~~~Concept of Wind-tunnel~~~

Wind tunnels date back to the 1870’s. Scientists realized it didn’t matter if an object was stationary and air was blown was over the object or if the object was moving through the air. The resultant forces over the object would be the same. The idea of blowing air over an object and determining the forces lead to the invention of wind tunnel.

As the name suggests, a wind-tunnel is a cylindrical tunnel in which air is blown from one side by a huge fan at high speeds. The middle part of the tunnel is called as throat. It is the place where the test model is placed. Various sensors are attached to the test model and the data is collected in the room near it. The data collected issued to reduce the aerodynamic drag and minimize fuel consumption and also increase the speed of the cars, trucks, planes, etc. also aerodynamic properties like lift, drag, forces, moments are measured with the use of wind-tunnel. In wind-tunnel air is blown over the test model which is kept stationary at the throat. This will produce the same effect as it would be produced if the vehicle is moving at high speeds on a road. For more accurate results the tunnel is sometimes equipped with rolling road to prevent the boundary layer forming on the floor which may affect the test results.

* What are wind tunnels?
Just as its name suggests, a wind tunnel is a tube or tunnel that has man-made windblown through it at a certain speed. Scientists and engineers put a model of an airplane in the tunnel and then study the way air moves around the model. By looking at the way this smaller model acts in the wind tunnel, they get a pretty good idea of how a real life-sized airplane of the same design will probably fly. It is a lot easier, cheaper, and safer to build and test a model than to build and fly a real airplane.

* How do wind tunnels work?
Wind tunnels work on the idea that a stationary model with air moving around it behaves the same way a real, full-scale airplane moving through stationary air does. Sometimes only a part of an airplane, like a wing or an engine, is tested in a wind tunnel. The models, usually made out of steel or aluminum, that are tested are loaded with many instruments and sensors that report back to the computers in the control room. It's there that scientists, engineers, and technicians can begin to understand how the airplane is performing.

* How are wind tunnels used in aerospace research?
Scientists and engineers use wind tunnels to study the pressures, forces, and air flow direction affecting an airplane. Pressure is measured by small devices called pressure taps that are placed at various locations on the surface of the model. Forces are recorded by sensors in the structures that support the model in the test section. The direction that air flows around the model can be seen by the way tufts, small yarn-like strands attached to the model, flap around. Sometimes smoke is blown into the test section to make it easier to see how the air is flowing. From these different kinds of measurements, a great deal can be learned about the model being tested.

Wind tunnels vary in size according to their function. Some of the smallest wind tunnels have test sections that are only a few inches large and therefore can only be used with tiny models. Other wind tunnels have test sections that are several feet big. The largest wind tunnel in the world is at the National Full-Scale Aerodynamics Complex at NASA Ames Research Center, in the United States. Its 80 foot by 120 foot test section can fit a life-sized Boeing 737 inside! Wind tunnels aren't just used to test airplanes. Anything that has air blowing around or past it can be tested in a wind tunnel. Some engineers have put models of spacecraft, cars, trucks, trains, even road signs, buildings, or entire cities in wind tunnels to see how to improve their designs.

~~~History of Wind-tunnel~~~

English military engineer and mathematician Benjamin Robins invented a whirling arm apparatus to determine drag and did some of the first experiments in aviation theory. Sir George Cayley also used a whirling arm to measure the drag and lift of various airfoils. His whirling arm was 5ft long and attained top speeds between 10-20 ft/sec. However the whirling arm does not produce a reliable flow of air impacting the test shape at a normal incidence. Centrifugal forces and the fact that the object is moving in its own wake mean that detailed examination of the airflow is difficult.

Francis Herbert Wenham, a council member of the aeronautical society of Great Britain, addressed these issues by inventing, designing and operating the first enclosed wind-tunnel in 1871. Once this breakthrough had been achieved, detailed technical data was rapidly extracted by the use of this tool. Wenham and his colleague Browning are credited with many fundamental discoveries, including the measurements of 1/d ratios, and their revelation of the beneficial effects of a high aspect ratio.

