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In this study, a conservative phase-field lattice Boltzmann model is examined for simulating immiscible two-phase flows with large density and viscosity ratios. This model is built upon the Allen-Cahn equation, incorporating a filtered collision operator and high-order corrections in the equilibrium distribution functions. The numerical model is also integrated with the volumetric boundary scheme and the immersed boundary method to achieve various stationary and moving boundary conditions on arbitrary geometries for different application scenarios. A comprehensive evaluation of this phase-field solver is conducted through a range of academic and industrial cases. First, droplet splashing cases at different Reynolds numbers are studied. The phase-field LB model effectively captures the surface wave motion and splashing of the droplet, although some smearing of small structures is observed due to the diffused interface property of the numerical model. Second, a tuned liquid damper is simulated, accurately capturing sloshing forces on solid walls. In the simulation of a dam-breaking case, predicted gauge pressure and free-surface elevation time histories compare well with experimental data. Lastly, a gearbox case under dip lubrication is simulated, with both the gearbox churning loss and oil distribution being well predicted across a range of rotating speeds. The validation studies demonstrate the effectiveness of the present phase-field lattice Boltzmann model in simulating immiscible two-phase flows with large and realistic density and viscosity ratios. The strengths, limitations, and weaknesses of the conservative phase-field LB model are discussed, and suggestions for the development and application of this numerical model are provided. [ABSTRACT FROM AUTHOR]

A viscous flux formulation at interior faces and at Dirichlet boundary conditions in the framework of unstructured vertex-centered k-exact finite-volume schemes is presented. It is based on a geometric decomposition of the viscous flux into orthogonal and nonorthogonal parts. Special emphasis is put on the truncation errors and the spectral properties of the scheme, and a connection is established to the well-known α-damping scheme. The latter is also used to introduce discretization coefficients into the method that allow a fourth and a sixth order of accuracy in space on Cartesian grids. The effect of the scheme is presented in terms of canonical diffusion problems, as well as test cases for laminar wall-bounded flows. It is shown that the method preserves a second-order accuracy for the viscous operator, even on highly distorted unstructured grids and in the presence of boundaries and that it strongly enhances the solution accuracy with respect to a conventional scheme for viscous fluxes. [ABSTRACT FROM AUTHOR]

This research presents an experimental assessment of exergy and entropy generation in a straight circular multi-microchannel stainless steel heatsink. Deionized water and water/alumina nanofluids with 1–4% (m/m) concentrations are used as cooling fluids in the heat sink, operating at low Reynolds numbers (10 ≤ Re ≤ 50). The primary goals of the exergy analysis are to evaluate the first and second laws of thermodynamics, including exergy output, gain, and loss. Entropy generation analysis encompasses both heat transfer and flow entropy. The study's main innovation is the examination of exergy and entropy generation in microchannel heat sinks using nanofluids at low Reynolds numbers. The results show that nanofluids with a 4% nanoparticle concentration achieve a higher exergy gain of 56% compared to DI water at Re = 40. Exergy loss increases with Reynolds numbers and with increase in the nanoparticles concentration up to 4%, the exergy loss increases up to 27% at Re = 10. Back conduction, significant at low Reynolds numbers, does not affect the second law efficiency. The highest second law efficiency occurs at Re = 10, with DI water achieving 7.2% under high heat flux, while nanofluids with 4% nanoparticle concentration show 5.9%. This efficiency decreases with Reynolds number and nanoparticle concentrations. However, introducing alumina nanoparticles into DI water reduces entropy generation; at Re = 50, the total entropy generation is 0.0013 W K−1 for DI water and 0.001 W K−1 for nanofluids with a 4% nanoparticle concentration. Nanofluids reduce entropy generation up to 27% at a 4% concentration of nanoparticles compared to DI water at Re = 40. These findings offer valuable insights for optimizing the design and performance of microchannel heat sink configurations for various thermal management applications, focusing on exergy and entropy generation. [ABSTRACT FROM AUTHOR]

Experimental investigations are carried out to explore the aerodynamic performance and vortex shedding characteristics of S5010 and E214 airfoil-based wings to provide guidance for the design of MAVs and other low-speed vehicles. Force and wake shedding frequency measurements are carried out in a subsonic wind tunnel in the Reynolds number (Re) range of 4 × 104 - 1 × 105. The measurements with increasing Re show that the slope of the lift curve in the linear region increases by 14% for S5010, while this increment is 11% for E214. The peak lift coefficient of both airfoils reduces with reducing Re. For lower pitch angles, the influence of Re on drag coefficients is less significant, but at higher angles, the drag increases as the Re drops. Unlike pre-stall mountings, the pitch-down propensity of the airfoil enhances in the post-stall region for high Re flows. Moreover, the frequency of shed vortices reduces with rising angle of attack at a given Re. In contrast, the Strouhal number almost remains constant with varying Re at a fixed angle of attack. For S5010 and E214 airfoils, the Strouhal number is noticed to vary between 0.68 - 0.36 and 0.58 - 0.36, respectively, for pitch angle variation of 12°- 28°. The airfoils show a higher Strouhal number than the bluff body wakes, but this difference decreases for high angles of attack mountings. This finding reveals that the wake structure of the airfoil at a high post-stall angle behaves as bluff body wakes. [ABSTRACT FROM AUTHOR]

