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American Scientific Research Journal for Engineering, Technology,  and Sciences  (ASRJETS) 
ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 

© Global Society of Scientific Research and Researchers  
http://asrjetsjournal.org/ 

 

Wind Tunnels: State of Art Survey and Future Scope for 

Testing Micro Air Vehicles 

Royson Donate Dsouzaa*, Samiya Salimb, Atul Shankarc, Mohammed Safwand, 

Sheldon D'sae 

a,b,c,d,eManipal School of Engineering and IT, Dubai, UAE. 
aEmail: roysonnitk@gmail.com 

bEmail: samiasalim26@gmail.com 
cEmail: shankar.atul22@yahoo.in 

dEmail: safwan69@live.com 
eEmail: dsasheldon@gmail.com 

 

 

 

Abstract 

Through the years the growth of Micro Aerial Vehicles (MAV) has gained increasing interest among engineers 

in the applications of military and civil domains. However obtaining accurate aerodynamic flying characteristics 

of MAV's was considered difficult due to their small size, the nature of their very low Reynolds's number as 

well as the lack of testing methods. To overcome these complexity, the MAV's can be tested by using a subsonic 

open circuit micro scaled wind tunnel. This paper presents the development of wind tunnels analyzed from 

research journals from the year 1937 to 2015 in order fabricate a micro scaled model of the tunnel to test the 

fundamental aerodynamics of a MAV flying at low speed and low Reynolds's number. The study clearly 

indicates that micro scaled wind tunnels are certainly bringing infinite possibilities to studying and 

understanding the in-flight characteristics of very small aircrafts flying at the low speeds and having low aspect 

ratio. 

Keywords: Subsonic Wind Tunnel; Low Cost; Test Section; Aerodynamics; Fluid Flow; Velocity Profile. 

 

 

------------------------------------------------------------------------ 

* Corresponding author.  

http://asrjetsjournal.org/


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1. Introduction  

The concept of a Micro Aerial Vehicle (MAV) was introduced by the Defense Advanced Research Projects 

Agency (DARPA) in the mid-1990s as an aircraft measuring less than 6 inches in all dimensions. The design of 

these vehicles were purely based on the experimental flight testing because of its inaccuracy of available 

aerodynamic design solvers at low Reynolds numbers and low aspect ratio flight [1]. Complete literature 

surveys of this area of research can be found in the reference [2,3]. Micro air vehicles are associated with low 

Reynolds number ranging approximately 200,000 or lower. In addition, due to limitation of size, fixed-wing 

MAVs are usually designed by very low aspect ratio wing.  MAVs that fly using flapping wings hold great 

potential for indoor investigation and hovering observation. 

Micro air vehicles have a large array of applications in military and civil domains. It is used as surveillance by 

defense forces to scout enemy activity, provide combat information and to take aerial photographs of the 

immediate area with no risk to the soldier's life. It has the potential of a high precision tactical weapon. In 

addition, the US Air Force Research Laboratory has set a goal to develop a bird sized MAV by 2015 and an 

insect-sized MAV by 2030 [4]. Apart from that, some of the stimulating research is taking its insight from 

nature where the development of a number of flapping-wing vehicles have been made that mimics a bird or an 

insect flight.  They are ideal for MAV's as the turbulence in the air affects them much more than larger aircraft. 

Flapping wing miniature air vehicles (MAVs) offer several advantageous performance benefits, relative to 

fixed-wing and rotary-wing MAVs. Additional applications of MAV's can be used by law enforcements for 

traffic control, riot control, hostage rescues. Moreover it has applications in bio chemical sensing, weather 

forecasting, and inspection of pipes. 

 

Figure 1: Harvard Micro Robotic Fly 

The aerodynamics and the wing shape of a MAV plays a crucial role in the design process. It is determined 

based on the experimental data of a wind tunnel test. However, a preliminary design method must be made by 

the designer in order to determine the overall size and the shape of the vehicle. A theoretical design procedure is 

known as follows [5]:  

• Determination of the components and take-off weight. 

• Evaluation of the cruise velocity  



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• Analyze the required lift co-efficient for a given area to maintain level flight: 

21
2

V

WC
Sρ

=ι.  (1) 

Where W is the weight of the aircraft, V is the estimated cruise speed, S is the wing area. 

