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Biomechanics of flexible wing drones usable for emergency medical transport operations

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Abstract - To warrant delivery by drone, a package needs to be of high value, lightweight and time sensitive. Blood, medical diagnostics and defibrillators fit that description. In urban environments where wind gusts are abundant, small fix-winged drones face difficult challenges on medical missions. The paper proposes the solution of flexible wings that passively adapt to the flow. To better understand the aerodynamic characteristics of flexible wings, the pressure distribution and the corresponding shape change of the lifting surfaces are tracked. In this paper, we use CFD simulations to underline the aerodynamic characteristics of a custom designed small aircraft wing.

Keywords: drones, medical missions, flexible wings, bio mechanics.

I. INTRODUCTION

How can you transport vital medicines, vaccines or medical diagnostics tools when roads either do not exist or are impossible for much of the year? How can you transport blood to an emergency auto accident site, during rush hour? How can you offer precise medical aid in an overpopulated are where time is an essential factor? The answer to all these questions and more could be provided by small, flexible, air transport systems, also known as drones.

To warrant delivery by drones, a package needs to be of high value, lightweight and time sensitive. Blood, medical diagnostics and defibrillators fit that description, and even more so, have been the main focus of proof of concept missions undertaken by several drones.

It is known that high maneuverability and agility are desired for small drone applications, and these have increased the interest in the use of membrane wings. Understanding of the passive flow control mechanisms using the flexible membranes is therefore of significant importance.

II. MOTIVATION

The aim of this research is to expand on the knowledge of aerodynamic characteristics of flexible membranes, through a sequence of investigations. It is a well-known that separation, unsteadiness and low lift-to-drag ratio are major problems in low Reynolds number aerodynamics. Yet, the characteristics of the membrane that help alleviate such issues are still largely unexplored.

With this in mind, one of the objectives of this work is to understand the characteristics of the membrane deformation and the flow field at varying angles of attack and freestream velocities. By varying the freestream velocities, not only the effect of Reynolds number but also the effect of membrane flexibility can be learned.

Another goal of this paper is to better understand the aerodynamic characteristics of small flexible wings and grasp their potential applications in the field of urban medical missions.

First we recognize our biologically inspired design aspects, and then track the pressure distribution and the corresponding shape change of the lifting surfaces. We use CFD (Computer Fluid Dynamics) simulations to underline the aerodynamic characteristics of a custom designed small aircraft wing. Moving towards the goal of computing and analyzing the dynamics of the coupled fluid and structure systems, we conduct the aerodynamic study for potential flow over a constant tensioned membrane. The presented results will be used to improve the overall design of the wing. Based on the CFD solution, a number of important conclusions regarding further studies in this field are drawn.

III. BIO INSPIRATION TO BIOMECHANICS

Nature has been the richest source of inspiration for adaptable wing designs, starting with Lilienthal's glider, continuing with the Wright brothers gliders and, not yet ending, with modern airplanes like the Grumann F-14. New insights in bird flight mechanics and the growing interest in small unmanned air vehicles (figure 1), led many researchers to reconsider birds as models for their designs. Insects and bats have also, been studied eagerly in the past years, as the drones have seen a decrease in dimension and an increase in missions.

There is much to be written about the differences and similarities concerning insects, birds and bats flight characteristics, morphology and so on, but this paper will focus only on flexible wings [1]. Therefore the following definitions, statements and observations are considered appropriate and useful in this context, and will offer a brief technical description of this ample domain:

' The term 'flexible wing' is used in this paper to describe a wing allowed to deform and bend as it is aerodynamically loaded in flight.

' Insects, birds and bats achieve their multi-point adaptability capability by using flexible lifting surfaces, actuated by moveable structural elements that are attached with muscles to the wing;

' The evolutionary trend in all groups previously mentioned is towards reduction of dimensional complexity of the wings;

' Many changes in wing shapes of flying animals are achieved as passive, automatic response to loading. Insects have smart wings that can adopt the needed twist and camber distributions in flight. Birds have the automatically-moveable flaps, known as feathers at their disposal, whereas bats use anisotropic properties of their membrane wings to achieve proper camber under aerodynamic loading.

As a quick drawn conclusion from has been presented here and from the studies and research information available, one can state that an adaptable wing design would have the look of complexity but would be built on simple principles. The wing would be actuated through structural elements with limited degrees of freedom, and, more importantly it would be built to conform to loading rather than resist it.

IV. THE VEHICLE CONCEPT

The concept developed for our flexible wing uses a small flying wing design with flexible surfaces designed for efficient control (Figure 2).

The wing is thin and under-cambered as are those used for small flying wings in order to provide lateral stability. The small air vehicle that we have designed will be built on carbon fiber skeleton and thin membrane materials.