The objective of this study was to measure the aerodynamic characteristics of the NACA 0018 airfoil across a range of low Reynolds numbers, with a particular focus on the unique behaviours observed in this regime. The experiments were conducted primarily in a low-turbulence wind tunnel using wall pressure taps, a pressure rake, and a force gauge. The Reynolds numbers investigated ranged from 30.000 to 160.000, with angles of attack spanning from -190 to +20 degrees. A major achievement of this research was the successful acquisition of data at Re = 30.000, where the lift behaviour significantly deviated from that observed at higher Reynolds numbers. The results demonstrated a strong dependency of aerodynamic characteristics on Reynolds number, with the drag coefficient at zero angle of attack decreasing by over 700 percent when Re increased from 30.000 to 160.000. Additionally, two independent techniques for measuring lift force were employed, both yielding consistent results despite the low Reynolds number. The experimental results were also compared with XFOIL and 2-D CFD simulations, which, although not perfectly accurate, provided reasonably good predictions. [ABSTRACT FROM AUTHOR]

Experimental investigations are carried out to explore the aerodynamic performance and vortex shedding characteristics of S5010 and E214 airfoil-based wings to provide guidance for the design of MAVs and other low-speed vehicles. Force and wake shedding frequency measurements are carried out in a subsonic wind tunnel in the Reynolds number (Re) range of 4 × 104 - 1 × 105. The measurements with increasing Re show that the slope of the lift curve in the linear region increases by 14% for S5010, while this increment is 11% for E214. The peak lift coefficient of both airfoils reduces with reducing Re. For lower pitch angles, the influence of Re on drag coefficients is less significant, but at higher angles, the drag increases as the Re drops. Unlike pre-stall mountings, the pitch-down propensity of the airfoil enhances in the post-stall region for high Re flows. Moreover, the frequency of shed vortices reduces with rising angle of attack at a given Re. In contrast, the Strouhal number almost remains constant with varying Re at a fixed angle of attack. For S5010 and E214 airfoils, the Strouhal number is noticed to vary between 0.68 - 0.36 and 0.58 - 0.36, respectively, for pitch angle variation of 12°- 28°. The airfoils show a higher Strouhal number than the bluff body wakes, but this difference decreases for high angles of attack mountings. This finding reveals that the wake structure of the airfoil at a high post-stall angle behaves as bluff body wakes.

The open lattice gas-cooled reactor presents a lightweight option for high-power space reactor power systems. The background of this study is a space reactor featuring a rod bundle core structure, with helium as the coolant. The typical Reynolds number at the core inlet is around 2 000. Reynolds average numerical simulation (RANS) is a commonly used computational fluid dynamics (CFD) method. The essence of the RANS method lies in turbulence models. Each turbulence model has its particular useful scenarios and needs to be chosen based on the specific working conditions. The helium-cooled rod bundle reactor is distinguished by its tight lattice structure and low flow Reynolds number. These features influence the flow and heat transfer characteristics in the reactor core. Consequently, when performing thermal-hydraulic analysis using CFD, it is essential to evaluate the applicability of turbulence models. Experiments of flow and heat transfer in 37-rod bundle structure were conducted, using electrically heated rods of the same size as the fuel rods and nitrogen as the experimental coolant. Based on these experiments, the convective heat transfer within the test section was numerically simulated using ANSYS Fluent, selecting four turbulence models: Realizable k-ε with enhanced wall treatment, SST k-ω, transition SST, and Reynolds stress model with enhanced wall treatment. The operating conditions for numerical calculations had inlet Reynolds numbers ranging from 688 to 2 986, all with uniform power distribution. By comparing the experimental measurements and calculated values of the heating rod cladding temperatures, the applicability of the four turbulence models was evaluated. Simultaneously, the differences in local flow field simulations by these models were observed, and an analysis was performed to understand the reasons behind the discrepancies in cladding temperature calculations among the different models. The results show that all four turbulence models generally underpredict the rod cladding temperatures. Among these models, the transition SST model exhibits the closest agreement with experimental data, with an overall average deviation of −2.0%. It effectively captures the crossflow characteristics between the rod bundle and is suitable for thermal-hydraulic simulations of open lattice gas-cooled reactor with Reynolds number around 2 000. This study confirms that the crossflow is an important factor affecting the flow and heat transfer in open lattice structures. Subsequent researches are needed to further investigate the factors and patterns influencing crossflow, in order to minimize its adverse effects on the heat transfer in the reactor core. The findings of this paper provide a reference for the numerical simulation and design of rod bundle gas-cooled reactors.