• Apply the wind tunnel data to create predictions of drag co-efficient and lift co-efficient vs. α for 

different platform profiles. 

• Calculation of the angle of attack that is needed to attain the lift co-efficient for changeable wing area 

and aspect ratio and for different plan form shapes. 

• Selection of the wing plan form shape which has the lowest required α and lowest drag co-efficient at 

that α.  

• Selection of the aspect ratio and wing area which is most suited for mission requirements. 

In the past years only a few literature reviews were published associated with the aerodynamics of the micro air 

vehicle.  Mueller and his colleagues [6] designed and constructed a new plan form having force and moment 

equilibrium to execute lift, drag and moment capacity on MAVs at the low Reynolds numbers. A finite element 

flow solver based on unstructured grid was designed by Ramamurti and his colleagues [7] to compute the lift 

and drag force distinction for two MAV models by means of 6-inch and 14.5 inch wingspans correspondingly. 

Shyy and his colleagues  [8] reviewed the scaling laws of biological and micro air vehicles and in turn exposed 

the kinematics of flapping wings and aerodynamic models for examining lift, drag and power. Waszak [9] and 

Ifju and his colleagues [10] came with the conclusion that the membrane wing tested in their wind tunnel 

demonstrated potential benefits to develop the design of the future flight vehicles. Kellogg and his colleagues  

[11] used CFD (computational fluid dynamics) to investigate the aerodynamic coefficients of a numerous small 

fixed wing vehicles having its wingspan of 8 ~ 18 inches. Jones and his colleagues [12] investigated the 

flapping-wing propulsion for small-scale vehicles. The lift and drag of four wing types at the three chords 

Reynolds numbers of 70,000, 100,000 and 140,000 were deliberated by Torres and his colleagues [13] based on 

a sensitive force balance in a wind tunnel test. 

The following section of the literature will discuss the design of wind tunnels that would facilitate the 

construction of a smaller scaled tunnel in order to test the application of MAV to evaluate its in-flight 

distinctiveness. The results based on the wind tunnel test will determine the appropriate MAV to be designed 

and fabricated with detailed discussions on ground and onboard components such as flight control unit, 

propeller, vision system, motor, battery and servos etc. Finally, the developed MAV prototype will be 

successfully tested in the real flight. See reference [14]. 

2. Wind Tunnels 

The field of wind engineering has been developing over centuries since 1871. The definition of a wind tunnel as 

is given by Pankhurst and Holder (1952), is, “A device for producing a moving airstream for experimental 



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purposes” [15]. Wind tunnel design has evolved with many fluid mechanics as well as engineering theories 

which are impossible to put out it in this paper. See reference [16]. Since the 1930s, when the strong effect of 

free stream turbulence on the shear layers became apparent, emphasis has been laid on wind tunnels with low 

levels of turbulence and unsteadiness [17]. Mehta and Bradshaw conducted a research on the whole design of 

low speed subsonic wind tunnel. 

 

Figure 2: Wind tunnel testing an airplane 

Wind tunnels can be classified into four groups which are differentiated by the Mach number or based on flow 

speed: 

i. Hypersonic wind-tunnels (M>5)  

The  development  of  new  methods  of  producing  hypersonic  wind-tunnel  flows  at  increasing  velocities  

during  the last  few  decades  is reviewed  with  attention  to  air breathing  propulsion,  hypervelocity 

aerodynamics  and  super orbital  aerodynamics [18]. These types of wind tunnels find their applications mainly 

in rocket and space vehicles. They are considered to produce a hypersonic flow having the velocity of the wind 

between Mach 5 and Mach 15 in the test section. It is the fastest operating wind tunnel in the world. It allows 

the physical properties of the flow to change rapidly. It operates continuously and air velocities of Mach 10 can 

be maintained for any length of time. In increasing the Mach number the hypersonic wind tunnel allows 

aerodynamicists to move on to designing robots, planes and missiles with greater speed. It should be designed or 

constructed to create the flow features of the given flow system. These flow features include the entropy layer, 

thin shock layer, viscous interaction and high total temperature of flow [19]. It is also obtained using 

convergent- divergent nozzles. New studies have found to reduce the noise of hypersonic wind tunnels.  They 

were developed to simulate hypersonic flow in flight, where the noise levels are very low. It requires a laminar 

boundary layer on the test section walls. [20] 

ii. Supersonic wind-tunnels (1<M<3)  

A supersonic wind tunnel consists of compressible flow effects when operating at normal conditions [21]. For 

supersonic wind tunnels, the Mach number is determined by the nozzle geometry and stagnation conditions [22]. 

