Essay: Open-loop subsonic wind tunnel

ABSTRACT
Wind tunnel is a device used for defining various flow phenomena for various objects like cars, airplanes etc. In automobiles and airplanes various forces affect the efficiency and speed which calls for a testing of new vehicle-scaled models in a wind tunnel testing machine. From this, aerodynamic estimates can be calculated and used for vehicle and component sizing (profile edge rounding, bonnet angle, rear end taper, under body rear end taper etc.), performance estimates and evaluation.
By looking at the way a smaller model acts in the wind tunnel, a pretty good idea of how a real-sized vehicle of the same design will probably work can be obtained.
In this project work, we have designed an open-loop subsonic wind tunnel on Autodesk-INVENTOR. We have also fabricated the experimental setup and successfully visualized the air flow over an Aerofoil model mounted in the test section. This project can be further used by the students to carry out various aerodynamic tests like measuring the lift and drag forces on an automobile model or checking the stability of architecture structures etc.
It is a lot easier, cheaper and safer to build and test a model than to build and test on real models. Also the experiment can be conducted in a well-controlled manner rather than real life environment condition
CHAPTER-1
INTRODUCTION
WIND TUNNEL
History
The earliest wind tunnels were invented towards the end of the 19th century. The wind tunnel was envisioned as a means of reversing the usual paradigm: instead of the air standing still and an object moving at speed through it, the same effect would be obtained if the object stood still and the air moved at speed past it. In that way a stationary observer could study the flying object in action, and could measure the aerodynamic forces being imposed on it. The would-be aeronauts of the nineteenth century closely studied the flight of birds and began building flying machines patterned after avian structures. Their birdlike craft failed miserably. They quickly realized that in reality they knew nothing about the lift and drag forces acting on surfaces cutting through the atmosphere. To fly, man first had to understand the flow of air over aircraft surfaces. This meant that he had to build instrumented laboratories in which wings, fuselages, and control surfaces could be tested under controlled conditions. Thus it is not surprising that the first wind tunnel was built a full 30 years before the Wrights’ success at Kitty Hawk.
English military engineer and mathematician Benjamin Robins (1707–1751) invented a whirling arm apparatus to determine drag and did some of the first experiments in aviation theory. His first machine had an arm 4 feet long. Spun by falling weight acting on a pulley and spindle arrangement, the arm tip reached velocities of only a few feet per second.
Sir George Cayley (1773-1857) also used a whirling arm to measure the drag and lift of various airfoils. His whirling arm was 5 feet long and attained tip speeds between 10 and 20 feet per second.
The whirling arm provided most of the systematic aerodynamic data gathered up to the end of the nineteenth century. Its flaws, however, did not go unnoticed. Test results were adversely influenced as the arm’s eggbeater action which set all the air in the vicinity in rotary motion. Aircraft models on the end of an arm, in effect, flew into their own wakes. With so much turbulence, experimenters could not determine the true relative velocity between the model and air. Something better was needed.
That something better was a “wind tunnel.” This utterly simple device consists of an enclosed passage through which air is driven by a fan or any appropriate drive system. The heart of the wind tunnel is the test section, in which a scale model is supported in a carefully controlled airstream, which produces a flow of air about the model, duplicating that of the full-scale aircraft.
Figure 1.1 Whirling Arm
Fracis H. Wenham (1824-1908), a Council Member of the Aeronautical Society of Great Britain, is generally credited with designing and operating the first wind tunnel in 1871.
In 1941 the US constructed one of the largest wind tunnels at that time at Wright Field in Dayton, Ohio. This wind tunnel starts at 45 feet (14 m) and narrows to 20 feet (6.1 m) in diameter. Two 40-foot (12 m) fans were driven by a 40,000 hp electric motor. Large scale aircraft models could be tested at air speeds of 400 mph (640 km/h).
By the end of World War Two, the US had built eight new wind tunnels, including the largest one in the world at Moffett Field near Sunnyvale, California, which was designed to test full size aircraft at speeds of less than 250 mph.
The advances in computational fluid dynamics (CFD) modelling on high speed digital computers has reduced the demand for wind tunnel testing.