Birds have traits that can induce better aerodynamic efficiency along with high manoeuvring capability during its flight, which could be shared with unmanned aerial vehicles for improving their aerodynamic performances. One such feature of the wing tip, i.e. the primary feathers of the birds could be an effective geometrical feature to reduce the wing tip vortices. This paper presents the bio-inspired wing tip devices, i.e. three-and four-tipped multiple winglets in reducing the strength of vortices emanating from the wing tip of the wing operating in the Reynolds number (Re) of 0. 9 7 9 4 × 1 0 5 and 0. 9 7 9 4 × 1 0 6 . Different combinations of both three- and four-tipped multiple winglets have been designed by varying the cant angle of each tip. Numerical simulations were carried out using Ansys-Fluent by solving three-dimensional Reynolds averaged Navier–Stokes formulations coupled with k- ϵ turbulence model to resolve the features of tip vortices. The simulation clearly indicates that there is a strong correlation between the size of the vortices and the aerodynamic performance parameters such as C L / C D , (C L) max ,   C L 0. 5 / C D , C L 1. 5 / C D . The three- and four-tipped multiple winglets are effective in reducing vortex drag by disintegrating large strength vortex which occurs in the tip of straight wing, into few numbers of small strength vortices. When compared to straight wing, three-tipped multiple winglet with the cant angle combination of 50, 30, 10 improves the aerodynamic efficiency by 22% to 23% and the four-tipped winglet with the cant angle combination of 60, 50, 40, 30 enhances the same by 21% to 22% in the Re of 0. 9 7 9 4 × 1 0 5 . Even the longitudinal static stability has seen considerable improvement for four-tipped multiple winglets than three-tipped multiple winglets and straight wing. [ABSTRACT FROM AUTHOR]

The cochlea, situated within the inner ear, is a spiral-shaped, liquid-filled organ responsible for hearing. The physiological significance of its shape remains uncertain. Previous research has scarcely addressed the occurrence of transverse flow within the cochlea, particularly in relation to its unique shape. This study aims to investigate the impact of the geometric features of the cochlea on fluid dynamics by characterizing transverse flow induced by harmonically oscillating axial flow in square ducts with curvature and torsion resembling human cochlear anatomy. We examined four geometries to investigate curvature and torsion effects on axial and transverse flow components. Twelve frequencies from 0.125 Hz to 256 Hz were studied, covering infrasound and low-frequency hearing, with mean inlet velocity amplitudes representing levels expected for normal conversation or louder situations. Our simulations show that torsion contributes significantly to transverse flow in unsteady conditions, and that its contribution increases with increasing oscillation frequency. Curvature alone has a small effect on transverse flow strength, which decreases rapidly with increasing frequency. Strikingly, the combined effect of curvature and torsion on transverse flow is greater than expected from a simple superposition of the two effects, especially when the relative contribution of curvature alone becomes negligible. These findings may be relevant to understanding physiological processes in the cochlea, including metabolite transport and wall shear stress. Further studies are needed to investigate possible implications for cochlear mechanics. [ABSTRACT FROM AUTHOR]

Dielectric barrier discharge plasma actuators, capable of creating a quasi-steady wall jet, are well suited for flow control over the microair vehicles at low Reynolds number. The plasma actuator is usually excited by a sinusoidal high-voltage power with a frequency of a few to tens of kilohertz. However, the investigations on the flowfield produced by a plasma actuator with a frequency below 200 Hz remain limited. Motivated by this demand, the formation and characterization of the vortices generated by a plasma actuator with a high-voltage frequency of 125 Hz are studied in detail. In addition to the starting vortex, it is of great importance that a train of vortices that shed periodically from the junction between the two electrodes and are quite different from the starting vortex are first observed. The shedding frequency of the periodic vortices is the same as the high-voltage frequency of 125 Hz. Combining the acoustic with the flow characteristics of the plasma actuator, the formation mechanism of these periodic vortices is discussed. Finally, a criterion for generating the periodic vortices is proposed based on the relationship between the scale of vortices and the separation distance between the two neighboring periodic vortices. [ABSTRACT FROM AUTHOR]

The lifting behavior of turbulent nonpremixed coaxial jet flames was investigated experimentally as a function of the center jet Reynolds number, annular-to-center jet velocity ratio, acoustic frequency, and acoustic amplitude, all with and without transverse acoustic disturbances acting on the flames situated at a pressure antinode. Global lifting regimes were mapped and consisted of attached flames, periodically lifted flames, and permanently lifted flames. With one exception, all flames were attached in the absence of acoustics and lifted only because of the applied acoustic excitation. The exception occurred at the highest Reynolds number and velocity ratio, where the flames were unconditionally lifted in all cases. Between these two extremes, a range of lifting regimes was observed. Lower applied frequencies and smaller amplitudes were found to generally promote attachment to the burner. Flow visualizations using high-speed Schlieren and OH* chemiluminescence imaging revealed a lateral spreading effect near the jet exit. The lateral spreading was hypothesized to be caused by an axially oriented counterflow collision between the downstream flowing jets and presumed local upstream portion of the acoustic velocity fluctuations. Physical mechanisms were hypothesized based on this observation, which appear to consistently explain most of the observed behavior. [ABSTRACT FROM AUTHOR]

A numerical investigation was conducted in this study utilizing Force Element Analysis to explore the vortex-induced vibration (VIV) mechanism of side-by-side dual cylinders under the conditions of Reynolds number Re = 100, mass ratio m* = 10, and spacing ratios L/D ranging from 3 to 6. The hydrodynamic forces by force element formulas were incorporated into the vibration response calculations of elastically supported rigid cylinders using a User-Defined Function (UDF) and the fourth-order Runge–Kutta method. A comprehensive analysis was performed to elucidate the combined effects of the spacing ratio L/D and reduced velocity Ur on the vibration responses, quantifying the hydrodynamic forces involved in the mutual interaction during VIV for side-by-side dual cylinders. The influence mechanisms of inter-cylinder interaction and their effects on the resultant hydrodynamic phenomena were discussed. It was revealed that for side-by-side arranged dual cylinders outside the "lock-in region", the lift and drag forces are predominantly supplied by the volume vorticity forces in conjunction with surface vortices (including frictional) forces. However, within the "lock-in region", the surface acceleration lift forces provide greater force contributions, and the volume vorticity lift force contributes significantly to negative values. Notably, alterations to the spacing ratio do not change the proportion of force element components. The amplitudes of the cylinders' mutual interaction forces are identical in magnitude but opposite in phase. Additionally, the "slapping" phenomenon near the "lock-in region" leads to "bounded" trajectories of cylinders. [ABSTRACT FROM AUTHOR]