The English man Osborne Reynolds of the University of Manchester demonstrated that the airflow pattern over a scale model would be the same for the full scale vehicle of a certain flow parameter were the same in both cases. This factor is now known as the Reynolds number. It is a basic parameter in the description of all fluid flowing situations including the shapes of flow patterns, the ease of heat transfer the onset of turbulence. This comprises the central scientific justification for the use of models in wind-tunnels to simulate real life phenomenon. However there are limitations on conditions in which dynamic similarity is based upon the Reynolds number alone. The Wright brothers’ use of a simple wind-tunnel in 1901 to study the effects of airflow over various shapes while developing their Wright flier was in some ways revolutionary. The wind-tunnel used by German scientist at Peenemunde prior to and during WWII is an interesting example of the difficulties associated with extending the useful range of large wind tunnels. It used some large natural caves which were increased in size by excavation and then sealed to store large volumes of air which could then be routed through the wind-tunnels. This innovative approach allowed lab research in high speed regimes and greatly accelerated the rate of advance of Germany’s aeronautical engineering efforts.

~~~Types of wind-tunnel~~~

Wind tunnels can be classified based on air flow speed in test section and based on shape.

Based on flow speed:

1. Subsonic or low speed wind-tunnels.
Maximum flow speeds in this type of wind tunnels can be 135 m/sec. this type of wind-tunnels are most cost-effective due to the simplicity of design and low wind speed.

2. Transonic Wind-tunnels.
Maximum velocity in test section of transonic wind tunnels can reach upto speed of sound i.e. 340m/sec.
This type of wind-tunnel is very common in aircraft industries as most aircraft operate around this speed.

3. Supersonic wind-tunnels.
Velocity of air in test section of such wind-tunnels can be upto 1700 m/sec. this is accomplished using convergent-divergent nozzles. Power requirements for such wind tunnels are very high.

4. Hypersonic wind tunnels.
Wind velocity in test section of such type of wind tunnels can measure between 1700m/sec to 5100 m/sec. This is also achieved using convergent-divergent nozzles.

Based on Shape:

1. Open-circuit wind-tunnel:
This type of wind-tunnel is open at both ends. The chances of dirt particles entering with air are more so more honey-combs are required to clean the air. Open type wind-tunnels can be further divided into 2 categories: a) Suck down tunnel b) Blower tunnel

a) Suck-down Wind-tunnel:
With the inlet open to atmosphere, axial fan or centrifugal blower is installed after test section. This type of wind-tunnel is not preferred because incoming air enters with significant swirl.

b) Blower tunnel:
A blower is installed at the inlet of wind tunnel which throws the air into wind-tunnel. Swirl is a problem in this case as well but blower tunnels are much less sensitive to it.

2. Closed circuit wind tunnel:
Outlet of such wind tunnel is connected to inlet so the same air circulates in the system in the regulated way. The chances of dirt entering the system are also very low. Closed wind tunnels have more uniform flow than open type. This is usually a choice for large wind tunnels as these are more costly than open-type wind tunnels.

~~~Construction of a Wind-tunnel~~~
In general, a standard wind tunnel can be divided into six components. There are some additional components which are sometimes employed to guide the flow by reducing turbulence.

Standard Components
There are six major components of a wind tunnel:
1. Filter
2. Settling Chamber
3. Contraction Chamber
4. Test Section
5. Diffuser section
6. Fan

1. Filter
Filters are installed at the inlet section of the turbine before settling chamber. As the name suggests, they are used to remove the dust and small dirt particles from the air. Dirt particles should not enter the system because it causes undesired turbulence in the flow. 2. Settling Chamber
Air cleaned through the filter enters the settling chamber. Main purpose of settling chamber is to straighten the flow. Turbulence from settling chamber is transferred to test section which is never desirable as it can result in undesired forces experienced in test section. For better simulation of experimental conditions, turbulence should be kept to minimum. Settling chamber section generally consists of honeycomb and wire mesh screens. This aids in smoothing the flow. Settling chamber should be designed long enough to accommodate honeycomb and mesh screens at appropriate intervals.