However, CFD results are still not completely reliable and wind tunnels are used to verify the CFD computer codes. For limited applications CFD can supplement or possibly replace the use of wind tunnels. For example, the experimental rocket plane Spaceship One was designed without any use of wind tunnels but where external turbulent flow is present; CFD is not practical due to limitations in present-day computing resources.
Wind tunnel
A wind tunnel is a device designed to generate air flows of various speeds through a test section. Wind tunnels are typically used in aerodynamic research to analyze the behaviour of flows under varying conditions, both within channels and over solid surfaces. The wind tunnel moves air around an object, making it seem like the object is really flying.
Figure 1.2 A wind tunnel test shows how a tennis ball moves through the air
Engineers can use the results obtained from the wind tunnel testing experiments to inexpensively tweak designs for aerodynamic performance without building numerous prototypes.
Types Of Wind Tunnels
Based on the path followed by the drawn air:-
Open return wind tunnel
Closed return wind tunnel
Based on speed of air in the test section:-
Subsonic wind tunnel High Supersonic wind tunnel
Transonic wind tunnel Hypersonic wind tunnel
Supersonic wind tunnel High Hypersonic wind tunnel
Based on the nature of the flow:-
Laminar flow wind tunnel
Turbulent flow wind tunnel
Boundary- layer wind tunnels are used to simulate turbulent flow near and around engineering and manmade structures.
Range of the Mach number, [M] Name of flow or conditions
M<1 Subsonic
M=1, or near 1 Transonic
1<M<3 Supersonic
3<M<5 High supersonic
M>5 Hypersonic
M>> 5 High Hypersonic
Table 1.1 Classification of various wind tunnels according to the Mach number.
PROBLEM STATEMENT
This report will focus primarily on the principle of working of a wind tunnel, the flow visualization inside a wind tunnel, materials required for fabricating a small scale, open loop, subsonic wind tunnel and various advantages, disadvantages and application of the wind tunnel.
OBJECTIVES
To design an open-loop, subsonic, suck-down type wind tunnel on Autodesk-INVENTOR.
To fabricate the experimental setup for educational and research purpose.
To get an impression of fluid flow around a scale model of a real object.
PROJECT SCOPES
To find the design fundamental for a small scale, open loop, subsonic wind tunnel.
Make the research for small wind tunnel background and construction.
To find the best material to be used and estimate the cost for model construction.
To study the flow visualization of an object design.
WORKING OF A WIND TUNNEL
Air is drawn through a duct equipped with a viewing window and instrumentation where models or geometrical shapes are mounted for study. For very large wind tunnels, several meters in diameter, a single large fan is not practical, and so instead a group of multiple fans are used in parallel to provide sufficient airflow.
The airflow created by the fans that is entering the tunnel is itself greatly turbulent due to the fan blade motion (when the fan is blowing air into the test section – when it is drawing air out of the test section downstream, the fan-blade turbulence is not a factor), and so is not directly useful for accurate measurements. The air moving through the tunnel needs to be relatively turbulence-free and laminar. To correct this problem, closely spaced vertical and horizontal air vanes are used to smooth out the turbulent airflow before reaching the subject of the testing.
Due to the effects of viscosity, the cross-section of a wind tunnel is normally circular rather than square, because there will be greater flow constriction in the corners of a square tunnel that can make the flow turbulent. A circular tunnel provides a smoother flow.
The inside facing of the tunnel is typically as smooth as possible, to reduce surface drag and turbulence that could impact the accuracy of the testing. Even smooth walls induce some drag into the airflow, and so the object being tested is usually kept near the center of the tunnel, with an empty buffer zone between the object and the tunnel walls.
Figure 1.3 Schematic of an open wind tunnel with a closed test section
The lighting is usually fixed into the circular walls of the tunnel and shines in through windows. If the light were fixed on the inside surface of the tunnel in a conventional manner, the light bulb would create turbulence as the air blows around it. Similarly, observation is usually done through transparent portholes into the tunnel. Rather than simply being flat discs, these lighting and observation windows may be curved to match the cross-section of the tunnel and further reduce turbulence around the window.
Various methods are used to study the actual airflow around the geometry and compare it with theoretical results, which must also take into account the Reynolds number and Mach for the regime of operation.
