Effect of Plasma-Based Control on Low-Reynolds-Number Flapping Airfoil Performance

For a number of manmade and natural flyers the aerodynamics of low Reynolds number plays a crucial role. Active study of birds and insects is growing rapidly for development of Micro Air Vehicles (MAVs). MAVs are capable of performing tasks like surveillance, environmental monitoring and threat assessment in hostile environment. When compared to general civilian aircrafts MAVs fly at low Reynolds number regime of 105 or lower. A considerable change of aerodynamic characteristics is seen between aircrafts that operate in low and high Reynolds number regimes. Low Reynolds number flyers are sensitive to wing gust and deformation because of being lightweight and operating at low speeds. The birds have nature’s finest locomotion system when it comes to maneuvering effectively through space. This review paper will assist further studies in the field of low Reynolds number flyers that can be used for future development on MAVs. A holistic review of past two decades of research work and studies done on low Reynolds number flyers has been presented in this paper. The Theoretical, Experimental and Numerical techniques followed by researchers have been highlighted so that the study on biological and manmade flyers can be systematically presented. It was found that researchers preferred Numerical and Experimental techniques while conducting aerodynamic analysis of low Reynolds number flyers. An attempt has been made in this paper to fill the research gap that is there in mimicking the flight of animals and thus developing man-made systems having higher efficiency.

Flow control effects of three types of nanosecond (NS) dielectric barrier discharge (DBD) plasma actuations in suppressing the flow separation of a compressor airfoil at low Reynolds number are investigated using large-Eddy simulation. It has been found that the NS DBD plasma actuation can effectively suppress the flow separation and two mechanisms behind the flow control effect have been uncovered. First, the NS DBD plasma actuation can induce distorted flow structure (DFS) within the flowfield, and the induced DFS of plasma actuations located upstream of the flow separation zone can bring the Kelvin–Helmholtz instability of the shear layer between mainstream flow and separated flow froward to the separation point. Then, a large-scale spanwise vortex, which promotes the mixing of the main flow and the separated flow, is induced within the flow separation zone, thus resulting in suppression of the flow separation. Second, the plasma actuation located at the blade leading edge can improve the momentum of the laminar boundary layer, owing to the propagations of compressive waves as well as DFS along the blade surface. The plasma actuation located within the separation zone fails to suppress the boundary-layer flow separation effectively because the induced DFS cannot influence the shear layer between mainstream flow and separated flow.

The aerodynamic design of micro air vehicles is challenging since previous studies have shown that the aerodynamic efficiency of airfoils and wings decreases substantially at low Reynolds-numbers. While many MAV approaches investigate biological designs, here a study is conducted on the aerodynamics of paper airplanes, which fly in the same Reynolds-number range as MAV, but have the advantage of simplicity compared to biological counterparts. Flow visualizations and force measurements in a water tunnel as well as large-eddy simulations are presented on one of the simplest paper airplane design: the dart. The results show that the high-sweep delta design of such an airplane provides high lift coefficients at low Reynolds-numbers. Furthermore, the centerfold of the airplane as a mean to improve the aerodynamic performance is identified.

A large-eddy simulation (LES) was carried out in order to numerically describe the use of plasma-based actuation for the control of turbulent boundary-layer separation. The configuration consisted of a flat plate section over which the boundary layer developed, followed by a curved convex rearward-facing ramp, corresponding to an experimental arrangement. A single dielectric-barrier-discharge (DBD) plasma actuator was then employed to reduce the extent of the separated flow region. Solutions were obtained to the Navier-Stokes equations, that were augmented by source terms used to represent plasma-induced body forces imparted by the actuator on the fluid. A simple phenomenological model provided the electric field generated by the plasma, resulting in the body forces. The numerical method utilized a high-fidelity time-implicit scheme, employing domain decomposition in order to perform calculations on a parallel computing platform. Simulations were first conducted for an isolated actuator in the absence of external flow, which were used to optimize the choice of parameters inherent in the plasma model. Subsequently, actuation was applied to the plate development section without the downstream ramp section. Following these computations, the complete plate/ramp configuration was simulated. In all cases, both continuous and pulsed operation of the actuator was considered. Comparisons are made with available experimental data, and with baseline flows where no control was enforced. The large separated flow region characterizing the baseline flow for the plate/ramp configuration was well captured by the simulation. When continuous control was applied, separation was almost entirely eliminated. Pulsing operation of the actuator using a 40% duty cycle was not as eective, but did result in a considerable reduction of the recirculating zone. The control cases compared reasonably well with experimental measurements. Although deficiencies in the plasma model were apparent in the near-wall region, it appeared to be adequate for use with LES in the exploration of plasma-based control for turbulent wall-bounded flows.