The effect of spanwise wing non-planarity, employed in conjunction with a Gurney flap, is presented. Testing was undertaken in a low-speed wind tunnel using a rectangular wing with an aspect ratio of three. The outer one-third of the wing was non-planar, which took the form of either dihedral or a circular arc. A 2% high Gurney flap was implemented such that it could extend over the entire span or the planar inboard section. The loads were measured using a sting balance. The data show that non-planarity increases the maximum lift coefficient and the wing's lift curve slope. Gurney flap lift modulation was enhanced in the presence of non-planarity. The addition of Gurney flaps caused a greater increment in the minimum drag coefficient for the non-planar wings. The Gurney flaps reduced the lift-dependent drag of the wings. As a whole, the Gurney flaps reduced the maximum lift-to-drag ratio (L/D)max for the non-planar wings; however, the flat wing exhibited a small L/D increment with flap addition. [ABSTRACT FROM AUTHOR]

In order to eliminate the nonlinear aerodynamic behaviour of a symmetric airfoil within the low angles of attack at low Reynolds number, this paper establishes a low Reynolds number airfoil optimization design procedure based on Gaussian process regression model and NSGA-II multi-objective genetic algorithm coupled with RANS numerical method. Then this paper carries out the airfoil optimization design with the average cruise factor in the range of small angle of attack of multiple states as the objective function. The results show that the optimized airfoil exhibits pronounced geometric bump and effectively alters the position and type of the laminar separation bubble. The aerodynamic characteristics of the airfoil are significantly improved, and the nonlinear phenomenon of the lift coefficient within the small angle of attack range at Re=4×104 basically disappears. Further analysis reveals that the bump of the optimized airfoil enhances the convective influence of the wall, which enhances the resistance of the incoming flow to the upward pressure gradient, thus improving the airfoil aerodynamic characteristics. [ABSTRACT FROM AUTHOR]

Copyright of Atomic Energy Science & Technology is the property of Editorial Board of Atomic Energy Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Chongqing University of Technology (Natural Science) is the property of Chongqing University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To achieve high-performance compressor cascades at low Reynolds number (Re), it is important to organize the boundary layer transition and separation processes efficiently and reasonably. In this study, the airfoil is focused on at a 5% blade height at the root of the orthogonal blade in the downflow passage of the high-load booster stage. The bionics modeling design is carried out for the leading edge of the original blade cascade; the response characteristics of laminar transition and separation to blades with different leading edge shapes at low Reynolds numbers are studied by using large eddy simulations combined with Omega vortex identification. The findings of this study demonstrate that bionic leading edge modeling can significantly improve the aerodynamic performance of blades at low Reynolds numbers. The blades effectively suppress the formation of separation bubbles at low Reynolds numbers and weaken or even eliminate large-scale flow separation at the trailing edge. In addition, the blades can weaken the vortex intensity on the blade surface, reduce the areas of high-velocity fluctuations, and minimize aerodynamic losses caused by turbulence dissipation. These results should serve as a valuable reference for the aerodynamic design and flow control of the high-load booster stage blade at low Re. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Magnetically driven microswimmers have recently received substantial recognition because of their ability to navigate in complex narrow environments and perform micromanipulation and cargo transport. They are envisioned as potential candidates for minimally invasive targeted drug delivery, microsurgery, and in-vivo sensing. Researchers are investigating and developing different variants of magnetic actuation systems based on the types of applications intended, but still, many challenges and issues need to be addressed. In this paper, starting from the very underrated issue which is the total cost of the whole actuation setup, accessibility of the workspace, and modularity of the design are addressed. Beginning with the geometrical designs, 3D printing of the parts, and assembly of the electronic control unit, we proposed a low-cost modular magnetic actuation system. The proposed actuation unit has a flexible (expandable) workspace and a provision to upgrade to different forms of electromagnetic units (paired coils systems, saddle coils, distributed electromagnets, and hybrid design) and mode of actuation. The economical cost of the unit lies in the fabrication of the modular parts by using a low-cost 3D printer (Creality Ender-3 S1), and readily available electronic components. After developing the actuation unit, by using the Hanging drop method and utilizing basic laboratory apparatus (Petri dish and syringes) a 0.98 mm diameter microsphere is fabricated to test the maneuvering capability using a joystick unit on a predesigned narrow channel fluidic environment in low Reynolds number condition ( R e ≈ 10 - 2 ) . In addition, using simulation we demonstrate the control of identical microspheres on a planar workspace. The OBJ files of all the modular parts of the unit and details of the electronic components used are also made available. [ABSTRACT FROM AUTHOR]