FLOW VISUALIZATION
Lift and drag are just two elements of aerodynamics forces that come into play inside a wind tunnel. For aircraft testing in particular, there are dozens of variables (like pitch, yaw, roll and many others), that can affect the outcome of experiments.
Figure 1.4 Smoke provides flow visualization so scientists can see how air is moving around the test object
Because air is transparent it is hard to directly observe the air movement itself. Several methods of visualizing the flow have been developed. Some of them include-
Tufts
Tufts are applied to a model and remain attached during testing. Tufts can be used to gauge air flow patterns and flow separation.
Figure 1.5 Wing with mini-tuft
Evaporating suspensions
Evaporating suspensions are simply a mixture of some sort or fine powder, talc, or clay mixed into a liquid with a low latent heat of evaporation. When the wind is turned on the liquid quickly evaporates leaving behind the clay in a pattern characteristic of the air flow.
Figure 1.6 China clay on a wing
Oil
When oil is applied to the model surface it can clearly show the transition from laminar to turbulent flow as well as flow separation.
Figure 1.7 Oil flow visible on a straight wing
Fog
The fog is transported inside the wind tunnel (preferably of the closed circuit & closed test section type). An electrically heated grid is inserted before the test section which evaporates the water particles at its vicinity thus forming fog sheets. The fog sheets function as streamlines over the test model when illuminated by a light sheet.
Figure 1.8 Fog wind tunnel
Sublimation
If the air movement in the tunnel is sufficiently non-turbulent, a particle stream released into the airflow will not break up as the air moves along, but stay together as a sharp thin line. Multiple particle streams released from a grid of many nozzles can provide a dynamic three-dimensional shape of the airflow around a body. As with the force balance, these injection pipes and nozzles need to be shaped in a manner that minimizes the introduction of turbulent airflow into the airstream.
High-speed turbulence and vortices can be difficult to see directly, but strobe lights and film cameras or high-speed digital cameras can help to capture events that are a blur to the naked eye.
High-speed cameras are also required when the subject of the test is itself moving at high speed, such as an airplane propeller. The camera can capture stop-motion images of how the blade cuts through the particulate streams and how vortices are generated along the trailing edges of the moving blade.
EXPERIMENTAL RIG
Figure 1.9 Wind tunnel available in the market
The unit is equipped with a bench, control panel, wind tunnel including an inlet cone, clear experiment section, outlet cone and screen; manual traverse unit, linear track with carrier; and main AC circuit breaker.
The lift and drag option on the H-6910 allows measurement of lift and drag forces on various shapes placed in the wind tunnels airstream. The readings are displayed on a digital meter and the selection between lift and drag is accomplished with the toggle switch located on the meter front panel.
DIFFERENT PARTS OF A WIND TUNNEL
Test Section
The test section is the “heart” of the wind tunnel where the model to be tested is located. The test section is the most delicate part of the tunnel, because it houses the model and includes the lift-and-drag sensory system. This part of the wind tunnel will have the highest air velocity. It will look like the following:
Figure 1.10 Test Section
Diffuser Assembly
This assembly houses the fan and the wind speed sensor. It is perhaps the easiest of the three assemblies to build. This is the largest assembly, and it is the only one that is made up of electronic components and wiring. It will look like the following:
Figure 1.11 Diffuser Assembly
Contraction Cone Assembly
This assembly will be at the forward end of the tunnel, into which the air will flow as it is drawn in by the fan at the back. This assembly consists of the Contraction Cone and the Settling Chamber. It has a provision for increasing the speed of air, taken from the room, before it enters the contraction cone. It will look like the following:
Figure 1.12 Contraction Cone Assembly
Honeycomb
It straightens the flow of air before it enters the contraction cone. A honeycomb naturally produces some turbulence of its own. The early wind tunnels which had a honeycomb but no screens (and usually a very small contraction ratio also) suffered from a very high turbulence intensity in the test section. Most of the modern tunnels have both honeycomb and screens.
Figure 1.13 Honeycomb
DEVELOPMENT OF PROTOTYPE
In this project work we have designed an open-loop, subsonic, suck-down type wind tunnel, fabricated the experimental setup and visualized the air-flow over an aerofoil model mounted in the test section.