Also longer length of settling chamber helps in killing turbulency. A very long settling chamber may result in boundary layer separation. 3. Honeycomb
Honeycomb is basically a wider mesh with significant cell length in flow direction. Out of the cross-section choices of square, circular and hexagonal, later is generally the preferred choice. Honeycomb reduces the larger eddies and lateral turbulence of the flow. They also reduce the cross flow component. 4. Screens
Screens are wire meshes are installed to reduce the mean flow non-uniformities and fluctuations of the flow. They also remove larger eddies and cross flow components similar to honeycomb. 5. Contraction Chamber
Contraction is the major part of a wind tunnel because this section decides the flow properties in the test section. Area of contraction chamber decreases gradually along the length. Contraction area ratio is the single most important factor that decides the flow uniformity, possibility of flow separation and flow speed in the test section. For wind tunnel designing, contraction area ratio is chosen so as to avoid the flow separation because this decreases the effective area available for testing. Contraction chamber length is kept sufficient long to minimize boundary layer separation at the exit. 6. Test Section
This part of the wind tunnel is used for carrying out experiments or simulating experimental conditions. In this section, flow is almost longitudinal with negligible lateral components. Each wind tunnel has a limit on maximum flow velocity. The whole wind tunnel is designed based on the test section area, test section length and maximum flow speed desired.
Working Section Configuration - 12 sq. m. working section for model tests - alternative working section for scale 1:1 tests

7. Diffuser
This section is designed to recover the static pressure that occurs from entry of contraction chamber to exit of test section. To achieve this, the area of diffuser increases gradually along the length. A long diffuser section results in more pressure loss due to viscous effects. The recovery of static pressure from kinetic energy reduces the power required to drive the wind tunnel. 8. Fan
Axial or radial fans can be used to power the wind tunnels. Generally axial fans are preferred because they produce static pressure rise with no significant change in axial velocity.

Fan diameter:
More than 5 m
Power demand: over 2000 kW.

~~~Working of Wind-tunnel~~~
The basic idea of a wind tunnel is crude and simple. It's like a giant drainpipe that wraps around on itself in a circle with a fan in the middle. Switch on the fan and air blows round and round the pipe. Add a little door so you can get in a test room in the middle. In practice, it's a bit more sophisticated than that. Instead of being uniformly shaped all the way round, the pipe is wide in some places and much narrower in others. Where the pipe is narrow, the air has to speed up to get through. The narrower the pipe, the faster it has to go. It works just like a bicycle pump, where the air speeds up when you force it out through the narrow nozzle, and like a windy valley where the wind blows much harder, focused by the hills on either side.

Air comes from the fans at the front of the wind tunnel and moves through the testing area at speeds of up to 180mph. The room is lined with heating elements to simulate different heat layers that can occur like at the site of a power station. The temperature in this part of the tunnel can be controlled 90°C. The instrumentation is mounted on a high tech robot which is full of data acquisition equipment and is operated by remote control from the outside of the tunnel. The results of these tests show the drag and lift the car would have for a better development of the design.
~~~Measurement of Aerodynamic Forces ~~~

Every wind tunnel is equipped with analysis room which has the state-of-the-art measuring equipments and instruments.
Each company or racing teams have their own expert aero dynamists working full time along with the research and development team.
There four vital elements that play a part when it comes to interaction of airflow with test vehicle: 1. Reynolds number 2. Drag 3. Lift 4. Pressure

Firstly the variables for Reynolds’s number and pressure are calculated using different methods: 1. The velocity of air through test section is calculated by Bernoulli’s theorem. 2. Pressure is measured using pressure transducers. 3. Direction of airflow is determined by using yarn threads ahead of the test section. 4. Pressure on the object is measured by beam balances attached to the beam or string on which the model is mounted. 5. Pressure distribution on the model was traditionally measured by drilling holes into the object surface which were connected to separate manometers. 6. Pressure distribution is also determined by using pressure sensitive paint. Higher local pressure is indicated by lower fluorescence of paint. 7. Another method is using pressure sensitive belts. These consist of pressure sensors mounted on the belt. This belt is taped along the profile of the test object. 8. It can also be done by doing a wake survey. In this a pitot tube is used to obtain multiple reading downstream from test mode. 9. Airflow over the model is studied by introducing colored smoke upstream and analyzing how it flows over the aerodynamic profile.