This type of wind tunnel has its Mach number between 1 to 3; however the velocity of the air in test section of 



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such wind tunnels can reach up to Mach 5 which is obtained by using convergent - divergent nozzles. While 

designing the supersonic wind tunnel, it is necessary to determine the generation of supersonic flows in a nozzle. 

Due to the air flow caused by pressure differential (i.e. the flow of air from high pressure area to low pressure 

area) the supersonic flow can be driven by producing large enough pressure differential. A supersonic wind 

tunnel additionally includes compressible flow effects when operating at normal conditions [23].The power 

requirements for these wind tunnels are also very high. Supersonic wind tunnels are further classified into three 

type’s i.e. continuous, blow down and in draft tunnels.  

iii. Transonic wind-tunnels (M=1) 

A transonic wind tunnel was designed, created and regulated in order to offer an important tool to study the 

phenomena of transonic flow. Transonic wind tunnels with high aspect ratio (2D) are generally operated at 

higher Reynolds number, it shares the same calibration problems as symmetrical tunnels.[22] Transonic flow 

occurs when there is mixed sub- and supersonic local flow in the same flow field. Usually the supersonic region 

of the flow is terminated by a shock wave, allowing the flow to slow down to subsonic speeds. This complicates 

both computations and wind tunnel testing. It also means that there is very little analytic theory available for 

guidance in designing for transonic flow conditions [24]. This wind tunnel uses flow properties adjoining the 

transmission of a shock wave along a tube in order to create the transonic flow. In the transonic wind tunnels, 

the maximum velocity in test section can reach up to speed of sound i.e. 340m/s or has Mach number equal to 1. 

These wind tunnels are widespread in the aircraft industry as most aircrafts operate around this speed. 

iv. Subsonic or low-speed wind-tunnels (M<1)  

The Mach number is less than 1 and generally comes out to be around 0.4 for this case [25]. The maximum flow 

speed of a subsonic wind tunnel is measured to be 135m/s. The air density remains constant while decreasing 

the cross-sectional area allows the flow to increase its velocity and decrease the pressure. Likewise by 

increasing the cross-sectional area the velocity decreases while the pressure increases. For a subsonic wind 

tunnel, the test section is placed at the end of the contraction section and upstream of the diffuser. The 

immediate advantage of this wind tunnel would be to provide the initial operational experience and evaluation 

of the magnetic balance at a considerably reduced operating cost [26]. They are the most inexpensive compared 

to the others because of its simplicity in design and the low wind speed. Generally such wind tunnels are found 

in schools and universities because of its low budget. 

 

Figure 3: Low Speed Subsonic Wind Tunnel, used for testing vehicles with Low Reynolds number 



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Table 1: Categorization of flows based on Mach number 

 

Type 

 

Gas 

Velocity 

 

Gas or Air 

Movement 

Technique 

 

Test Duration 

 

Model 

Movement 

(during test) 

 

Test Characteristics 
References 

 

Subsonic 

 

Under 

600  

mph 

 

Motor-driven 

propellers or 

jet engine 

exhaust 

 

Up to hours in 

length 

 

Can be 

extensive 

 

Half low-speed data 

acquisition (100 sps) 

and half high speed (to 

40 KC sps). Variations 

of tunnel operations. 

Variations of 

aerodynamic model 

altitudes. Essentially 

steady-state testing 

IBM 1800 for 

Wind Tunnel 

Data 

Acquisition, 

pg. 3. 

 

Supersonic 

 

Mach 1 to 

Mach 3-5 

 

Pumped 

chambers or 

motor-driven 

 

A few minutes, 

depending on 

velocity, tunnel 

size and tank 

storage 

 

Usually 

limited to 

one or two 

model 

changes 

High speed data 

acquisition (5 to 50 

KC sps). Some 

variation of tunnel 

operations. Few model 

altitude changes. 