We have used Autodesk-Inventor (CAD software) to develop the design of the wind tunnel. We have also used Autodesk-CFD software to analyze the design made on Inventor. This software was a great help to ensure the stability of the design through the various static-stress, pressure and velocity profile which could easily be obtained form it.
Entire body of the wind tunnel has been made from 18mm thick ply-wood sheets. And for the test section 5mm thick Plexiglass sheet (Acrylic Sheet) has been used. The flow visualization will be done using a High Speed Camera.
Figure 1.14 Wind tunnel design made on INVENTOR
CHAPTER-2
ADVANTAGES & DISADVANTAGES OF WIND TUNNEL
ADVANTAGES OF THE WIND TUNNEL
It is a lot easier, cheaper and safer to build and test a model than to build and test on real model.
Experiment can be conducted in a well-controlled manner rather than real life environment conditions.
Data acquisition and processing is simpler with direct connection to ground base equipment.
Dangerous, uncontrollable flight conditions will be safely investigated in a wind tunnel.
If one intends to run internal combustion engines or do extensive flow visualization via smoke, there is no purging problem provided both inlet and exhaust are open to the atmosphere.
DISADVANTAGES OF THE WIND TUNNEL
The main disadvantage of wind tunnel is that it is seldom possible to reproduce the condition of full scale motion exactly. This is mainly due to the use of scaled models for reason of tunnel cost and power consumption.
Also if located in a room, depending on the size of the tunnel to the room size, it may require extensive screening at the inlet to get high-quality flow. The same may be true if the inlet and/or exhaust is open to the atmosphere, when wind and cold weather can affect operation.
For a given size and speed the tunnel will require more energy to run. This is usually a factor only if used for developmental experiments where the tunnel has a high utilization rate.
In general, open circuit tunnels tend to be noisy. For larger tunnels (test sections of 70 ft. and more) noise may cause environmental problems, limit hours of operations, and/or require extensive noise treatment of the tunnel and surrounding room.
CONCLUSION: Because of the low initial cost, an open circuit tunnel is often ideal for schools and universities where a tunnel is required for classroom work and research and high utilization is not required.
APPLICATIONS/USES OF WIND TUNNEL
To determine aerodynamic loads:-
Wind tunnels are used to determine aerodynamic loads on the immersed body. The loads could be static forces and moments or dynamic forces and moments. Examples are forces and moments on airplane wings, airfoils, and tall buildings.
To study how to improve energy consumption by automobiles:-
They can also be used on automobiles to measure drag forces with a view to reduce the power required to move the vehicle on roads and highways.
To study flow patterns:-
To understand and visualize flow patterns near, and around, engineering structures. For example, how the wind affects flow around tall structures such as sky scrapers, factory chimneys, bridges, fences, groups of buildings, etc. How exhaust gases emitted by factories, laboratories, and hospitals get dispersed in their environments.
Other uses include:-
To teach applied fluid mechanics, demonstrate how mathematical models compare to experimental results, demonstrate flow patterns, and learn and practice the use of instruments in measuring flow characteristics such as velocity, pressures, and torques.
Figure 2.1 Testing of Different Types of Vehicles in a Wind Tunnel
CHAPTER-3
LITERATURE REVIEW
Design Construction and Performance Test of a Low Cost Subsonic Wind Tunnel
Md. Arifuzzaman, Mohammad Meshed
Wind tunnel is a device, by artificially producing airflow relative to a stationary body that measures aerodynamic force and pressure distribution to simulate with actual conditions. Wind Tunnels offer a rapid, economical, and accurate means for aerodynamic research. The most important aspect of wind tunnels is their ability to accurate recreate the full complexity of full fluid flow. In the current study, a low cost subsonic wind tunnel is designed, constructed and its performance is tested. The main focuses were to reduce the cost of construction and to erect it in a laboratory room.
The main design criteria used are listed in the table below:
Open circuit wind-tunnel.
Good flow quality (mean flow variation, turbulence intensities & temperature variation).
Contraction ratio, CR, of 8.
Test section is square and the maximum test section length possible in the available space.