After pressure is calculated it is put into the Drag and lift equation which already has the projected area of the particular model
Once the magnitude of these forces is established the different moments that act on the model can be calculated
The moments are: 1. Rolling 2. Pitching 3. Yawing
Apart from vehicle dynamics some other aspects that are tested are: 1. Heat transfer from the cooling system 2. Heat developed due to friction of air 3. Wind noise around the vehicle
The heat transfer is calculated using temperature sensors mounted on the model and also by infrared cameras.
The wind noise is recorded in decibels by mounting microphones at different spots on the test model.
The main types of test data in any wind tunnel are lift and drag. The design team used a sting and thrust balance system to measure the effects of lift and drag on their models. Lift and drag are measured by using a series of springs, a pivot, and a strain gauge, which are all part of the thrust balance system. When the air passes over the model, the pivots and springs create a tighter or looser strain on the gauge. Strain gauge sensors electrically respond to the change and to its intensities, and send a voltage change to the A/D board. The A/D board converts the electric signal into a digital computer language. The computer collecting the data will be an IBM 486SX, with sixteen input channels. Total and static pressure readings will be another important source of data. This data provides a way to find the true air velocity in the tunnel. The wind tunnel will have four pitot tubes, which will be able to measure static pressure and total pressure. By substituting in appropriate pressure-velocity equations, one can determine the velocity from the two pressures. true air speed = square root of ((2 x dynamic pressure) / air density) dynamic pressure is measured in pounds / square foot air density is measured in slugs / cubic foot Flow patterns are the most easily viewed test. The design team plan on using smoke, tufts, and paints to visualize flow patterns in the wind tunnel. Smoke will be used to show turbulence around the model. Paint and tufts will be the least expensive and most effective way to illustrate flow on the surface.