The fuselage uses carbon fiber pre-impregnated cloth and on the control surface of the wing there is attached a custom latex rubber membrane.

This configuration uses a blended wing-body: the fuselage blends into the wing using an initial standard E184 airfoil, and then blending it with the custom designed airfoil that has a thicker leading edge and a latex membrane. At about a third of the length of the chord, the latex membrane comes into play, thinning the airfoil significantly as can be seen also in Figure 3.

One of the most important choices in airplane design is the selection of the airfoil. The wing section chosen must have characteristics suited to the flight pattern of the type of model being designed. For urban medical missions the flight pattern would normally include the following stages: take-off, climbing to desired altitude (for building avoidance), speed dash to target (patient, hospital roof etc.), loiter (for package delivery or target spotting), descent towards target or return to base and finally landing. Therefore the structure must be rigid enough to sustain shock damage from landing, but also employ the use of flexible wings. The flexible wing design is presented in Figure 3.

V. FLEXIBLE WING DESIGN

Flexible wings can offer several interesting advantages over the conventional rigid ones. The flexible wings have the ability of adapting to the airflow and providing smoother flight. This is done because of the passive adaptive washout. For example in sail vessels, this passive mechanism of adaptive washout is produced by twisting the sail. Thus the range of wind speeds that truly affect the sail and decrease its lift is reduced considerably, and the vertical flexible wing produces more constant thrust or lift, even in gusty wind conditions.

The flexible wing that was designed is actively changing as a function of the airspeed and the angle of attack. The mechanism of the adaptive washout comes into play via the membrane extension and framework twisting. Thus the angle of attack changes as well as the camber of the airfoil along the length of the wing in response to air speed and angle of attack. Considering the fact that the small aircraft hits a head-on wind gust, the airspeed will increase.

The increased airspeed causes a shape change in the wing that decreases the lifting efficiency, but because the airspeed in the gust is higher, the wing maintains almost the same lift. With the reducing of the airspeed, the wing recovers and changes back to the original configuration [2].

If the relative airspeed is reduced, the angle of attack increases and the wing becomes more efficient and constant lift is restored.

This process ends with an extraordinary smooth flying wing, even in gusty wind conditions. Problems arise however, when the mechanism for adaptive washout is tuned to the wing to work effectively.

When aircrafts have small inertia, as is the case for small UAVs, any modification of the wing loading can affect the flight path. It is a known fact that the effects of wind gusts become critical with the decrease in size of the aircrafts and the more so, as the platform would have a camera on board or a medical sensitive package. Moreover, with the decrease of the UAVs airspeed, wind gusts tend to become a growing percentage of the mean airspeed of the vehicle. For example, a 1m aircraft flies between 15 and 25 m/s. On a typical day the wind speed can vary by more than 5 m/s. For rigid wings, the lift can vary by 50% or more over the short period of time. And you have to take into serious consideration the fact that gusts are not always head-on. Because control of these aircraft is one of the most important hurdles, it is critical to solve the aerodynamics problem in order to track the pressure distribution and the corresponding shape change of the lifting surfaces.

VI. AERODYNAMIC ASSESSMENT/ EVALUATION OF THE PROPOSED CONCEPT

Looking at a rigid drone's wing, we can safely say that the pressure distribution is determined by the wing shape and free-stream flow properties only. Now, considering a flexible wing, we note that its shape changes under the load of the aerodynamic forces, and, obviously, the angle of attack and surface pressure distribution change too. To better understand the aerodynamic characteristics of flexible wing, we need to solve the aerodynamics to track both the shape change as well as the pressure distribution on the wing.

The first ones to use the Navier-Stokes equations as the flow dynamics model in a flexible wing theory are Smith and Shyy [7]. They presented a CFD procedure that models the interaction of a laminar steady fluid flow at high Reynolds number and a two-dimensional flexible wing. The results obtained from the flexible wing model in viscous flow were compared with a potential flow based flexible wing theory. (Figure 5)

The current study approaches both potential and viscous solutions [3] for a two-dimensional flexible wing with no mass. An iterative procedure that iterates between the equilibrium equation of the flexible wings shape, and the flow equations that provide the pressure and shear stress distributions along the profile, is used for both solutions.

The equations used are those below:

' The equilibrium equation of the flexible wing:

(1)

where   is the pressure difference between the upper surface of the wing and the lower surface, T is the tension in the membrane and

' The total shear stress along the profile [4], [7] is

  (2)

By using the above equations and   and   from the computational study, we have an iterative solution, which, when converged, provides the flexible wing equilibrium shape, and the complete viscous flow solution.