Maximum flow speed in the test section of 40 m/s.
Low noise level.
Low cost.
Various design rules have been provided in here along with proper explanation and calculations for the important parts of a wind tunnel and pressure losses for a particular design that had been followed in this research paper were given.
The exact design on which they simulated the velocity profile at different position of the test section is given below.
Parameters Value
Type Open circuit
Test section length 1.35 m
Test section cross section 0.90 m × 0.90 m
Mean air velocity range 28 m/s
Overall length 7.35 m
Effective region in the test section 76% of width or height
Boundary layer region 12% of width or height from every wall
Contraction ratio 8
Honeycomb cell diameter, length 0.02 m, 0.125 m
Number of screens 2
Settling chamber cross section 2.55 m × 2.55 m
Motor and Fan 3-phase 20 kW, 10 blades
Table 3.1 Specifications of the newly designed wind tunnel
In a wind tunnel, pressure losses occur as consecutive pressure losses in the different sections. Overall pressure loss (pglobal) equals the pressure gain due to the fan. In a wind tunnel component, i, pressure loss (pi) can be written as the product of constant Ki and the dynamic pressure at the entrance of the component as shown in the following equation;
K_i = (∆p_i)/(1/2 ρ_i C_i^2 )
Where, Ci is the mean flow velocity in the concerned section at the entrance of component i
With the above criteria, the loss coefficients for each wind tunnel component can be calculated. Table 3.2 shows pressure drops for each wind tunnel component. Summing all the wind tunnel section pressure drop values produces the total pressure drop.
Components Pressure loss, ∆p [Pa]
Test section 9.76
Diffuser 61.42
Honeycomb 3.1
Screen 1 7.65
Screen 2 9.46
Contraction cone 2.05
Total Pressure loss 93.44
Table 3.2 Component pressure loss at cts = 40 m/s (cts = test section air speed)
Static pressure variation within the wind tunnel in ideal and real cases is reported in Fig. 3.1, while Fig. 3.2 shows incremental pressure loss.
Fig. 3.1 clearly shows lower pressure values in the real case compared to the ideal case up from test section to the fan section. From inlet to the honeycomb, the real pressure curve is always smaller than the ideal one which is due to the pressure losses throughout the wind tunnel.
The wind tunnel sections’ contribution to pressure loss is shown in Fig. 3.3. Clearly maximum occurs in the diffuser section and minimum occurs in the contraction cone.
Figure 3.1 Relative static pressure in the wind tunnel
Figure 3.2 Cumulative pressure losses in the wind tunnel
Figure 3.3 Pressure losses in the wind tunnel
Different data have been taken to construct the velocity profile in the test section and also the pressure readings at different point have been recorded. All data have been taken at maximum air velocity.
Figure 3.4 Vertical velocity profiles at three positions
From the velocity profiles it is clear that the mean velocity in the test section is almost linear. At 5 cm from the test section inlet, air velocity close to the wall is much less than the other positions (at 60cm and 115 cm). The maximum velocity is also found at 5 cm position. These happen because of the effect of the contraction outlet due to which vena contracta is formed. In case of 60 cm and 115 cm positions, velocity profile is almost similar which indicates that mean flow velocity throughout the test section is identical. For all cases, velocity is gradually increasing near the wall because of the boundary layer formed. This boundary layer region is approximately 12% of the total height of the test section in each side.
So, the effective flow height is found to be approximately 76% of the total height of the test section. The effective flow region is 10 cm from the bottom wall to 10 cm below the top wall. In case of vertical measurements, the mean flow velocity in the effective flow region is about 28 m/s.
Outcome of this Research
Comparison was made between this design and the wind tunnel built at NASA and MIT (USA) of approximately same test section is shown in table. From the comparison it is clear that the overall length of newly designed wind tunnel is much shorter than the other two. Besides this, the construction cost of the wind tunnel is about $8500 which is much less than the one available in the market of the same size.
