Ultra-low Reynolds Number Research Articles

Aerodynamic performance study of propellers for Mars atmospheric environment

In terms of flight conditions, the Martian atmospheric environment is undoubtedly much harsher than the Earth atmospheric environment, with extremely low air density, ultra-low Reynolds number, and raging Martian sandstorms, all of which have serious impacts on the design of Martian unmanned aerial vehicles (UAVs), especially for the design of the power system. The purpose of this paper is to study the impact of the Martian atmospheric environment on the aerodynamic performance of the propeller system. The computational fluid dynamics numerical simulation was used to study the impact of varying degrees of sand and dust accumulation on the aerodynamic performance of Martian propellers caused by Martian sandstorms and widespread dust. It was found that in the Martian atmospheric environment, the rough surface of propeller blades after sand and dust accumulation has better aerodynamic performance than the smooth surface, which is greatly different from the Earth environment; The aerodynamic characteristics of the propeller are tested in the simulated Martian atmosphere vacuum chamber environment, and the experiment has shown that the propeller has better aerodynamic performance under the condition of 8° angle of attack; under the condition of 8° angle of attack, the aerodynamic performance of the propeller is simulated by different Reynolds numbers and different Mach numbers, so as to obtain the influence of the Reynolds number and the Mach number on lift–drag characteristics under the Martian atmospheric environment, and then, the modified blead element theory suitable for the Martian atmospheric environment is derived, which provides a detailed reference for the accurate modeling of the Mars UAV. It has important guiding significance for the design of flight controller.

Airfoil optimization for rotors operating in the ultra-low Reynolds number regime

Ultra-low Reynolds number aerodynamics has gained significant attention in recent times, primarily due to the proliferation of micro-aerial vehicles and their utilization in low-density environments such as the Martian atmosphere and high altitudes in Earth's atmosphere. The Martian atmosphere presents a unique combination of low Reynolds numbers and high subsonic Mach numbers, necessitating the use of unconventional airfoil designs in this regime. This study focuses on optimizing airfoils for rotors operating in the compressible ultra-low Reynolds number regime. We demonstrate the capability of XFOIL, a panel method code incorporating an integral boundary layer formulation, to accurately predict airfoil loads when the flow remains attached. We determine the optimal camber and maximum camber positions by analyzing the four digits, two percent thickness, National Advisory Committee for Aeronautics XX02 airfoil family using XFOIL. Subsequently, we employ a multi-objective optimization approach, utilizing a Class Shape Transformation airfoil parameterization to maximize lift and minimize drag. We select several points from the resulting Pareto front and evaluate their performance through unsteady compressible Navier–Stokes simulations. Our findings reveal that incorporating sharp leading-edge variations in these airfoil designs enhances the peak efficiency by over 10%, primarily attributable to the development of laminar separation bubbles on the suction side of the airfoils. Importantly, these modified airfoils maintain favorable performance at low angles of attack.

A Boundary-Vorticity Perspective on Lift Enhancement at Ultra-Low Reynolds Numbers

The ultra-low Reynolds number flow over airfoils is considered from the perspective of boundary vorticity dynamics in this paper. The dynamic balance between the streamwise pressure gradient and vorticity creation rate at the wall is highlighted by the boundary vorticity flux. It is a localized feature with a high peak generally indicating impending boundary layer separation. Two interesting features of ultra-low Reynolds number flows are considered. Firstly, the higher values of lift coefficient with decreasing Reynolds number is seen to be due to delayed boundary layer separation. The peak in boundary vorticity flux clearly predicts the separation event. Secondly, the lift enhancement achieved by certain optimal airfoils due to a vortex trapped in the airfoil cavity is considered. An interesting double-peak in boundary vorticity flux seen for the optimal airfoil enhances the lift. An alternative interpretation of the lift enhancement is also provided by examining the Lamb vector-field.