 
 
 Flapping-Wing Micro Air Vehicles (FW-MAVs) are small hand-held flying vehicles that can maneuver in constrained space owing to their lightweight, low aspect ratio and the ability to fly in a low Reynolds number environment. In this study, the aerodynamic characteristics such as time-averaged lift (CL avg) and time-averaged drag (CD avg) of camber wings with different five wind tunnel test models with 6, 9, 12, and 15 per cent camber were developed and the results were compared with a flat wing to assess the effects of camber wing on the aerodynamic performance for flapping flight applications. The experiments were performed in an open chamber with non-return airflow with a test section of (0.3 x 0.3) m and capable of speeds from 0.5 to 30 m/s. The (CL avg) and (CD avg) as functions of the angle of attack at 100, associated with flapping frequency at 9 Hz and low Re = 3600 of the flapping motions concerning the incoming flows are measured by using a strain gauge balance and KYOWA PCD- 300A sensor interface data acquisition system. It is found that camber would bring significant aerodynamic benefits in a higher lift in comparison to the flat wing and increase with increasing camber instead of drag shows contrary a decreasing of performance with increase drag when the camber raised.
 
 

This paper presents an experimental parametric study to maximize the hover efficiency of a coaxial rotor system for a micro air vehicle (MAV) that could be launched from a 40 mm grenade launcher. Towards this, isolated rotor experiments were first conducted to optimize the performance of a single rotor at low Reynolds numbers (Re) through varying parameters including blade pitch angle, thickness-to-chord ratio (t/c), chord length, and Reynolds number. Results showed that t/c had minimal impact on the figure of merit FM) below 4%, while increasing blade chord length significantly improved hover efficiency until a chord length of 16.6 mm (solidity = 0.10). The optimal pitch angle for the isolated rotor was around 16 degrees, and the maximum FM was 0.59 at Re = 70,000. Coaxial rotor experiments were performed using the optimal isolated rotor as the baseline. The vertical separation between the upper and lower rotors had negligible impact on performance for a separation distance range from 0.5R to 3R. However, below a separation distance of 0.5R, the thrust and FM of the lower rotor increased and that of the upper rotor decreased. The highest FM obtained for the coaxial rotor was around 0.60 at Re = 30,000, which remained relatively constant across all vertical separations tested. The coaxial rotor system produced almost 1.66 times the thrust of an isolated single rotor. When compared at the same disk loading, the power loading (thrust/power) for the coaxial rotor was similar to that of an isolated single rotor. A comprehensive analysis (RCAS) was used to model the MAV-scale isolated rotor in hover and the analytical predictions agreed well with the experiments. In order to generate the airfoil lookup table for the RCAS model, water tunnel experiments were conducted at Re = 44,000 using a scaled-up wing with the exact same circular-cambered plate airfoil used in the coaxial rotor blades.

Micro air vehicles (MAVs) with a wingspan of 15cm or shorter, and flight speed around 10m∕s have attracted substantial interest in recent years. There are several prominent features of MAV flight: (i) low Reynolds number (104-105), resulting in degraded aerodynamic performance, (ii) small physical dimensions, resulting in certain favorable scaling characteristics including structural strength, reduced stall speed, and impact tolerance, and (iii) low flight speed, resulting in order one effect of the flight environment and intrinsically unsteady flight characteristics. Flexible wings utilizing membrane materials are employed by natural flyers such as bats and insects. Compared to a rigid wing, a membrane wing can better adapt to the stall and has the potential for morphing to achieve enhanced agility and storage consideration. We will discuss the aerodynamics of both rigid and membrane wings under the MAV flight condition. To understand membrane wing performance, the fluid and structure interaction is of critical importance. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble, and tip vortex, as well as structural dynamics in response to the surrounding flow field are discussed. Based on the computational capabilities for treating moving boundary problems, an automated wing shape optimization technique is also developed. Salient features of the flexible-wing-based MAV, including the vehicle concept, flexible wing design, novel fabrication methods, aerodynamic assessment, and flight data analysis are highlighted.

