is driven by the exchange between variations in the orientation and degree of order in the nematic that predominantly occurs during defect creation and annihilation. We then show that the primary features of energy transfer during defect chaos scale with the active length scale. Finally, we identify a second regime that forms extended bend walls instead of point like topological defects, which we refer to as bend wall chaos. The bend walls grow and extend over the whole system and result in an energy transfer from short to long length scales.

Micro air vehicles (MAVs) operating at low Reynolds numbers face aerodynamic and structural challenges that differ significantly from those encountered by conventional aircrafts. Nature provides effective solutions to these constraints, as insects, birds, and bats demonstrate highly efficient flight through integrated interactions between morphology, kinematics, and unsteady aerodynamic mechanisms. This review examines how biological flight principles can inform the design of next-generation MAVs. The study first analyzes biological flight strategies across insects, birds, and bats, with emphasis on scaling laws and physiological adaptations relevant to small-scale flight. It then reviews key unsteady aerodynamic phenomena governing low-Reynolds-number flight, including leading-edge vortex stability, wing-wake interactions, tandem-wing effects, and ground influence, as well as current modeling approaches ranging from quasi-steady methods to high-fidelity Navier-Stokes simulations. Building on these principles, the paper discusses biomimetic design strategies for MAV wings, structural-aerodynamic coupling, and actuation technologies used to replicate flapping flight. Existing MAV demonstrators inspired by biological flyers are analyzed, including concepts relevant to planetary exploration environments. Finally, the review identifies current technological limitations and research gaps in materials, actuation, aerodynamic modeling, and system integration. By synthesizing insights from biology and engineering, this work highlights key directions for the development of efficient, adaptable biomimetic MAV platforms capable of operating in complex environments.

• Experimental investigation of twin propellers configuration: aerodynamic loads (load sensors) and wake flowfield (stereoscopic particle image velocimetry (SPIV)). • Application of a phase-reconstruction methodology based on the propeller blade position extracted from the raw images obtained from the SPIV measurements. • In depth analysis of the propeller-motion correlated and uncorrelated flow fields deriving from triple decomposition to the measured test cases. • Discussion on the reduced propeller performance and on the flow field topology in a side-by-side propellers configuration. An experimental investigation is conducted to study the interaction of side-by-side propellers operating in forward flight at low Reynolds numbers. The effect on performance is first evaluated by means of load cell measurements, while the flow field is studied employing a stereoscopic particle image velocimetry (SPIV) setup. Three different configurations are tested: single propeller, co-rotating and counter-rotating cases at varying advance ratios. The results indicate that the performance of the single propeller is decreased due to aerodynamic interaction, leading to an average 3.2% reduction in propulsive efficiency, evaluated across all the tested operating conditions. The effect is stronger at lower advance ratios, owing to a greater interaction between the two streamtubes in such conditions. SPIV measurements indicate a widening of the wake as well as a reduction in the turbulence intensity for the cases with two propellers, with a stochastic fluctuations approximately 15% lower for the twin propeller cases than the single propeller. The data is then sorted and phase-ordered a posteriori via a data-driven approach, effectively reconstructing phase-averaged flow fields. This enables the decomposition of the velocity field into phase-correlated and purely turbulent components. The results show that the stochastic (turbulent) component of the velocity field increases when the propellers operate at advance ratios different from the maximum efficiency condition.

Vortical structures and the associated flowfield on finite-span, cantilevered NACA 0015 wings were observed experimentally. The effects of semi-aspect ratios of 1, 2, and 4, sweep angles of 0, 15, 30, and 45 deg, at angles of attack of 12, 18, and 22 deg were explored at a Reynolds number of 600. On each wing, there are three regions along the span: a wall region, a tip region, and wake region between the two. At low semi-aspect ratios, end effects played a significant role by mitigating the shedding, resulting in a small separated region. At higher semi-aspect ratios, the wings ends had less of an influence on the overall flowfield, allowing the developments of separation and shedding in the wake. For the unswept wings, secondary structures of alternating rotation developed as a result of the interaction between the separated flow and the end regions. In the swept cases, an outboard spanwise velocity component developed, which interacted with the separated region forming secondary structures and a large dominant swirl vortex along the span. Both time-averaged and instantaneous flowfields were used to understand the formation of the structures. The present study shows the complexity of three-dimensional flow separation even on simple geometries.