Analysis of Aerodynamic Characteristics of Propeller Systems Based on Martian Atmospheric Environment

Compared to detection methods employed by Mars rovers and orbiters, the employment of Mars UAVs presents clear advantages. However, the unique atmospheric conditions on Mars pose significant challenges to the design and operation of such UAVs. One of the primary difficulties lies in the impact of the planet’s low air density on the aerodynamic performance of the UAV’s rotor system. In order to determine the aerodynamic characteristics of the rotor system in the Martian atmospheric environment, a rotor system suitable for the Martian environment was designed under the premise of fully considering the special atmospheric environment of Mars, and the aerodynamic characteristics of the rotor system in the compressible and ultra-low Reynolds number environment were numerically simulated by means of a numerical calculation method. Additionally, a bench experiment was conducted in a vacuum chamber simulating the Martian atmospheric environment, and the aerodynamic characteristics of the UAV rotor system in the Martian environment were analyzed by combining theory and experiments. The feasibility of the rotor system applied to the Martian atmospheric environment was verified, and the first generation of Mars unmanned helicopters was developed and validated via hovering experiments, which thereby yielded crucial data support for the design of subsequent Mars UAV models.

Numerical Investigation on Aerodynamics of a Rectangular Blade Rotor under Mars Air Conditions Using Large Eddy Simulation

The study of the aerodynamics of a flight vehicle under Martian air conditions is experimentally difficult due to its characteristics such as low air density and temperature, making the vehicle operate at an ultra-low Reynolds number, which in turn introduces a complex flow field. In this paper, to find a proper computational fluid dynamics (CFD) method with which to better understand the aerodynamics of rotor-type aircraft under Martian air conditions, the aerodynamic performance of a rotor with rectangular blades under ultra-low density Martian air conditions is studied. A simulation scheme using a large eddy simulation solver and sliding meshing technology is established, and the method is verified based on experimental results from a Mars Air Simulator (MAS). In addition, the influence of the test bench and chamber is investigated through flow field analysis. The results show that the established method can predict lift in a very accurate manner, but that the torque prediction is not so promising. The study also determines that the fixture and the chamber wall of an MAS has little influence on the prediction of aerodynamic performance due to the quickly decreasing of flow speed and dissipation of vortexes. The test bench has about 5% influence on lift prediction, possibly due to the ground effect of the bench. In addition, simulation under actual Martian air conditions shows that the results agree well with the MAS experiment’s results, indicating that the temperature difference has little influence on the lift performance, and therefore that the MAS is a good tool for the lift prediction of Martian helicopters.

Lift Enhancement by Trapped Vortex at Ultra-low Reynolds Numbers

A means of passively trapping a vortex to enhance the aerodynamic characteristics of an airfoil is explored in this paper. The flow over an airfoil with a cavity on the upper surface, an NACA 0004 airfoil, and a flat plate airfoil is studied computationally in the ultra-low Reynolds number regime. At low angles of attack a vortex is seen to be trapped in the cavity. With increasing angle of attack the trapped vortex grows in size and occupies the entire cavity and the effective geometry resembles a smooth airfoil. The vortex circulation also increases and enhances the lift characteristics of the airfoil. The drag coefficient of the cavity airfoil is comparable to the NACA 0004 airfoil. The lift-to-drag ratio of the cavity airfoil is found to be higher than the smooth airfoils in the entire range of angles of attack considered in the present study. The cavity airfoil, with a trapped vortex, offers an interesting prospect for engineering nano-air vehicles.

Aerodynamic optimization and analysis of quadrotor blades operating in the Martian atmosphere