<div class="htmlview paragraph">The propeller-induced flow highly effects to the aerodynamic characteristics of Micro Air Vehicle (MAV) because of large propeller size and low flight speed. The tractor propulsive induced flow may decrease the lift-to-drag ratio up to 50%. This is due to that wing’s local angle of attack is reduced. In the pusher-propulsive configuration, the local angle of attack is not changed but it decreases the propulsive efficiency. This paper objects to demonstrate and compare the aerodynamics advantage and disadvantage of propeller installation, pusher and tractor on a very low aspect ratio MAV. The study was done by an experiment in wind tunnel.</div>

In this study, effect of plasma actuator on a flat plate and manipulation of flow separation on NACA0015 airfoil with plasma actuator at low Reynolds numbers were experimentally investigated. In the first section of the study, plasma actuator which consists of positive and grounded electrode couple and dielectric layer, located on a flat plate was actuated at different frequencies and peak to peak voltages in range of 3-5 kHz and 6-12 kV respectively. The induced air flow velocity on the surface of flat plate was measured by pitot tube at different locations behind the actuator. The influence of dielectric thickness and unsteady actuation with duty cycle was also examined. In the second section, the effect of plasma actuator on NACA0015 airfoil was studied at Reynolds number 15000 and 30000. Four plasma actuators were placed at x/C = 0.1, 0.3, 0.5 and 0.9, and different electrode combinations were activated by sinusoidal signal. Flow visualizations were done when the attack angles were 0°, 5°, 10°, 15° and 20°. The results indicate that up to the 15° attack angle, the separated flow was reattached by plasma actuator at 12kV peak to peak voltage and 4 kHz frequency. However, 12 kVpp voltage was insufficient to reattach the flow at 20° angle of attack. The separated flow could be reattached by increasing the voltage up to 13 kV. Lift coefficient was also increased by the manipulated flow over the airfoil. Results showed that even high attack angles, the actuators can control the flow separation and prevent the airfoil from stall at low Reynolds numbers.

MAVs (micro air vehicles) with a maximal dimension of 15 cm and nominal flight speeds of around 10 m s−1, operate in a Reynolds number regime of 105 or lower, in which most natural flyers including insects, bats and birds fly. Furthermore, due to their light weight and low flight speed, the MAVs' flight characteristics are substantially affected by environmental factors such as wind gust. Like natural flyers, the wing structures of MAVs are often flexible and tend to deform during flight. Consequently, the aero/fluid and structural dynamics of these flyers are closely linked to each other, making the entire flight vehicle difficult to analyze. We have recently developed a hummingbird-inspired, flapping flexible wing MAV with a weight of 2.4–3.0 g and a wingspan of 10–12 cm. In this study, we carry out an integrated study of the flexible wing aerodynamics of this flapping MAV by combining an in-house computational fluid dynamic (CFD) method and wind tunnel experiments. A CFD model that has a realistic wing planform and can mimic realistic flexible wing kinematics is established, which provides a quantitative prediction of unsteady aerodynamics of the four-winged MAV in terms of vortex and wake structures and their relationship with aerodynamic force generation. Wind tunnel experiments further confirm the effectiveness of the clap and fling mechanism employed in this bio-inspired MAV as well as the importance of the wing flexibility in designing small flapping-wing MAVs.

Micro Air Vehicles (MAVs) have become more attractive for various missions including surveillance or reconnaissance in recent years. MAVs should be capable of maintaining their attitudes through either inherent passive stability or active feedback in order to successfully perform their directives. Stability and Controllability Augmentation Systems (SCASs) are usually employed to enhance the flight performance of conventional aircrafts and Unmanned Aerial Vehicles (UAVs). However, it is no simple task to obtain an accurate numerical model for the flight dynamics of a MAV. An alternative approach for SCASs would be to incorporate reinforcement learning in order to address this numerical complexity. Such implementation has already been successful in other vehicles, such as unmanned ground vehicles (UGVs), because of their bettered stability compared to aerial vehicles. However, in order to train MAVs to learn how to fly, they must first be airborne. Similar to teaching infants how to walk, this paper presents a new method to provide an effective environment where a MAV can learn how to fly. A test setup was constructed to enable magnetic levitation of a MAV embedded with a permanent magnet. This apparatus allows for flexible experimentation: the position and the altitude of the MAV, the constraint forces, and the resulting moments are all adjustable and fixable. This ‘Pseudo Flight Environment’ was demonstrated with a fixed wing MAV model. In order for the model to maintain a constant altitude, a height hold control system was devised and implemented.