Air foils of various designs will be used in the wind tunnel. Additional experiments such as small models of automobiles have been considered. The sting, which is the support for the models, will be adaptable to almost any model. Proper placement of the sting and model in the test bed will be essential to quality data collection. In addition to the sting placement, there are also proper component placements. The pressure rakes, pitot tubes, thermocouples, and thermometer probes must be placed in way to avoid disturbing the air flow. An analog to digital (A/D) sixteen channel, 100 kHz converter board, a terminal connector, and a multiplexer are necessary in the data collection process.
Appropriate flow analysis software will be selected that is simple to operate, has a large range of functions, and that also has graphic display capabilities. The five important measurements that need to be found in the low speed wind tunnel are dynamic pressure, static pressure, total pressure, temperature and turbulence. Thermocouples, temperature probes, pressure rakes, pitot tubes and a force balance on the mounting sting are required for the wind tunnel to generate accurate data.
~~~Concepts of Automotive Aerodynamics~~~
We all know a little about aerodynamics. Stick your hand out of the window of a moving car and you can immediately feel the effects of airflow-lifting your hand up or pushing it down, in addition to moving it backwards. Place your hand perfectly flat in the wind, parallel to the ground, and you should only feel your hand being tugged backwards a little. That tugging force is drag. As you increase the angle your hand makes to the oncoming wind, your hand is forced upwards. This is lift. Decreasing the angle of your hand from the flat position will cause the wind to force it down, hence the term downforce. In both situations, the effects of drag increase because the surface area exposed to the airflow increases.

As applied to cars, the broad logic of these concepts is pretty easy to understand. Generate enough lift and you can fly, which is about the last thing you want a car to do. Turning lift upside down, however, creates downforce, which race cars use to corner faster. But all of this comes at the cost of drag, just like in the hand experiment. The trick is to find that magic balance of getting the most downforce with the least amount of drag. Doing so requires digging a little deeper into aerodynamic theory.

The point of automotive aerodynamics is to shape a car so that it disturbs the air it moves through as little as possible. Literally, aerodynamics means the study of the forces acting on a body moving through air. In our case, the body happens to be a car and the forces will depend on many factors such as the shape and size of the car, in addition to some of the properties of air.

Engineers measure power in units such as watts (W) or horsepower (hp). Torque is measured in Newton-meters (Nm) or pound-feet (lb-ft). Aerodynamicists determine how well an object slices through the air by using coefficients, which differ from other measurements in that they lack units. This is because unit less coefficients allow for the comparison of cars of different sizes and shapes. For example, a Honda Civic is shorter and has a smaller frontal area than a Mack truck, but by using the coefficient of drag (CD) as a standard of measure, the aerodynamic efficiency between the two can be fairly evaluated. In addition to CD, other commonly used measures are the lift coefficient (CL), and side force coefficient (CY).

By now you might be wondering what all the fuss is all about. The hand-out-the-window example is useful for explaining the concepts of lift and downforce, but doesn't do justice to the negative aspects of poorly designed aerodynamics. Aerodynamic drag hurts performance. It is chiefly responsible for the top-speed limitation on any car since it takes exponentially more power to maintain higher speeds. For example, it takes almost five times more power to maintain your car at 100 mph than it does to drive it at 60 mph. More drag (a higher CD) magnifies this effect, negatively impacting top speed and fuel consumption by requiring the engine to do more work.

There are three primary ways to reduce drag. One is to reduce the frontal or cross-sectional area of the vehicle. Think of a car punching a hole in the atmosphere while in motion. Cars with less frontal area punch smaller holes, thus creating fewer disturbances. Of course, there are few things you can do to reduce frontal area on a car beyond taking off the side mirrors and running on skinny doughnut tires.

A better way to reduce a car's Cd is to make it more streamlined. This happens at the design phase and involves implementing smooth curves on body panels, rounded lights, soft windshield-to-roof transitions, smooth A-pillar-to-side window treatment, and more.

The third method is a modification of streamlining and centers around using a flat or smooth tray under the car to allow air to escape with less drag. Without using an underbody tray, airflow beneath the car is highly turbulent, due to the clutter of mechanical bits like the exhaust system, driveshaft, fuel tank, shift linkage, and suspension. A smooth underbody also reduces lift by allowing the air to move faster and therefore at a lower pressure. The added stability from the reduced lift is why most supercars and race cars now have smooth underbellies. It's the most practical way to improve performance compared to reducing frontal area or streamlining the body.

Like drag, lift becomes a problem for cars at high speeds. Think about airplanes. To fly, airplanes have wings that generate a lifting force equal to or greater than their massive weight. A car body has a shape similar to that of an airplane wing, with a curved upper surface and a relatively flat underside. When moving, air going over the top of the car accelerates, while the air going under stays at approximately the same speed as the car. Given enough speed the car will lift like an airplane.

Bernoulli's principle states that faster air above the car has a lower static pressure than slower air below it, and so the car is literally being pushed upwards. The result is lift, a curse to almost all production cars. For example, the 1995 BMW M3 has a lift coefficient (Cl) of about 0.34, which means that a lifting force of approximately 500 pounds is generated by the body at 100 mph.