Selection of RANS turbulence model for calculating thermal hydraulics of fuel assemblies of LMC reactors at low Reynolds numbers

• An enhanced analytical model has been developed to characterize the velocity field within a corrosion fatigue crack, and validated through (CFD) simulations. • Analytical and numerical analyses have demonstrated that, at extremely low Reynolds numbers, the velocity field of the crack is independent of the fluid properties. • Experimental observations from the literature indicate that the loading ratio significantly affects the corrosion rate due to the refreshing of the corrosive liquid inside the crack. This phenomenon is explained mathematically. This study presents a semi-analytical model to characterize fluid flow within narrow corrosion fatigue cracks, based on crack geometry and loading conditions. The model is validated through computational fluid dynamics simulations. Results show that the velocity field is explicitly dependent on the loading ratio R , while remaining insensitive to the loading amplitude and fluid properties, assuming that low Reynolds number conditions are fulfilled. Near the crack tip, the velocity magnitude approaches zero, whereas towards the crack mouth, the along-crack-length velocity component increases linearly and exhibits a parabolic distribution across the crack width. Furthermore, the model reveals that fatigue-induced crack face motion leads to periodic exchange of seawater within the crack, offering a mechanistic explanation for experimentally observed correlations between loading conditions and corrosion rates. The model describes how loading drives fluid flow within the crack, thereby refreshing the corrosive medium and governing its velocity under corrosion fatigue conditions.

Experimental study on Venturi pressure loss under particle-laden air flow with varying dust types and loadings at low Reynolds numbers

Under overland flow with low Reynolds numbers, the resistance parameters generated by the soil and vegetation are crucial under current climate change conditions. The inter-rill erosion occurring under this overland flow can lead to inter-rill erodibility as a resistance parameter, and each type of plant generates different vegetal drag coefficients and hydraulic resistance parameters. A set of sixteen simulated rainfall events capable of generating overland flow on an Fluvisol tilled with a semiarid agroforest, cactus, under a litter layer, and under bare conditions was applied. The overland flows generated were laminar tranquil flows with very low Reynolds numbers varying from 6 to 25 on bare soil. • Environmentally sustainable indicators from soil and semi-arid plants. • Hydraulic resistance and soil erodibility linked to overland flow at low Reynolds number. • Semi-arid crop systems and shrub generating environmental service.

With the rapid development of micro air vehicles (MAVs) in low-altitude application scenarios, biplane airfoils are introduced to conceptual designs for novel multi-wing configuration of aircraft. However, the aerodynamics of biplane airfoils is inevitably subject to strong wake interference between the upper and lower wings in the unsteady flow with high angle of attack (AOA) and low Reynolds number. It is urgent to find a new method for flow separation control of biplane airfoils. In this paper, the effects of the feather-inspired flexible flap on unsteady aerodynamics of biplane airfoils at high angle of attack is studied employing lattice Boltzmann–finite element method. The coupling mechanism of flow separation and vortex interaction between biplane airfoils at different spacing is systematically investigated, and the difference of unsteady flow control between rigid flap and feather-inspired flexible flap is compared. The results shown that the feather-inspired flexible flap can effectively optimize the lift distribution of biplane airfoils, reduce the lift discrepancy between the upper and lower wings, mitigate abrupt lift transients, reduce the total drag of the upper wing, and improve flight stability without significantly sacrificing aerodynamic efficiency. Moreover, the stiffness of the flexible flap has a notable impact on aerodynamic response. The fluid–structure interaction effect of the flexible flap in unsteady flow separation plays an extremely important role in improving the autonomy and adjustability of flow control, which improves the flow control strategy of the aerodynamics of biplane airfoils and provides support for the efficient development of MAVs in complex flows.

An adaptive isogeometric boundary integral method using analysis-suitable T-splines for fluid-shell interactions at low Reynolds numbers