 

Hypersonic 

 

Mach 3-5 

to Mach 8 

or more 

 

Pumped 

chambers 

 

Up to one or 

two minutes, 

depending on 

velocity, tunnel 

size and tank 

storage 

 

Usually 

limited to 

one or two 

model 

changes 

 

Same as supersonic 

 

Shock 

wave 

  

High 

pressure 

change in 

sealed 

chamber 

 

Very short 

fractions of a 

second 

 

No model 

changes 

High speed data 

acquisition only (10-

50 KC sps). Start- stop 

test only 



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Wind tunnel configurations are based on their shape and there are two major types i.e. the open type circuit 

tunnel and the closed type circuit tunnel. It also includes an intake and exhaust.  

The open type wind tunnel has both the ends open. It consists of one or more honeycombs that are used to clean 

the impurities in the air as there are higher chances of dirt particles entering the system through the air. The 

power needed to drive the wind tunnel is usually high because of the loss of energy in the outgoing air. The 

open circuit wind tunnel is the simplest and most affordable to build. In these tunnels air is expelled directly into 

the laboratory and typically reinvested after circulating through the lab, though some tunnels utilize instead a 

compressed gas source. The open system however has the problem with excessive noise production limiting the 

hours that it can be run, however since this wind tunnel is not expected to require utilization late at night this 

should not be a problem [27].  In addition to their low costs, open circuit tunnels are also advantageous because 

they have are relatively immune to temperature fluctuations and large disturbances in return flow, provided that 

the volume of the laboratory is much greater than that of the tunnel [28]. 

Open type wind tunnels are divided further into two categories which are differentiated by the location of the 

fan: 

i.Suck down tunnel: It consists of a centrifugal blower or an axial fan that is placed after the test section to draw 

the air in, due to the inlet being open to the atmosphere. This type of wind tunnel is usually not preferred since 

the air that is entering is of momentous swirl.  

Suck down tunnels are typically more susceptible to low frequency unsteadiness in the return flow than blowers, 

though some claims have been made that intake swirl is less problematic in these tunnels because it does not 

pass through the fan before entering the test section [29]. 

ii.Blower tunnel: A blower tunnel consists of a blower that is mounted at the inlet which is used for driving the air 

flow into the wind tunnel. However, swirling takes place in this tunnel as well but blower tunnels are much less 

sensitive to it. 

 

Figure 4: Diagram of an Open Type Wind tunnel 

In the closed circuit wind tunnel since the outlet is connected to the inlet, the same air is allowed to circulate 

throughout the system in a regulated way. It has low chances of dirt entering the system and the power required 



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to obtain a given flow speed is less compared to open circuit tunnels. It forms an enclosed loop in which exhaust 

flow is directly sent to the tunnel inlet. Closed wind tunnels are proposed warily to make the most of the 

uniformity of return flow. Based on research by the Tatman, these tunnels are usually large and more difficult to 

build. These tunnel need to be designed carefully in order to maximize the uniformity of the return flow [30]. 

The closed system can be fitted with heat exchangers so that it can be used for experiments that are temperature 

sensitive or require thermal modeling. Because most of the anticipated research would not be temperature 

sensitive or need thermal modeling it would be suggested to use the open circuit design to save on total costs 

[31].  

 

Figure 5: Diagram of a Closed Type Wind Tunnel 

3. Settling Chamber 

The settling chamber is the largest rectangular or a square cross section as well as the most important 

component of a wind tunnel.  The usual arrangement consists of a honeycomb (with about 25000 cells) followed 

by screens, the number and K-value depending on the turbulence level requirements [32].  It is placed at the 

entrance of the contraction cone and is used to smooth the flow by eliminating swirl and unsteadiness from the 

flow. The less turbulent, the better will the wind tunnel simulate flying conditions.  When a high quality flow is 

required, some devices can be installed to increase the flow uniformity and to reduce the turbulence level at the 

entrance of the contraction. The most commonly used devices are screens and honeycombs. Both devices 

achieve this goal by producing a relatively high total pressure loss; however, keeping in mind that the local 

dynamic pressure equals to 1/N2 of the reference dynamic pressure, such pressure loss will only be a small part 

of the overall one, assuming that N is large enough [33]. 

i. Honeycomb: The honeycombs are made of hexagonal, circular and square cells as shown in the figure. 

These cells can are aligned in the flow direction in the settling chamber and is used to reduce mean or 

fluctuating variations in transverse velocity with little effect on stream wise velocity because the 

pressure drop through a honeycomb is small. They are used when flows yaw angles are not greater than 

about 100. The hexagonal honeycomb is most widely used compared to other shapes as it reduces 

turbulent flow. See reference [34]. 