The design of unmanned aerial systems (UAS) for flight in the Martian atmosphere is a relevant and current topic. The successful flight of the Ingenuity helicopter recently proved its applicability. We present a numerical approach to the aerodynamic optimization of blades for rotors working in flow regimes with Reynolds numbers smaller than 15,000. Due to the atmospheric gas density and viscosity on Mars and the required rotation speed and rotor diameter, rotary-wing works mainly in the so-called ultra-low Reynolds number regime (103<Re<104), where the flow is likely laminar. The tip region is in the lower range of the very-low Reynolds number regime (104<Re<105), where separation-induced laminar-turbulent transition may occur. Such conditions characterize rotor blades of UAS operating in the Martian atmosphere or at very high altitudes (30 km) in the Earth's atmosphere. The blade design procedure consists of a two-step optimization process. The process starts with an initial two-dimensional airfoil design analysis followed by three-dimensional Blade Element Momentum (BEM) simulations to achieve the chord and twist radial distributions necessary to define the blade geometry. Then, a procedure to perform three-dimensional adjoint-based CFD simulations enhances the performance. The aerodynamic characteristics of optimal blades are evaluated with Navier-Stokes (N-S) and Large Eddy Simulations (LES), demonstrating the negligible effect of turbulence in blade performance for these Reynolds numbers when the boundary layer is attached. The implemented BEM method and high-fidelity CFD simulations show a good agreement.

Intelligent Micro-/Nanorobots for Cancer Theragnostic.

Cancer is one of the most intractable diseases owing to its high mortality rate and lack of effective diagnostic and treatment tools. Advancements in micro-/nanorobot (MNR)-assisted sensing, imaging, and therapeutics offer unprecedented opportunities to develop MNR-based cancer theragnostic platforms. Unlike ordinary nanoparticles, which exhibit Brownian motion in biofluids, MNRs overcome viscous resistance in an ultralow Reynolds number (Re << 1) environment by effective self-propulsion. This unique locomotion property has motivated the advanced design and functionalization of MNRs as a basis for next-generation cancer-therapy platforms, which offer the potential for precise distribution and improved permeation of therapeutic agents. Enhanced barrier penetration, imaging-guided operation, and biosensing are additionally studied to enable the promising cancer-related applications of MNRs. Herein, the recent advances in MNR-based cancer therapy are comprehensively addresses, including actuation engines, diagnostics, medical imaging, and targeted drug delivery; promising research opportunities that can have a profound impact on cancer therapy over the next decade is highlighted.

Study of Airfoil Characteristics on NACA 4415 with Reynolds Number Variations

This study uses computational methods to analyze fluid flow characteristics NACA 4415 in ultra-low, low, and moderate Reynolds number categories. The turbulence model chosen is the standard k-ε. The ultra-low Reynolds number used is 5×10^3, while the Reynolds number for the low category is 4.17×10^4. For the moderate category, the Reynolds number is 3×10^6. The variation of the grid being tested is the grid with the number of elements 5×10^4, 10^5 and 2×10^5. The grid with the number of elements 2×10^5 gives the best results. At the ultra-low Reynolds number, there is no stall condition. The stall condition at the low Reynolds number is AoA=12°. At the moderate Reynolds number, the stall occurs at AoA=18°. The largest Cl is in the moderate Reynolds number. Meanwhile, the smallest Cd is at the ultra-low Reynolds number. Based on the analysis of pressure contours, velocity contours and velocity streamlines, the fluid flow patterns seen are not much different. However, there is a significant difference in their values.

Effect of free stream turbulence intensity on corrugated airfoils at ultra low Reynolds numbers

Effect of free stream turbulence intensity on corrugated airfoils at ultra low Reynolds numbers

Active matter dynamics in confined microfluidic environments.

The field of active matter is a nascent area of research in soft condensed matter physics, which is drawing on the expertise of researchers from diverse disciplines. Small scale active particles-both inorganic and biological-display non-trivial emergent dynamics and interactions that could help us understand complex biological processes and phenomena. Recently, using microfluidic technologies, several research groups have performed important experimental and theoretical studies to understand the behavior of self-propelled particles and molecular active matter within confined environments-to glean a fundamental understanding of the cellular processes occurring under ultra-low Reynolds number conditions. In this chapter, we would like to review applications of microfluidics in active matter research, highlighting a few important theoretical and experimental investigations. We will conclude the discussion with a note on the future of this field mentioning a few open questions that are at the forefront of our minds.