Low Reynolds number refers to a specific range of values of the dimensionless parameter known as the Reynolds number (Re) in fluid dynamics. The Reynolds number is a critical factor used to characterize and predict fluid flow behaviour around objects or within fluid systems. Low Reynolds numbers (typically Re < 2000), laminar flow prevails, where fluid particles move in smooth, orderly layers with minimal mixing or turbulence. Low Reynolds number aerodynamics is a focal point of interest, especially for micro air vehicles (MAVs), drones, and small-scale aircraft. Understanding the nuanced aerodynamic performance of aerofoils and wings in this regime is pivotal for designing efficient and controllable flight systems, ushe ring in innovations in the field of aviation. The unique characteristics of low Reynolds number flow to advance technology, science, and our understanding of fluid dynamics.

Missions envisioned for Micro Air Vehicles (MAVs) such as Intelligence, Surveillance and Reconnaissance (ISR) in urban environments require exceptional agility. These missions typically involve ight through or in close proximity to buildings, tunnels and trees, and require the MAVs to maneuver using sharp dives and climbs along with small-radius turns. The small size of MAVs allows such ight operations within con ned space but the control e ectors must provide su cient maneuverability, while maintaining stability, with only limited sensing capability onboard the platform. To meet these challenges, researchers have long been attracted by the amazing attributes of biological systems, such as those exhibited by birds and insects. Birds can y in dense ocks, executing rapid maneuvers with g-loads far in excess of modern ghter aircrafts, and yet never collide with each other, despite the absence of air tra c controllers. This paper introduces a novel framework for the design and control of a Micro Air Vehicle. The vehicle's conceptual design is based on biologically-inspired principles and emulates a dragon y (Odonata-Anisoptera). The author is borrowing ight mechanism features from the dragon y and merging them with indigenous designs in creating a novel version of a Flapping Wing MAV, called the QV. The QV design incorporates the simplicity of mechanical construction and the complexity of a sophisticated multi-layered Hybrid & Linear / Non-Linear controller to achieve extended ight times and improved agility compared to other Rotary and Flapping Wing MAV designs. The paper talks about the design and control features of the proposed QV design and gives an overview on the developmental e orts been made towards the prototyping of the QV yer. The potential applications for such a high endurance vehicle are numerous, including air-deployable mass surveillance in cluster and swarm formations. The disposability of the vehicle would help in battleeld deployment as well, where such a MAV would be made available to soldiers for proximity sensing and threat level assessment. Other applications would include search and rescue operations and civilian law-enforcement.

In recent years Micro Air Vehicles (MAV) for disaster aerial video are developed vigorously. In order to improve aerodynamic performance of MAV wing performance in low Reynolds numbers (Re) need to be improved, but research on the theme are very rare. In category of Hand Launch Glider, a kind of model aircraft, glide performance are competed, as a result high performance airfoils in Re is around 20,000 are developed. Therefore for MAV's aerodynamic performance improvement airfoils of Hand Launch Gliders should be referred and aerodynamic characteristics of the airfoils desired to be studied. So in this research, aerodynamic characteristics of the gliders are measured in wind tunnel. And also consistency between wind tunnel test and glide test in calm air is examined to confirm reliability of wind tunnel test. Comparison of different airfoils and flow visualization are also performed.

Single-dielectric barrier discharge (SDBD) plasma actuators have gained a great deal of world-wide interest for flow-control applications. With this has come the need for flow-interaction models of plasma actuators that can be used in computational flow simulations. SDBD plasma actuators consist of two electrodes: one uncovered and exposed to the air and the other encapsulated by a dielectric material. An AC electric potential is supplied to the electrodes. When the AC potential is large enough, the air in the region over the encapsulated electrode ionizes. The ionized air in the presence of the electric field results in a space–time dependent body force vector field. The body force is the mechanism for flow control. This study describes a semi-empirical model that has been developed to capture the dynamic nature of the local air ionization and time-dependent body force vector distribution. Validation of the model includes comparisons to experimentally measured space–time charge distribution and the time-resolved and time-averaged body force. Two flow simulations are then used to further validate the SDBD plasma actuator model. These involved an impulsively started plasma actuator in still air, and the flow around a circular cylinder in which plasma actuators were used to suppress the Karman vortex street. In both cases, the simulations agreed well with the experiments.