The flow solution is obtained by a combination of the vortex sheet method [3], which produces the pressure difference along the profile, with the non-linear equilibrium equation. The tension is considered to be constant along the airfoil in this case. Using CFD simulations of the Navier Stokes equations [3] to obtain the pressure difference and shear stress along the profile we also obtained the viscous flow solution. The representation of tension is given a particular attention as the membrane is considered to be linearly elastic. The tension in the membrane may be described by the following equation [4]:

  (4)

where T0,E,l,l0 and gr are the membrane pretension, modulus of elasticity, length, initial length and thickness respectively. All of the above mentioned quantities were non-dimensionalised by the freestream dynamic pressure and the chord length of the membrane airfoil, in accordance to the quantity dimension. Another important parameter, used to define the initial length of the membrane, which affects the equilibrium shape through the equation (4), is the initial slack of the membrane, represented by the normalized excess length of (  is the excess length [4] and c is the chord [3]):

  (5)

In the viscous case, shear stress is also applied to the membrane. Thus, when the shear stress cannot be neglected, the tension can no longer be treated as constant along the profile (figure 4.). In this case an additional equilibrium equation relates the local change of tension to the local shear stress according to

  (6)

The CFD solver used in this study is Fluent and the study is based mostly on laminar Navier Stokes simulations.

VII. NUMERICAL SOLUTION

The computational procedure is iterative, alternating between solution of the flexible wing equilibrium equation and the flow solution. Figure 5 depicts a comparison between the applied solution for the designed airfoil with small camber at a high 100 angle of attack, and a validation model provided by the Nielsen method [8].

For the numerical solution, the first equation (1) was computed using finite differences, and the aerodynamic solution was obtained by CFD simulations, for potential and viscous flow, respectively. When shear stress effects were studied, the final equation (6) was added to the shape solution procedure. In all cases the tension was assumed to be constant along the profile.

The potential flow case considered is assumed to be fully attached and steady. The flexible wing potential flow solution is relevant only to steady cases. Similarly, steady CFD solutions are obtained only for small angle of attacks, for which the flow is mostly attached.

Even though the flow remains attached to the membrane surface at low degrees of angle attack, a small leading-edge separation bubble is evident. This mild separation bubble is not expected to affect the pressure distribution significantly. When the flow remains attached, oscillations of the membrane are very small.

Figure 6 shows the magnitude of the time-averaged velocity field superposed on the streamline pattern. For selected angles of attack (120, 160,200, and 250) the data were taken in two different regions (one near the leading-edge and the other near the trailing-edge).

VIII. CONCLUSIONS

Understanding of the passive flow control mechanisms and using the flexible membrane wings is considered an opportunity for urban medical sensitive packages aerial transport. Therefore in this study we point out the medical missions available for small aircrafts, as well as the importance of gust effects relief for the success of these missions. We recognize the biologically inspired designs and apply similar ideas to classical delta wing drone. Rethinking the classical E184 airfoil, we conduct an aerodynamic study, tracking the pressure distribution and the corresponding shape change of the bi-dimensional flexible airfoil. We used CFD simulations to study the velocity field characteristics and the influence of the separation bubble. The results obtained are validated by applying the Nielsen method and flow visualization gives us the opportunity to account for the low influence of the separation bubble on the flow over the flexible airfoil, unlike the rigid one. These results will be used to improve the overall design of the wing and shared over the internet [5], [6]. Another important conclusion is that the study needs to continue for unsteady flow and oscillatory behavior of this custom designed flexible airfoil.

References

[1] Barbarino, S., Bilgen, O., Ajaj, R.M., Friswell, M.I., Inman, D.J. A Review of Morphing Aircraft. JIMSS, June 2011.

[2]. M. Boscoianu, R. Pahonie, A. Coman, The optimization of CUMULUS micro aerial vehicle aerodynamics by using the adaptive flexible wing concept, 6th IASME/ WSEAS International Conference on FMA08

[3] Katz J. , Plotkin A., Low-speed Aerodynamics; From Wing Theory to Panel Methods,  San Diego St. Bookstore, 1999..

[4] Timoshenko S.P., Woinowsky-Krieger S., Theory of Plates and Shells,  McGraw-Hill, 1959.

[5] I. Chiuchisan, H. N.  Costin, O. Geman, 'Adopting the internet of things technologies in health care systems,' Electrical and Power Engineering (EPE), 2014 International Conference and Exposition on, p.532 ' 535, 16-18 Oct. 2014.

[6] D. A. Antonovici, I. Chiuchisan, O. Geman, A Tomegea 'Acquisition and management of biomedical data using Internet of Things concepts,' Fundamentals of Electrical Engineering (ISFEE), 2014 International Symposium on, p. 1-4, 28-29 Nov. 2014.

[7] Smith, R. W. and Shyy, W., Computational model of exible membrane wings in steady laminar flow," AIAA Journal, Vol. 33, No. 10, 1995.

[8] Nielsen, J. N., Theory oflexible aerodynamic surfaces, Journal of Applied Mechanics, Vol. 30, 1963.

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