Computations of Flow past the Corrugated Airfoil of Drosophila Melanogaster at Ultra Low Reynolds Number

The study of corrugated wings has become acquainted in the field of insect flight in recent times. Recent studies on the aerodynamic effects of a corrugated wing are based on insects like the Dragonfly; whereas the likes of Fruitfly (Drosophila Melanogaster) usually go unobserved due to their smaller size. Consequently, the behaviour of these corrugations is found to be anomalous especially in the low and ultra-low Reynolds number region. Therefore, the main aim of this study is to understand the aerodynamic effects of the corrugated airfoil present in the wing of a Fruitfly; by conducting a geometric parametric study during a static non-flapping flight at 1000 Re. In this computational study, a 2-D section of the corrugated wing along the chord is considered. The parametric study helps in understanding the effects of varying number of corrugations, angle of corrugations and the presence of a hump at the trailing edge. The dimensions were scaled to a suitable reference value to additionally compare the corrugated airfoil of Fruitfly to that of a Dragonfly. The present study shows that the aerodynamic performance of the corrugated wing in terms of cl and cd are predominantly governed by the subtle geometric variations that can largely impact the formation of bubbles, vortex zones, and their mutual interaction. The reduction in the number of leading edge corrugations improved the cl/cd ratio and reduction in the corrugation angle helped produce higher lift. The presence of a trailing edge hump also improved the stall angle with a better flow re-attachment. The presence of corrugation at the trailing edge proved to be more beneficial compared to the model with corrugations at the leading edge. This also helped in understanding, the aerodynamic superiority of the trailing edge corrugations present in the Dragonfly's wing when compared to the Fruitfly's.

Investigation of Bioinspired Flow for a Nano Coaxial Rotor in Hover

AbstractA computational study of a nano coaxial rotor with bioinspired blade motion was conducted at an ultralow Reynolds number to determine the principle of improving rotor performance via an ind...

Parametric study of a corrugated airfoil in a forward flight at ultra-low Reynolds number

In the current study, the mid cross section of the dragonfly forewing was simulated at ultra-low Reynolds number. The study aims to understand better the contribution of corrugations found along the wing on the aerodynamic performance during a forward flight. Different flapping parameters were employed. FLUENT solver was used to solve unsteady, two-dimensional, laminar, incompressible Navier–Stokes equations. The results revealed that any stroke amplitude less than 1cm generated no thrust force. The stroke amplitude had to be increased to form the reversed Kármán vortices responsible for generating thrust force. The highest propulsive efficiency was found in the Strouhal number range 0.2 < St < 0.4 with a peak efficiency of 57% at St = 0.39. Changing the phase difference between pitching and plunging motions from advanced to synchronized caused lift force to drop 91% and thrust force to increase by 15%. On the other hand, changing the phase difference from synchronized to delayed caused lift and thrust forces to increase by 89% and 36%, respectively, and propulsive efficiency to deteriorate significantly. In all performed simulations, the airfoil was assumed to start motion from rest with no initial angle of attack. The increase in initial angle of attack generates a very high lift force with a fair loss for both thrust force and propulsive efficiency. The decomposition of flapping motion into its elementary motions revealed that the aerodynamic forces generated are a non-linear superposition from both pure pitching and pure plunging aerodynamic forces. This can be attributed to the non-linear interaction between unsteady vortices generated from these decomposed motions.

An Experimental Comparison between Wing Root and Wingtip Corrugation Patterns of Dragonfly Wing at Ultra-low Reynolds Number and High Angles of Attack

This study presents the empirical comparison between the wing root and wingtip corrugation patterns of dragonfly wing in the newly-built wind-tunnel at the IAUN. The main objective of the research is to investigate the effect of wingtip and wing root corrugations on aerodynamic forces and the flow physics around the cross-sections at Re=10000 and the angle of attack of 0° to 30°. For this aim, two cross-sections are extracted from wing root (first cross-section) and wingtip (second cross-section). The first cross-section has corrugations with higher density than the second cross-section. The comparison of lift coefficients obtained from pressure distribution and force measurement indicates an acceptable agreement between the results. Also, Particle Image Velocity (PIV) technique is used to measure the velocity field. The results show that all corrugation patterns do not have positive effects on the aerodynamic forces. The second cross-section can generate considerable aerodynamic forces compared to the first cross-section. At α=25°, the lift coefficient generated by the second cross-section is 90% and 25% higher than that of the first cross-section and the flat plate, respectively. Based on results, corrugations in the wing root's vicinity have a crucial role in the solidity of insect wings; however, corrugations in the wing tip's vicinity play a vital role in generating adequate aerodynamic forces. The comparison conducted in the current research reveals the second cross-section is an appropriate replacement for the flat plate in MAVs due to generating more essential forces for flight.