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ii. Screens: The screens are placed downstream of the honeycomb or sometimes at the inlet section to 

reduce turbulent intensity in the flow and to avoid any unwanted objects from entering the tunnel. For 

large swirl angles, the screen meshes should be placed before the honeycomb. It is characterized by its 

open area ratio, which is defined as:  

2

1 d
L

β  = − 
 

                          (2) 

Where 

 d = wire diameter 

 L= length of the screen. 

At least one screen in the settling chamber (ideally the last) should have an open-area ratio of ß<0.57, as screens 

with lower ratios are known to produced non uniformities in the flow. This is presumable due to the formation 

of small vortices created by the random coalescence of tiny jets emitted from the screen. [35] The number of 

screens required depends on the flow quality requirements in the test section. As the number of screens increase, 

the power requirement increases as well. The effectiveness of screens and honeycomb in reducing turbulence is 

greatly affected by relatively minor physical damage; therefore, extreme care must be exercised in installing and 

maintaining honeycomb or screens if the turbulence reduction performance is to be maintained [36].  

4. Motor/ Fan Drive Unit 

The motor or the supply unit is used to drive the flow of air in the wind tunnel. In general, either an axial or a 

centrifugal type of fan is used, with the axial fan being the better selection for closed circuit tunnels as it 

increases the static pressure which is necessary to balance out the total pressure loss in the rest of the circuit. In 

the case of an open-circuit tunnel, swirl present in the flow out of the fan may be dissipated before the flow 

reaches the intake, but a remaining advantage of pre rotation vanes is that the flow velocity relative to the fan 

blades is larger than if the stator is absent or located downstream of the fan [37]. The fans having a high ratio of 

tip speed to the axial velocity typically produces the necessary pressure rise in the small blade area. The 

centrifugal impeller of the squirrel-cage type drives the wind tunnels that are fitted with a blower. The fan is 

used to draw the air in from the atmosphere through the honeycomb mesh or screen section. A control box with 

a pull chain control drives the fan motor between the off position and a low and high speed setting. This limited 

the range of possible experimental velocities [38].  

5. Contraction Cone 

The most important purpose of the contraction section is to accelerate the incoming flow from the settling 

chamber and supply it to the test section at preferred velocity. In designing the contraction cone for a wind 

tunnel, the usual design condition is that the velocity at the end of the cone must be fairly uniform [39]. This 



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section basically diminishes the cross-sectional velocity deviation and upholds the uniformity of flow. The size 

and shape of the contraction dictate the final turbulence intensity levels in the test section [40]. In general, a 

small radius of curvature is used at the entrance of the section and a large radius of curvature is measured at the 

exit of the contraction section. On the other hand, the boundary-layer division should be avoided at both the 

ends of this section. The contraction length is likely to be small so that large contraction area ratios are favored. 

Most analytical methods of contraction design have been based on Stokes' Stream Function for the steady 

axisymmetric flow of an incompressible in viscid fluid i.e. [41].Most analytical solutions were found by the 

method of separating variables in equation 1. 

1U
r r
∂Ψ

=
∂                                 (3) 

1V
r x
∂Ψ

= −
∂                                 (4) 

Where x, y are coordinates in the axial and radial direction and U, V are respective fluid velocities i.e. given by: 

2 2

2 2
1 0

x r x r
∂ Ψ ∂Ψ ∂ Ψ

− + =
∂ ∂ ∂                  (5) 

6. Test Section 

The test section is the basic element of wind tunnel on which the designs on different vehicles are usually made. 

All the aerodynamic models are mounted in the test section when the tunnel is activated with desired flow 

velocity. Aerodynamic performance of the models can be better matched to full-scale performance in a closed 

test section; however, an open jet permits far-field acoustic measurements at the cost of potential test section jet 

deflection, jet/collector interactions, and shear layer refraction [42]. The test section can be of various shapes for 

constructing the wind tunnel i.e. hexagonal, octagonal, rectangle etc. The ratio of width to height of a test 

section is generally chosen with the intended purpose of the wind tunnel in mind. For example, a wind tunnel 

intended for testing wing sections will usually be wider than it is tall (often in a ratio of 3:2), whereas a wind 

tunnel designed for testing of architectural models will normally be taller than it is wide [43]. It is generally 

designed on the foundation of effectiveness and aerodynamic considerations since cost of construction depends 

on the test section area. Length of the test section is typically equivalent to main dimension of the cross-section 

of the similar or twice of it. Moreover, the test section should also be supplied with facilities as per the testing 

necessity. A common rule of thumb in test section sizing is to have rectangular dimensions with a ratio of about 

1.4–1 [44]. The test section velocity is commonly determined as percentage variation from the average of the 

cross-section. The ideal test section has steady uniform velocity at the inlet having no cross flow with less or no 

turbulence and low operating cost. 