Among the critical factors contributing to the decline in the aerodynamic performance of near-space aircraft under low Reynolds number conditions, a significant one lies in the occurrence of laminar separation bubbles forming on the wings. Within the scope of this investigation, the primary research methodology adopted involves utilizing an unsteady numerical simulation technique rooted in a spring-smoothed dynamic grid system. This study meticulously examines the aerodynamic attributes and flow patterns exhibited by an airfoil undergoing partial oscillation, thereby elucidating the underlying mechanisms through which such oscillations lead to enhanced lift and diminished drag forces. The outcomes of this research reveal that the imposition of partial oscillation engenders a noteworthy augmentation of 4.9% in the lift coefficient of the airfoil, concurrent with a substantial diminution of 15.3% in its drag coefficient when juxtaposed against the non-deforming counterpart. The oscillation frequency exerts a profound influence on both the onset location of transition and the extent of the laminar separation bubble's development. As the oscillation frequency escalates, it follows an initial ascending trend in the lift coefficient of the airfoil, followed by a subsequent decline, whereas the drag coefficient exhibits an initial decrement prior to a rising tendency, thus indicating the existence of an optimal frequency point where the airfoil achieves its most favorable aerodynamic characteristics. It is observed that the flow control effects are optimally pronounced when the region subjected to partial oscillation is proximate to the airfoil's leading edge or situated precisely at the centroid of the laminar separation bubble. [ABSTRACT FROM AUTHOR]

Microballoon actuators as a potential active flow control device have been studied for years. However, most studies have relied on experimental methods to investigate its effects. In this paper, we utilized the numerical method of steady-state RANS to explore the feasibility of applying microballoon actuators to suppress flow separation on a wing section and a high-altitude propeller. The geometric design, including shapes and positions for microballoons, is introduced, and these microballoons are fully resolved for the numerical models to better assess the influence of sensitive parameters. The turbulent model used in simulations is well validated in comparison with experimental data. In the wing section model, computational results show that at Re=2×105, placing nonrotation microballoons close to the separation point can suppress separation bubbles and decrease drag by 12% before the stall angle of attack. In the propeller model, computational results show that placing a microballoon actuator array with a proper dimension and position on the blade can also effectively suppress the crossflow separation appearing at the trailing edge. At a rotational speed of 450 rpm, the efficiency enhancement can reach a maximum of 1.6%. [ABSTRACT FROM AUTHOR]

Using computational models and low-speed wind tunnel tests, the aerodynamic characteristics of the NACA 0012 airfoil with low Re numbers of (8 × 104{10}^{4}, 2 × 105{10}^{5}, 3 × 105{10}^{5}, and 4 × 105){10}^{5}) and angle of attack (AOA) ranging from 0° to 18° by two steps are examined. Using the same 3-D wind tunnel dimensions, numerical simulations were run. The software program ANSYS FLUENT was used to solve the mathematical model using the continuity equation, the Navier‒Stokes equations, and the k–ω shear‒stress transport turbulence model. Findings demonstrate that at all AOAs, there is a direct relationship between Reynolds numbers (Re), lift and drag coefficients, kinetic energy, and stall angle. The lift coefficient rises linearly as the AOA increases, peaking at 14°, the stall angle at higher Reynolds number. The lift coefficient was found to decline when the AOA was increased further, reaching its minimal value at an AOA of 18°. With a greater AOA, the airfoil’s drag coefficient rises, creating turbulent flow. The eddies produced by the turbulence cause the flow to start separating from the airfoil surface as turbulence increases. As a result, the airfoil lift coefficient drops, and its drag coefficient rises at the same time, leading to poor performance. The validation of the numerical results through wind tunnel experiments provided confidence in the findings of the study.

The aerodynamic performance of wings degrades severely at low Reynolds number; lift often becomes non-linear, while drag increases significantly, caused by large extents of separation. Consequently, a non-conventional wing design approach is implemented to assess its ability to enhance performance. The design methodology is that of wing segmentation, where the wing is divided into spanwise panels that can be separated, thereby yielding small gaps between the panels. A moderate aspect ratio wing comprised of four separate wing panels was manufactured and wind tunnel tested through a Re range from 40,000 to 80,000. Force balance data and surface flow visualization were used to characterize performance. The results indicate that segmentation is effective in significantly augmenting efficiency at Reynolds numbers at which the fused wing (i.e., no gaps) shows large extents of open separation. Drag is greatly reduced, while lift is increased, and stall is delayed. The benefit of segmentation was noted to diminish at higher Re where the fused wing's performance improves markedly. Wing segmentation could find application in micro-unmanned-aerial-vehicle and drone design. Further study would entail the effects of AR and the number of spanwise panels on performance. [ABSTRACT FROM AUTHOR]

The tiltrotor design is favored for urban air mobility (UAM) prototypes due to the combination of vertical takeoff and landing (VTOL) capability and efficient forward flight. With rising UAM air traffic at low altitudes, noise from these aircraft is a crucial design factor. Most tiltrotor noise research focuses on high disk loading and Reynolds number setups, leaving smaller aircraft configurations less explored. This study investigates aero-acoustic trends from rotor-wing interaction at low disk loading (< 100 N/m2) and Reynolds number (Re < 100,000). While prior literature suggests lowering disk loading and reducing rotor wake interference can mitigate rotor noise, such ideas lack empirical validation. The setup involves an anechoic chamber housing a two-blade rotor, along with flat and NACA 0012 airfoil wings. Microphones and a rotation stage capture acoustic data for analysis. Factors like flow recirculation, isolated rotor noise, rotor height, rotation direction/rate, and wing curvature are assessed for impact on noise signature. It is found that the deflected rotor wake in rotor-wing interaction significantly increases low-frequency broadband noise and overall sound pressure level (OASPL), compared to an isolated rotor. Dominant tonal noise diminishes based on the strength of the deflected rotor wake. These findings offer insights into reducing noise from rotor wake impingement on the wing. [ABSTRACT FROM AUTHOR]