Numerical simulations of ultra-low-Re flow around two tandem airfoils in ground effect: isothermal and heated conditions

The advent of pico-aerial vehicles (PAVs) for thermal surveillance has necessitated a better understanding of the flow field around airfoils at ultra-low Reynolds numbers (102 to 103). Previous studies have shown that two airfoils arranged in a tandem configuration can exhibit better aerodynamic performance than two identical airfoils in isolation, but this improvement has only been confirmed at relatively high Reynolds numbers (105 and above). In this parametric study, we numerically simulate the two-dimensional flow field around two tandem NACA 0012 airfoils in ground effect, at a Reynolds number low enough to be relevant to PAVs (Re = 500). With the angle of attack fixed at α = 5° on both airfoils, we investigate the effects of three control parameters, namely the stagger distance, the gap height and the ground clearance, for both isothermal airfoils and fore-heated airfoils. Results show that consistent with previous studies at higher Re, two tandem airfoils are more aerodynamically efficient than two identical airfoils in isolation, especially when the gap height is positive, i.e., when the fore airfoil is higher than the aft airfoil. The aerodynamics of the tandem-airfoil system are strongly influenced by the airfoil-to-airfoil interference arising from the downwash generated by the fore airfoil. The presence of a laminar separation bubble on the suction surface of both airfoils is found to alter the lift and drag coefficients as well as the overall lift-to-drag ratio. The wake of the fore airfoil is often seen impinging on the aft airfoil, which is a key mechanism by which the lift and drag forces are altered. The gains in aerodynamic efficiency achieved by the tandem airfoils become smaller as the stagger distance increases owing to weakened airfoil-to-airfoil interference. The effect of ground clearance on the tandem airfoils is found to be similar to that on two isolated airfoils, with both the lift and drag coefficients increasing with decreasing ground clearance. Heating the fore airfoil of a tandem-airfoil system in ground effect is found to decrease the lift coefficient without much affecting the drag coefficient, resulting in a drop in the lift-to-drag ratio. Overall these results lend new insight into the ultra-low-Re aerodynamics of tandem airfoils under both isothermal and heated conditions, advancing the development of the next generation of PAVs for thermal surveillance and other assorted applications.

Bio‐inspired nano rotor investigation based on UVLM

Nano rotor is of great value in military and civilian applications. Due to its nano size, it works at an ultra-low Reynolds number and aerodynamic performance deteriorates dramatically. The bio-inspired nano rotor is carried out to improve the rotor propulsive performance. Unsteady vortex lattice method (UVLM) model is established fully considering the influence of induced drag and wake vortex distortion on aerodynamic forces. The aim is to quickly and accurately simulate the flow field around the bio-inspired nano rotor and to efficiently perform the aerodynamic calculation to optimise the design of the bio-inspired rotor. The rotor parameters and motion parameters such as aspect ratio, taper ratio and camber are studied using UVLM. It is found that the aerodynamic performance of the rotor increased with the aspect ratio. The quality factor changes parabolically with the taper ratio and camber, and there is an optimal value for the ratio and camber, respectively. The influences of pitching angle and frequency are investigated as well. Results show that the bio-inspired motion improves the propulsion performance of nano rotor.