Parameters New Tunnel NASA(Small) Tunnel MIT (USA) Tunnel
Test section 0.9 m × 0.9 cm 0.9 m × 0.9 m 0.85 m × 0.85 m
Mean velocity 28 m/s 25 m/s 40 m/s
Test section length 1.35 m 3 m 2.8 m
Overall length 7.35 m 13 m 11 m
Table 3.3 Comparison of newly designed wind tunnel with existing tunnels
Also measurements of the velocity in the empty wind tunnel showed a uniform field which is essential for using it for aerodynamic researches.
This research provided certain rules to be followed for the betterment of design of basic important components of wind tunnel. This will also help us to compare our design with the one provided in this research to know where the deviation is coming so that it can be sorted out.
Boundary-Layer Predictions for Small Low-Speed Contractions
James H. Bell and Rabindra D. Mehta
Contraction sections form an integral part of all wind tunnels, whether designed for basic fluid flow search or model testing. The main effects of a contraction are to reduce both mean and fluctuating velocity variations to a smaller fraction of the average velocity and to increase the flow mean velocity. The most important single parameter in determining these effects is the contraction ratio c. Contraction ratios of between 6 and 10 are found to be adequate for most small, low-speed wind tunnels – defined here as tunnels with a test section cross-sectional area of less than about 0.5 m2 and free stream velocities of less than about 40m/s.
The wall shape design of a contraction, of given area ratio and cross section, centers on the production of a uniform and steady stream at its outlet. These conditions generally can be met by making the contraction section sufficiently long. On the other hand, another desirable flow quality, namely a min¬imum boundary-layer thickness (in a laminar state) at the contraction exit, suggests that the contraction length should be minimized. However, the risk of boundary-layer separation near the two ends of the contraction increases as the length is reduced. In general, the boundary layer is less liable to sepa¬rate at the contraction exit, due to its reduced thickness caused by passage through the strong favorable pressure gradient. Also, the concave curvature at the contraction inlet has a destabilizing effect on the boundary layer, in contrast to the convex curvature near the exit that has a stabilizing effect.2 In addition to unnecessary thickening of the boundary layer, separation also generally leads to flow unsteadiness, which cannot be easily eliminated from the test section flow.
Figure 3.5 Typical Calculated Wall Velocities
A three-dimensional potential flow code (VSAERO) was used to compute the velocity distributions along the contrac¬tion walls. VSAERO uses a singularity panel method employ¬ing sources and doublets to solve the Laplace equation.
It was hypothesized in this study that for small, low-speed wind tunnels the boundary layers enter the contraction in a laminar state. In most small wind tunnels, the flow entering the contraction comes through a honeycomb and a series of screens (usually at least three). The effect of a screen on a turbulent boundary layer is to significantly reduce its thickness and turbulence stress levels and scales.
Table 3.4 Details of Contraction Used For Comparison
.2 .4 .6 .8 1.0
Figure 3.6 Wall contour shapes of contractions used for verification of the computational scheme (L is the contraction length and H, the inlet height)
Outcome of this Research
A scheme is proposed for the prediction of boundary-layer development in small, low-speed contraction sections. The wall pressure distributions, and hence the wall velocity distri¬butions, are first calculated using a three-dimensional poten¬tial flow method. Although a panel method was used in this investigation, in principle, any potential flow solver should be acceptable. Once the wall velocities have been obtained, the boundary-layer behavior can be adequately calculated, rather than relying on some separation criterion. For the family of contractions discussed in this Note, the assumption of a lami¬nar boundary layer originating at the contraction entrance and remaining laminar in passage through it seems justified. The measured boundary-layer momentum thicknesses at the exit of four existing contractions, two of which were three-dimen¬sional, were found to lie within 10% of the predicted values, with the predicted values generally lower. The present results indicate that the relatively simple Thwaites method is probably adequate for most purposes. If the prediction accuracy of within 10% on 0 is acceptable, then the present results also suggest that an iterative process, accounting for the boundary-layer displacement thickness, is not necessary.
This helps us to properly design the contraction cone for proper flow of the fluid to remove any errors.