The aim of this research work was to explore how modifying the design of small-scale HAWT rotor blades through the backward-facing step technique affects their efficiency under varying wind speeds. The study involved altering step parameters such as location, length, and depth to create four distinct stepped blade shapes and enhance the aerodynamic performance of a rotor with a diameter of 280 mm. A specific blade profile, NREL S822, was selected to meet both aerodynamic and structural criteria. The rotor models were examined at a Reynolds number of 4.7 × 104 for wind speeds between 8.5 and 15.5 m/s and tip-speed ratios between 2 and 5. The experimental results indicated that for certain geometric step parameter values, the efficiency of the rotor model (B3) increased by approximately 47% compared to the base model (B1), particularly for tip-speed ratios lower than around 3.2. However, beyond this point, the rotor efficiency dropped significantly, reaching approximately 60% in one case. Additionally, a hybrid rotor model (B6) was generated by combining the shape of the rotor model (B4) with the most efficient rotor model from the literature, generated using the leading-edge wavy shape technique. This hybrid rotor model enhanced rotor efficiency for specific values of tip-speed ratio and also ensured its smoother operation. Overall, the rotor model (B2), distinguished by smaller step parameter values and a shift as well as broadening of the power coefficient curve towards lower tip-speed ratio values, exhibited a higher peak power coefficient, approximately 1.4% greater than the base rotor (B1). This increase occurred at a lower tip-speed ratio, allowing the rotor to operate with higher efficiency across a broader range of tip-speed ratios. [ABSTRACT FROM AUTHOR]

The study of microswimmers in viscoelastic fluids is important due to their essential role in many biological systems and various industrial and medical applications. This review article summarizes the behavior of microswimmers in complex viscoelastic fluids. Interestingly, both an increase or decrease in the speed and the power efficiency of microswimmers may be observed depending on the swimming pattern and the rheology of the polymeric fluid. First, it introduces the continuum frameworks for a complex fluid in which the microswimmers are present. This study briefly discusses how the reciprocal motion can propel a swimmer in a viscoelastic fluid contrary to the scallop theorem. This is followed by a section on different types of individual swimmers and their theoretical framework. A finite‐sized swimming sheet can show the resonance effect when relaxation time matches the characteristic time of the swimming. Lastly, progress on the topic of squirmers in the complex environment is presented, and it is seen that the interaction of the squirmer with the complex fluid is unpredictable. [ABSTRACT FROM AUTHOR]

The main objective of the present analysis is to characterize the transient buoyancy-motivated free convection turbulent flow and heat transfer characteristic features of an incompressible viscous fluid past a vertical cylinder with low-Reynolds-number (LRN) k–ε turbulence model in a two-dimensional coordinate system numerically. The Reynolds averaged Navier–Stokes equations (RANS) such as continuity, momentum, and energy are considered in terms of cylindrical coordinate system. The extra stress tensors obtained from the RANS model are closed using the eddy diffusive model. The local value of turbulent kinematic viscosity ( ν t ) is determined by utilizing the kinetic energy (k) and dissipation rate (ϵ) equations. The resulting system of partial differential equations (PDEs) with high nonlinearity, governing the turbulent boundary layer flow are solved using the implicit Crank–Nicolson technique. The discretized set of dimensionless tridiagonal algebraic equations are simplified by utilizing Thomas algorithm. Also, the simulated results are expressed in terms of graphs to analyse the average velocity, temperature, kinetic energy, dissipation rate, and also average momentum and heat transfer rates for the varying values of turbulent Prandtl ( Pr t ), Grashof (G r t) and Reynolds ( Re t ) numbers. It is noted that the average velocity, kinetic energy, dissipation rate of kinetic energy fields suppressed, and temperature field enhanced with increasing Re t . Also, the rising turbulent Prandtl parameter decreased the average velocity, temperature, turbulent kinetic energy, and dissipation rate profiles. Further, the increasing turbulent Grashof number decreased the kinetic energy and dissipation rate profiles. Further, the obtained results from the present turbulent investigation are compared with the existing results and observed an excellent agreement. [ABSTRACT FROM AUTHOR]

In the present study, numerical simulation for a two-dimensional NACA0005 is performed for angles of attack ${\alpha } = {0^{\circ }-15^{\circ }}$ . The Reynolds numbers 1000, 2000 and 5000 are considered. Simulations show that the flow fields can be classified due to characteristics of vortex shedding based on Reynolds number and angle of attack. Furthermore, this study reveals that in the case of thinner airfoils, several modes (Mode I, Mode II, Mode III) are present at the same angle of attack for varying Reynolds number. Maximum lift to drag ratio is found at Reynolds number 5000 and angle of attack 6° as well as gradual stall is observed. Onset of oscillations to determine critical angles of attack is reported for NACA0005 with the dominant frequencies. A subcategory of mode III termed as mode IIIa/c, and recently reported mode 4i is also observed in present study. A new mode termed as k-mode is discovered for higher angles of attack at Re = 5000. In addition, numerical simulations of two-dimensional periodic flows around NACA0005 profile at Reynolds number Re = 5000 demonstrate that unsteady periodic flows reach different saturation states at angles of attack of $10^{\circ }-15^{\circ }$ . The coexisting periodic states and period-doubling (with higher and lower intensities) in this range indicate that the wake undergoes substantial changes from the Von-Kármán vortex street.