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7. Diffuser 

Barlow, Rae and Popestates that for a conical diffuser, the divergence half angle of the diffuser walls should be 

less than 3.5◦ for a “good” design [45]. Mehta states that the diffuser-included angle for a conical diffuser 

should be between 5◦ (for best flow steadiness) and 10◦ (for best pressure recovery) [46]. It is usually a duct 

with larger area that is installed downstream of the test section. This allows the air to pass smoothly out of the 

test section and to decrease the flow velocity while increasing the pressure. In the case of an open wind tunnel, 

the exit pressure of the air should be greater than the atmospheric pressure in order to avoid reverse of flow. 

This is a very crucial division in design as the acquired pressure rise diminishes the power constraint for the 

wind tunnel which is directly proportional to the cube of velocity. Therefore the foremost objective of diffuser 

design is to achieve the maximum pressure recovery at the least possible distance. To avoid the boundary layer 

separation, the cone angle of the diffuse is made 7º or lesser. There are two main types of the diffuser:  

i.Exit Diffuser: These diffusers are installed at the downstream of the working section and consist of a diffuser 

with gentle expansion having an angle not more than 5 degree. The design of these diffusers is well catered for 

by existing methods [47]. 

ii.Wide- angle Diffuser: This type of diffuser is normally placed between the blower and the settling chamber or 

between the fourth corner and settling chamber of a closed-circuit tunnel [48]. It is used for decreasing the 

length for a given area ratio rather than having an effect on the pressure recovery. Usually the total pressure rise 

in a wide angle diffuser is negative but small.  

8. Turning Vanes 

Turning vanes are generally chambered aerofoil of bent planes that are provided at the corners of the wind 

tunnel having two bends aligned 900 to each other in a closed circuit wind tunnel. It is used to allow the air to 

circulate smoothly in a controlled manner. These vanes are deliberately made as adaptable for even operation 

thus avoiding under/over tuning. The vanes were tested with a flap in a recent study where on one hand, the 

presence of flaps increases the pressure loss due to the increase of the friction surface, whereas on the other 

hand the flaps improve the quality of the upstream flow into diffuser 1 and might also affect the test chamber if 

the damped pressure perturbation is intense enough. The presence of flaps reduces the pressure loss by 

diminishing the turbulence level in the corner [49].  

9. Optical Components 

For an open-jet test section, a collector is required at the downstream end of the test section to capture the jet. 

The resulting flow pattern is highly dependent on the room characteristics and the test configuration/conditions 

[50]. By using Computational Fluid Dynamics the design of the area of the collector can be best determined. 

However it has disadvantages like the noise produced due to the boundary-layer scrubbing and the turbulent 

shear layer impingement on the collector. 

For closed-circuit tunnels where the ducts are fixed together by 90 degree bends, there will be flow separation 

that would lead to significant pressure drop, flow unsteadiness, and noise generation. Collar and Salter 



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developed some early designs for turning vanes [51]. Gelder and his colleagues . developed turning vanes with a 

low loss factor [52]. Mechanical vibrations generated by the drive system can propagate into the test section 

through ductwork and the ground. These vibrations must be minimized if aero-optic, transition, or acoustic 

measurements will be conducted. Vibration isolation devices consist of passive and active mounts, as well as a 

flexible bellows section [53]. 

10. Facility Characterization 

In order to determine if the design criteria of a wind tunnel is satisfied, it must be characterized. The most 

obvious test is to check that the facility operates safely at desired flow speeds, but beyond this a series of careful 

experiments are recommended [54]. Some of the types of characterization experiments are given below: 

i. Flow uniformity: There are a large number of experimental techniques available to measure the mean 

velocity profile [55] including transversing a pitot-static or pitot probe through the test section. It is 

proven and cost effective but a time consuming experiment. The flow uniformity is usually 

characterized as either as a min–max or an rms deviation from the mean velocity in the inviscid core 

flow of the test section [56]. 

ii. Turbulence characterization: The low turbulence in the test section for a numerous facilities are critical. 