Design Rules for Small Low Speed Wind Tunnels
R. D. MEHTA and P. BRADSHAW
Even with today’s computers, a wind tunnel is an essential tool in engineering, both for model tests and basic re¬search. Since the 1930s, when the strong effect of free-stream turbulence on shear layers became apparent, emphasis has been laid on wind tunnels with low levels of turbulence and unsteadiness. Consequently most high performance wind tunnels were designed as closed-circuit types to ensure a controlled return flow. How¬ever, as will be seen below, it is possible with care to achieve high performance from an open-circuit tunnel, thus saving space and construction cost. ‘Blower’ tunnels (with the fan at entry to the tunnel) facilitate large changes in working section arrangements; to cope with the resulting large changes in operating conditions, a centrifugal fan is preferable to an axial one. For ease of changing working sections the exit diffuser is often omitted from small blower tunnels, at the cost of a power factor greater than unity.
Outcome of this Research
Once the tunnel power factor has been estimated and the required fan static pressure rise determined, one can set about the selection of the optimum fan size. The dynamic pressure rise through a blower is usually ignored and can be thought of as a safety factor in the calculations.
The fan outlet flow will be least turbulent when the fan is operating near maximum efficiency. Fan efficiency is a function of the dimensionless flow rate; the pressure rise coefficient is a (weak) function of the dimensionless flow rate also, so that requiring maximum efficiency specifies both dimensionless flow rate and pressure rise coefficient. So for a given required flow rate and pres¬sure rise, two equations are obtained which can be solved to give the fan size and optimum operating rpm. In practice the manufacturer’s performance charts should be searched for a fan size (and rpm) giving near maxi¬mum efficiency for the required flow rate and pressure rise.
Various types of fans, blower have been discussed and the pros and cons of each are given and proper selection process has been shown for particular type of application. Also the blade design calculation is obtained.
The design parameters and effect of each individual components of wind tunnel such as screens, settling chamber, honeycomb, contraction cone, working sections etc. Different parameters such as wall shape, spacing between screens, length, open area ratio, contraction ratio for different components have been made known.
Review of Design and Construction of an Open Circuit Low Speed Wind Tunnel
Mansi Singh, Neha Singh & Sunil Kumar Yadav
One of the most important parts of a wind tunnel is the flow visualization it provides. Sure lift, drag and efficiency can all be calculated with complex equations. However, it is the visual aspect of a wind tunnel and the controllable environment it provides that allows you to physically see what will happen in multiple real life situations. You can create an environment where you can see how a plane will react when it is taking off, cruising and landing all in the confines of a test lab. Then, with the same machine, you can see how air flows over the body of a race car when it is zooming around a track to maximize its efficiency. The versatility and tangibility of a wind tunnel is what makes it such an important part
Test Section Length – 50 cm
Test Section Diameter – 30cm
Settling Chamber Diameter – 60cm
Contraction Ratio – 4
Honeycomb Thickness/Cell Size/Cell Count – 10cm
Max Mean Air Velocity – 5.7 m/s
Max Mean output- 4.024 m/s
Diffuser angle- 2.54⁰
Drag and Lift calculations and various observations were done on this designed wind tunnel.
Lift and drag coefficients calculated for the test section with velocity 5.7m/s has been calculated as-
Cl = 1.30634
Cd = 6.14588
Figure 3.12 Velocity Profile of the Airfoil
Outcome of this Research
The profile shows that the fluid i.e. smoke flowing inside the tunnel has high turbulence.
Lift and drag coefficients for the test section with velocity 5.7m/s has been calculated for the aerofoil of the weight 0.42 kg.
To show the effect of the velocity on the aerofoil we need a velocity more than 10 m/s. Thus after achieving variant velocities by attaching a drive to the exhaust and obtaining a velocity above 10m/s we can show the effect of the variation in velocity on the aerofoil inclination through this model.
This model is suitable for an aerofoil of weight less than 0.15 kg. And the study can be done using different aerofoils with variant weights, materials and designs.
CHAPTER-4
DESIGN & CALCULATION
BASIC DESIGN CRITERIA
The main design criteria used are as follows-
According to construction: Open loop type of wind tunnel.
According to position of the fan: Suck down type of wind tunnel.
According to the Mach number: Subsonic type of wind tunnel. (Mach Number < 1)
The fluid considered for the design process is compressible .The fluid is atmospheric air at room temperature.