Certain high-end sports cars have found ways to eliminate lift. The Ferrari F430 actually generates downforce to the tune of about 300 pounds at 124 mph and 616 pounds at 186 mph. This is due to a reduced cabin height, low ground clearance of the smooth underbody, and very effective diffusers. Surprisingly, the F430 doesn't use add-on downforce producers such as front splitters or rear wings. It's a testament to Ferrari's racing pedigree that they can achieve this level of downforce and still maintain a relatively low Cd of 0.32.

The third aerodynamic factor to contend with the side force, which is similar to lift but acts upon the automobile from the side. Side force is often ignored on street cars, but for F1 and rally cars that experience yaw at high speeds, lift caused by air passing over and under the car from the side becomes a significant factor in handling and stability.

But why is lift so bad? You'd be right in assuming that no car produces such a dramatic amount of lift to be dangerous to drive. But lift does negatively affect performance in two ways. First, lift reduces the load acting on the tires. Since the maximum amount of traction available from each tire is a function of the load acting upon it, a reduced load means less available traction. Lift also causes extra drag called "induced drag," which is a good percentage of the overall drag on the car.

Now that we know a bit about the aerodynamic forces that act on moving vehicles and the coefficients that define them, it is helpful to understand how aerodynamicists collect and use this information. Aerodynamics is an incredibly complex field. Even with the use of complicated equations, the effects of aerodynamic forces can only be determined for simple scenarios. For something as complex as a car with rolling wheels, vents, and spoilers, all on a moving roadway, aerodynamicists have to rely on two general methods.

One is a computer simulation method called Computational Fluid Dynamics (CFD). With CFD, computer algorithms approximately solve aerodynamic equations for a given car design and airflow velocity. Extracting usable data requires extremely powerful super-computers and accurately digitized three-dimensional car models. Most large car companies and top Formula 1 teams use CFD to understand how air moves around the various parts on the car and improve troublesome areas where the air is not flowing smoothly. Although expensive and time consuming, the CFD method allows for many virtual designs to be explored before time and money is spent building an actual prototype.
The other method is the wind tunnel testing. For this kind of simulation, a one-third to one-half scale model is usually made. Air is then blown over the stationary model, which is positioned on digital force-measuring transducers. These force transducers measure the drag, lift, and side forces acting on the model. These results are used to calculate the drag and lift coefficients that we referred to earlier.

With the data gathered from CFD simulations and wind tunnel testing, engineers sculpt modern production cars with blended sleek curves, gently sloping windshields, and smooth underbodies. As a result, these vehicles possess performance features only dreamed about just a few years ago. Some of the benefits include improved fuel economy, better high-speed handling and acceleration, enhanced airflow to the engine, and higher top speeds.
Drag:

Aerodynamic efficiency of a car is determined by its Coefficient of Drag (Cd). Coefficient of drag is independent of area; it simply reflects the influence to aerodynamic drag by the shape of object. In theory, a circular flat plate has Cd 1.0, but after adding the turbulence effect around its edge, it becomes approximately 1.2. The most aerodynamic efficient shape is water drop, whose Cd is 0.05. However, we cannot make a car like this. A typical modern car is around 0.30.
Drag is proportional to the drag coefficient, frontal area and the square of vehicle speed. You can see a car travelling at 120 mph has to fight with 4 times the drag of a car travelling at 60 mph. You can also see the influence of drag to top speed. If we need to raise the top speed of Ferrari Testarossa from 180 mph to 200 mph like Lamborghini Diablo, without altering its shape, we need to raise its power from 390 hp to 535 hp. If we would rather spend time and money in wind tunnel research, decreasing its Cd from 0.36 to 0.29 can do the same thing.

Lift:
Another important aerodynamic factor is Lift. Since air flow above the car travels longer distance than air flow underneath the car, the former is faster than the latter. According to Bernoulli’s Principle, the speed difference will generate a net negative pressure acted on the upper surface, which we call "Lift".
Like drag, lift is proportional to area (but surface area instead of frontal area), the square of vehicle speed and Lift Coefficient (Cl), which is determined by the shape. At high speed, lift may be increased to such an extent that the car becomes very unstable. Lift is particularly serious at the rear, you can easily understand, since a low pressure area exists around the rear screen. If the rear lift is not adequately counter, rear wheels will become easy to slip, and that is very dangerous for a car travelling at something like 160 mph.

Fastback is particularly bad in this aspect, because it has a very big surface area in contact with air flow. It seems that good drag and good lift are mutually exclusive; you can't have both of them.

However, as we did more research on aerodynamics, we found there are some solutions to achieve both of them.
Cd World Record:
Cd Year Model Cd | Year | Model | Remark | 0.137 | 1986 | Ford Probe V | Concept Car | 0.19 | 1996 | GM EV1 | Electric Car | 0.25 | 1999 | Honda Insight | Hybrid Car | 0.25 | 2000 | Audi A2 “3-Litre” | --- | 0.26 | 1989 | Opel Calibra 2.0i | Base Model | 0.26 | 2000 | Mercedes C180 | --- | 0.27 | 1996 | Mercedes E230 | --- | 0.27 | 1997 | VW Passat | --- | 0.27 | 1997 | Lexus LS400 | --- | 0.27 | 1998 | BMW 318 | --- | 0.27 | 2000 | Mercedes C-class | C200 upto C320 |