The effect of spanwise wing non-planarity, employed in conjunction with a Gurney flap, is presented. Testing was undertaken in a low-speed wind tunnel using a rectangular wing with an aspect ratio of three. The outer one-third of the wing was non-planar, which took the form of either dihedral or a circular arc. A 2% high Gurney flap was implemented such that it could extend over the entire span or the planar inboard section. The loads were measured using a sting balance. The data show that non-planarity increases the maximum lift coefficient and the wing’s lift curve slope. Gurney flap lift modulation was enhanced in the presence of non-planarity. The addition of Gurney flaps caused a greater increment in the minimum drag coefficient for the non-planar wings. The Gurney flaps reduced the lift-dependent drag of the wings. As a whole, the Gurney flaps reduced the maximum lift-to-drag ratio (L/D)max for the non-planar wings; however, the flat wing exhibited a small L/D increment with flap addition.

A numerical investigation was conducted in this study utilizing Force Element Analysis to explore the vortex-induced vibration (VIV) mechanism of side-by-side dual cylinders under the conditions of Reynolds number Re = 100, mass ratio m* = 10, and spacing ratios L/D ranging from 3 to 6. The hydrodynamic forces by force element formulas were incorporated into the vibration response calculations of elastically supported rigid cylinders using a User-Defined Function (UDF) and the fourth-order Runge–Kutta method. A comprehensive analysis was performed to elucidate the combined effects of the spacing ratio L/D and reduced velocity Ur on the vibration responses, quantifying the hydrodynamic forces involved in the mutual interaction during VIV for side-by-side dual cylinders. The influence mechanisms of inter-cylinder interaction and their effects on the resultant hydrodynamic phenomena were discussed. It was revealed that for side-by-side arranged dual cylinders outside the “lock-in region”, the lift and drag forces are predominantly supplied by the volume vorticity forces in conjunction with surface vortices (including frictional) forces. However, within the “lock-in region”, the surface acceleration lift forces provide greater force contributions, and the volume vorticity lift force contributes significantly to negative values. Notably, alterations to the spacing ratio do not change the proportion of force element components. The amplitudes of the cylinders’ mutual interaction forces are identical in magnitude but opposite in phase. Additionally, the “slapping” phenomenon near the “lock-in region” leads to “bounded” trajectories of cylinders.

Biohybrid actuators aim to leverage the various advantages of biological cells over artificial components to build novel compliant machines with high performance and autonomy. Significant advances have been made in bio-fabrication technologies, enabling the realization of muscle-powered bio-actuators. However, the mechanics of muscle-scaffold coupling has been relatively understudied, limiting the development of bio-actuators to intuitive or biomimetic designs. Here, we consider the case of implementing muscle-based actuation for soft robotic swimmers operating at low Reynolds numbers. We develop an analytical model to describe the elasto-hydrodynamic problem and identify key design parameters. Muscle contraction dynamics is characterized experimentally and the implications of nonlinear amplitude-frequency relationship of muscle-based actuation are discussed. We show that a novel bio-actuator with high performance can be developed by introducing compliant flexural mechanisms undergoing large deflection. Geometric nonlinearities are accounted for in the analysis of the force-deflection relationship for the flexural mechanism. Our results show that for expected muscle contraction forces, this novel bio-actuator can outperform previous muscle-powered swimmers by up to two orders of magnitude in swimming speed. [ABSTRACT FROM AUTHOR]

To realize a propeller-driven Mars airplane, the propeller-wing flow interaction characteristics should be clarified. Thus, we conducted numerical simulations of the propeller-wing interaction at low Reynolds number conditions (Re = 3.0 © 104) corresponding to Mars atmosphere, focusing on the propeller slipstream effects on the fixed wing. Solutions for the tractor case (propeller in front of the fixed wing) and the wing only case were compared. The results showed that in the tractor case, the lift coefficient (CL) of the fixed wing increased with the angle of attack (¡) more linearly than for the wing only case and showed delayed stall because the propeller slipstream favorably suppressed the expansion of the recirculation region over the fixed wing in the time-averaged flow fields, supporting the experimental result. The flow over the fixed wing separated from near the leading-edge in both low and high thrust conditions even at a moderate angle of attack (¡ = 5°), whereas previous studies at relatively high Reynolds numbers (Re = O(105)) showed that almost attached flow fields were formed under the propeller slipstream influence. Additionally, it was observed that the CL fluctuation of the fixed wing increased even when the time-averaged CL was lower than that of the wing only case. [ABSTRACT FROM AUTHOR]