DESIGN PROCESS
First step was to decide the type of wind tunnel. We have designed a sub-sonic, open loop wind tunnel for which the Mach number has to be less than 1. For prototype making purposes, the Mach number has to be less than 0.3. Beyond that the effect of compressibility has to be taken under considerations.
The next step was to design the test section. This can be designed as per the user’s criteria. The only parameter to be kept in mind is that the length should be 0.5 to 3 times the hydraulic diameter.
Now, the contraction cone was designed. The contraction ratio has to be between 6-10.
We have chosen 7 which is the most optimum value. Contraction section consists of three parts viz. settling chamber, contraction nozzle and settling chamber before test-section.
For contraction ratio 7, the length of settling chamber before the test section varies from 0.5-1.5 times the exit radius. We have chosen 1.5
Figure 4.1 Open loop subsonic wind tunnel
We have chosen 7 which is the most optimum value. Contraction section consists of three parts viz. settling chamber, contraction nozzle and settling chamber before test-section.
For contraction ratio 7, the length of settling chamber before the test section varies from 0.5-1.5 times the exit radius. We have chosen 1.5
Settling chamber is always taken as 0.5 times the inlet diameter.
L/D of contraction cone varies from 0.67 to 1.79. This consists of the contraction nozzle and the settling chamber before the test section. We have taken this as 1.038
Now comes the diffuser. The outlet diameter of the diffuser depends on the bore diameter of the tube-axial fan selected. The area ratio should be between 2-3 with lower values being more preferable.
The equivalent cone angle should be between 2-3.5, again with lower values being more preferable. Our design has the area ratio of 2.25 and equivalent cone angle 3°.
COMPONENTS OF THE WIND TUNNEL TO BE DESIGNED
Test section
Contraction section
Diffuser section
Settling chamber
Fan
DESIGN AND CALCULATION OF VARIOUS COMPONENTS
Test section
It is the most important part of the wind tunnel which is to be designed first based on the model to be tested.
Cross section – 200mm x 200mm based on the model to be tested.
Length – it is taken as 200mm based on the variable length of the model to be tested.
A exhaust fan is used for suck down type based on following specification of heavy duty exhaust fan
So, Reynolds’s number is given by – (QD_H ρ)/(μ A) = 4.064 x 105
Figure 4.2 Test section design in Inventor
Contraction section
We are considering the contraction ratio of 7.0025. Generally it ranges from 6-10 to take into consideration the parameters like boundary layer conditions, fabrication cost, size even though the energy ratio increases.
Contraction ratio = Area of inlet of contraction section
Area of the exit of contraction section
So, the dimension of inlet of the contraction section is 530 x 530mm.
The length can be determined as 1.5 times the radius (considering hydraulic length). This maintains the angle below 45o as per research of the contraction section. Or L/D=1.5
Where,
D is the hydraulic length = 530mm
So, length of contraction section = 400mm.
Test section settling length is also considered as 1.5 times the outlet radius of contraction cone. This is placed so as to avoid the effect of eddies formed due to change in the cross section.
Length of settling chamber before the test section = 150mm.
Figure 4.3 Contraction cone design in Inventor
Diffuser section
For diffuser section, one cross section is 200 x 200mm and other is 300 x 300mm based on the dimensions of the fan and maintaining the diffuser angle as 3, based on Jewel B. Barlow, William H. Rae, Jr. and Alan Pop “LOW-SPEED WIND TUNNEL TESTING” Third Edition.
So as to avoid the turbulent boundary layer conditions due to sudden increase in the cross section of the diffuser.
Equation used for calculation of the length is-
θ = tan-1 (R_2- R_1)/L = tan-1 1/2 (√((A_R – 1)))/(L⁄D_1 )
Where,
AR = A_exit/A_inlet
D1 = hydraulic diameter at inlet
R1 and R2 are the hydraulic radius at inlet and exit respectively.
Θe = diffuser angle
Figure 4.4 Diffuser section design in Inventor
So, the length for the diffuser is 950mm.
Settling chamber
The settling chamber has two parts-
Screens
Honeycomb: These are placed 2 cm apart, this makes the flow streamlined in the wind tunnel. The honeycomb has a size as of the cross section of settling chamber.
Length = 0.5 x hydraulic radius = 250mm