Aerodynamic Shape Optimization of a Wavy Airfoil for Ultra-Low Reynolds Number Regime in Gliding Flight

The effect of the number of waves and the width of the ridge and valley in chord direction for a wavy airfoil was investigated at the angle of attack of 0 ∘ and Reynolds number of 10 3 through using the two-dimensional direct numerical simulation for four kinds of wavy airfoil shapes. A new method for parameterizing a wavy airfoil was proposed. In comparison with the original corrugated airfoil profile, the wavy airfoils that have more distinct waves show a lower aerodynamic efficiency and the wavy airfoils that have less distinct waves show higher aerodynamic performance. For the breakdown of the lift and drag concerning the pressure stress and friction stress contributions, the pressure stress component is significantly dominant for all wavy airfoil shapes concerning the lift. Concerning the drag, the pressure stress component is about 75 % for the wavy airfoils that have more distinct waves, while the frictional stress component is about 70 % for the wavy airfoils that have less distinct waves. From the distribution of pressure isoline and streamlines around wavy airfoils, it is confirmed that the pressure contributions of the drag are dominant due to high pressure on the upstream side and low pressure on the downside; the frictional contribution of the drag is dominant due to large surface areas of the airfoil facing the external flow. The effect of the angle of attack on the aerodynamic efficiency for various wavy airfoil geometries was studied as well. Aerodynamic shape optimization based on the continuous adjoint approach was applied to obtain as much as possible the highest global aerodynamic efficiency wavy airfoil shape. The optimal airfoil shape corresponds to an increase of 60 % and 62 % over the aerodynamic efficiency and the lift from the initial geometry, respectively, when optimal airfoil has an approximate drag coefficient compared to the initial geometry. Concerning an fixed angle of attack, the optimal airfoil is statically unstable in the range of the angle of attack from − 1 ∘ to 6 ∘ , statically quasi-stable from − 6 ∘ to − 2 ∘ , where the vortex is shedding at the optimal airfoil leading edge. Concerning an angle of attack passively varied due to the fluid force, the optimal airfoil keeps the initial angle of attack value with an initial disturbance, then quickly increases the angle of attack and diverges in the positive direction.

Propulsion performance investigation of bio-inspired nano rotor base on fluid–structure interaction

In this paper, the bio-inspired blade motion is introduced to improve the propulsive performance of nano rotor at an ultra-low Reynolds number. However, the complex flow interacts with the flexible composite blade structure resulting in the change of nano rotor propulsion performance and the vibration of blade structure. A composite nano rotor with blade-pitch motion is investigated computationally with a computational solvers based on fluid–structure interaction. The finite element model for the composite rotor is created and verified with a non-contact modal test. It is found that the simulation results matched well with the experimental results. Successively, the propulsive performance of a rigid nano rotor is studied. The propulsive performance of the nano rotor is analysed at different bio-inspired pitch frequencies. The results show that the figure of merit of the bio-inspired pitch rotor increases because of the bio-inspired blade pitch motion. And it is also found that the improvement of the propulsive performance of the nano rotor varies with the pitch frequency. The propulsive performance of the flexible bio-inspired nano rotor is also studied with by using fluid–structure interaction method. It is found that the computational results for flexible nano rotor are lower than that for rigid nano rotor. It is evident that it is necessary to consider the flexibility of the composite nano rotor when investigating the propulsion performance of bio-inspired nano rotor. And the response of blade structure is also studied. Structural dynamic analysis shows that the blade structure vibrates with small amplitude. And two peak values are found at the rotation frequency and the fundamental frequency of the nano rotor structure.

Moving Surface Boundary Layer Technique For NACA 0012 Airfoil At Ultra-Low Reynolds Number

Application of moving surface boundary layer control technique has been confined to relatively high Reynolds numbers. The present paper reports a numerical study of application of the above flow technique in the ultra-low Reynolds number range. A two dimensional incompressible unstructured grid based Navier Stokes solver has been used for conducting the numerical studies. Moving surface has been applied at three different portions on the airfoil surface, firstly, in the form of a rotating leading edge portion of the airfoil, secondly, a continuous moving surface from leading edge of airfoil to 57% of the chord along the leeward surface of the airfoil and thirdly a continuous moving surface from leading edge to 97% of the chord along the leeward surface of the airfoil. All the moving surface configurations show improvement of aerodynamic performance of the airfoil through enhancement of lift and decrement of drag as compared to a fixed surface one.