A 2-D airfoil shape optimization in transonic low-Reynolds number regime is conducted. A Navier–Stokes flow solver with a transition model (k-ω SST γ-Reθ) is used to evaluate the fitness function. Single-point and multi-point formulations of the optimization results are compared. In addition, the effects of Mach number and angles of attack on aerodynamic characteristics of the optimized airfoils are investigated under low Reynolds number (Re = 17,000) and high-subsonic-flow ( M a , ∞ = 0.6 − 0.9) conditions. The results show that the corresponding drag divergence Mach number curves of the conventional airfoil present almost a parallel shifting at the entire Mach number range. By contrast, the unconventional airfoil starts showing a significant drag reduction when Mach number is greater than 0.75. Besides, the maximum lift-to-drag ratio is highly influenced by the Mach number because of the formation, movement, type, and strength of a shock wave. In addition, the distinguishing difference in the conclusion between two airfoils is that the lift fluctuation of the conventional airfoil amplifies with the increase of the Mach number. However, the unconventional airfoil shows an opposite trend. [ABSTRACT FROM AUTHOR]

The flight of a migratory bird-like flapping-wing aircraft is characterized by a low Reynolds number and unsteadiness. The selection of airfoil profiles is critical to designing an efficient flapping-wing aircraft. To choose the suitable airfoil for various wing sections, it is necessary to calculate the aerodynamic forces of the unsteady two-dimensional airfoil with a Reynolds number in the range of 105. While accurate, calculating this by solving the Navier–Stokes equations is impractical for early design stages due to its high consumption of computing resources and time. The computational demands for extending it to 3D aerodynamic calculations are even more prohibitive. In this paper, a relatively simple method is proposed. The two-dimensional unsteady panel method is utilized to derive the inviscid flow field, the unsteady integral boundary layer method is utilized to solve the boundary layer viscous flow, and the eN transition model is adopted to predict the position of the transition. These models are coupled with the semi-inverse interaction method to solve the aerodynamics of the unsteady low-Reynolds-number two-dimensional airfoil. The unsteady aerodynamics of the symmetric and cambered airfoils at different wing sections are calculated respectively by the proposed method. Mechanism analysis of the calculation results is conducted, and a symmetrical airfoil or a slightly cambered airfoil is recommended for the wing tip, a moderately cambered airfoil is suggested for the outer-wing section, and a highly cambered airfoil is suggested for the inner-wing section. [ABSTRACT FROM AUTHOR]

Ice accretion is inevitable on fix-wing UAVs (unmanned aerial vehicles) when they are applied to surveillance and mapping over colder climates and arctic regions. Subsequent aerodynamic profile changes have caused the current interest in the better prediction of the effect of icing shapes/sizes/distribution patterns on the aerodynamic performances of an aircraft. This study employs a numerical model which investigates the RG-15 aerofoil's response to various icing scenarios at a Reynolds number of  R e = 2 × 10 5 . Under icing conditions, compared to a clean aerofoil, a reduction in the lift coefficient and an increase in the drag coefficient are observed. Lower temperatures and reduced liquid water content lead to a decrease in the maximum thickness of ice accretion on the RG-15 aerofoil. Particularly noteworthy is the 10.85% reduction in the lift coefficient at a 10° angle of attack, which is in the icing condition at −10 °C with a mean volume diameter of 15 μm. Power consumption increases in the range of 0.46% to 26.5% under various icing conditions, showing synchronization with the rise in drag coefficient. This study underscores the need for future research to investigate various cloud conditions comprehensively and deeply in the context of aerofoil icing. [ABSTRACT FROM AUTHOR]

A methodology based on sound non-equilibrium thermodynamics principles is developed to estimate the extremum dissipation point for steady-state incompressible flow past a sphere at low Reynolds numbers. It is shown, that the extremum dissipation point appears at the point when both the shear stress and the pressure at the surface of the sphere are equal to zero. The Reynolds number and the position of the extremum dissipation flow past a sphere were further estimated with the aid of a mathematical model for pressure distribution on the sphere surface, accounting for both creeping and ideal flow. The parameters of the model were determined by comparison of the calculated pressure distribution at the surface with the available literature data. The conditions at which the separation angle and the extremum dissipation angle coincide were also investigated. It is believed that this work could be used to further elucidate the flow past a sphere. [ABSTRACT FROM AUTHOR]

Micro Air Vehicles (MAVs) airfoils usually operate at low Reynolds number conditions, where viscous drag will consume a large amount of propulsion power. Due to the small dimensions, many drag reduction methods have failed, resulting in limited current research. To develop an effective method of reducing viscous drag, transverse grooves were placed on the surface of MAVs airfoils in this study, and a numerical investigation was implemented to uncover the corresponding flow control law as well as the mechanism. Research has shown that transverse grooves have an impact on the drag and lift of airfoils. For drag, properly sized transverse grooves have the effect of reducing drag, but under high adverse pressure gradients or when the continuous arrangement of grooves is excessive, the optimal drag reduction effect achieved by the grooves is weakened, and even the drag increases due to the significant increase in pressure difference. In severe cases, it may also cause strong flow separation, which is not conducive to MAV flight. For lift, the boundary vortex in the groove has the ability to reduce the static pressure near the groove. However, high adverse pressure gradients or too many grooves will thicken the boundary layer and increase the blockage effect, resulting in a large static pressure on the grooved side of the airfoil (with an increase in drag). From the perspective of circulation, the static pressure changes on the suction and pressure surfaces have opposite effects on lift. Considering the comprehensive aerodynamic performance of the airfoil, we designed a high lift-to-drag ratio airfoil with grooves, which increased the lift-to-drag ratio by 33.747% compared to the smooth airfoil. Based on the conclusions, we proposed preliminary design criteria for grooved airfoils, providing guidance for subsequent research and applications. [ABSTRACT FROM AUTHOR]