Wind tunnels often isolates the effects of incoming free stream of turbulence. In aero acoustic studies, 

ingested turbulence can interact with model components and generate additional “leading edge” noise, 

contaminating measurements of other components of interest, such as airfoil trailing edge noise [57]. 

iii. Acoustics and vibration : The background noise of a facility must be assessed if the facility is used for 

acoustic measurements. It should have a background noise levels at least 10 dB below the acoustic 

source of interest in a test, constraints and requirements. [58]. 

11. Flow Visualizations 

Flow visualizations are qualitative examination of flow that are important in understanding the aerodynamic 

characteristics of MAV models in wind tunnels. Two different methods can be used to generate smoke for flow 

visualization. With the first method, smoke is generated by a device which allows kerosene to drip onto 

electrically heated filaments; the smoke is then funneled to a smoke rake. The rake has a filter bag and cooling 

coils which reduce the smoke temperature to approximately ambient before passing through the anti-turbulence 

screens and into the test section. With the second method, a fine wire is placed upstream of the model. This wire 

is coated with oil and an electric current is applied to the wire. As the wire gets heated, the small beads of oil 

formed on the wire burn, which gives rise to fine smoke streak lines. This technique, which has been described 

in more details in the reference [59]. This is referred to as the smoke-wire technique. 

Fluid that creates colored fog has been tried but never satisfactorily worked without seriously damaging the 

smoke machine and is not available anywhere. In order to get the colored smoke for flow visualization, we'll be 

using normal smoke fluid (white smoke) and wash the smoke with colored lighting. The white fog particles will 

defract the light and will make it look like colored smoke. Another alternative is to use theatrical colored 



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smoke pyrotechnics that actually make proper, real colored smoke - but they have the potential to stain fabrics 

and materials. The smoke when comes into contact can be hazardous and should be avoided breathing the 

smoke. 

12. Review Discussion  

While the technology development in the field of MAVs are drastically increasing, the demand for such wind 

tunnels are also enhanced at the same time. There are many domains of interest in MAVs in which the 

prospected effort can offer further approach to designers. For example: 

• Improvement of different mechanism models by using miniaturized strain gages, and a variety of 

dynamic characterization tests 

• 3-D motion tracking of wing deformation by filming the motions of the wing or using multiple camera 

angles.  

• Improvements in the load cell measurement techniques that measures the lift and thrust forces for each 

set of wings.  

• Unvarying Construction of Wing Techniques.  

As for the future works of a wind tunnel, these micro scaled apparatuses can be available for a broad range of 

testing and can be presented for educational and research purposes that can be accessed for future experiments. 

The construction of this tunnel is inexpensive and easy to use. It has the possibility to support the perceptive of 

aerodynamics implicated in upcoming student projects as well. 

13. Conclusion 

A survey of literature on the history of low cost wind tunnels and the local sections of the wind tunnel were 

discussed thoroughly in this paper. It is customary to construct a model prototype wind tunnel before the full-

scale wind tunnel to work out design problems early in the design cycle. The low cost wind tunnel testing will 

focus on flow stability and uniformity in the test section as well as the maximum attainable wind speed of 

various micro air vehicles.  

By constructing a wind tunnel the main goal is to carry out experiments on different types of airfoils in the 

laboratory. The current research can be extended to include other parameters like flow visualization over test 

specimens of different materials through smoke, installing six force sensors for measurement of yaw, pitch and 

roll & automation of tunnel for data acquisition. The idea of the design and development of a subsonic micro 

scaled wind tunnel that would generate on low speed will have high potential in studying the aerodynamic 

characteristics of these minute air vehicles like the flapping wing. These micro scaled wind tunnels are strongly 

recommended for the validation of concept and the design of micro air vehicles. Furthermore, the experimental 

analysis of a flapping wing MAV, will further determine if the prototype will be developed or not. Upcoming 

research with wind tunnels should embrace the expansion of a more sensitive measurement apparatus. 

 



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Acknowledgements 

We would like to acknowledge the authors and our assistant professor for their relevant contribution to the 

accomplishment of this study.  

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