~~~Computational Fluid Dynamics~~~
(CFD)

To understand CFD lets break down the word CFD:
Computational - having to do with mathematics, computing.
Fluid Dynamics - the dynamics of things that flow.

CFD uses computers to simulate fluid (e.g., air and water) flow over digital models, such as those produced by Computer-Aided Design (CAD) software. The nature of the CFD analysis techniques means that full-size models can be simulated directly, avoiding the Reynolds number matching and wall problems faced when using a wind tunnel. Using CAD models means that CFD can simulate flow over virtual models without the need to make costly physical models.

So far so good, CFD sounds like a great replacement for the wind tunnel. However, CFD makes assumptions and approximations in solving the governing flow equations, called the Navier-Stokes equations.

In so doing the accuracy of CFD results suffers, especially relative to turbulence. Modeling turbulence in CFD is problematic - many turbulence models are tuned for specific flow regimes and are not generally applicable. Another issue with CFD is the need for a mesh to cover the entire 3D flow domain. Generating a mesh that adequately resolves boundary layers on surfaces and yet doesn't over-resolve regions of little interest without producing degenerate elements is a non-trivial, often time-consuming, task. The inevitably large meshes necessary for complex configurations, such as F1 cars, require relatively long run times to obtain results. Once a model is manufactured and mounted in the wind tunnel, an automated alpha sweep can be orders of magnitude faster than performing a series of equivalent CFD simulations.

Once you have the results from a CFD simulation, it is relatively easy to visualize flow features throughout the flow domain and extract plots of flow variables. In contrast, extracting data (other than forces) from a wind tunnel simulation requires prior (and often expensive) model instrumentation.

Wind tunnels have advanced in the period since CFD appeared (possibly in response to competition from CFD), with innovations such as: * Pressure-Sensitive Paint (PSP), which is a match for the colorful pressure contours produced by CFD visualization * Particle Image Velocimetry (PIV), which allows wind tunnels to produce non-intrusive velocity field visualization, mimicking those from CFD * Rapid manufacturing techniques, such as stereo-lithography (using the same CAD geometry that feeds CFD) making wind tunnel model construction faster and cheaper - making it more competitive with CFD model generation
Often the pairing of wind tunnels and CFD simulations can be used to gain advantage, such as: * Using wind tunnel data to validate CFD for a specific application * Wind tunnels can perform detailed investigations into CFD anomalies and vice versa * CFD can calculate wind tunnel wall correction factors * Using CFD to narrow down design constraints and parameters for further detailed analysis in wind tunnel tests * CFD can identify important flow regions to study further in wind tunnel tests using instrumented models

To many observers the rivalry between wind tunnels and Computational Fluid Dynamics (CFD) is a zero-sum game - as CFD matures it simply replaces wind tunnels. However, this is far from the truth. Often you'll find wind tunnels and CFD used together in a symbiotic process where one technique fills in knowledge gaps left by the other.

For example, as well as having the latest CFD software running on some of the world's most powerful computers, most Formula 1 (F1) teams also either have their own, or have access to, state-of-the-art wind tunnels. The same is true of NASA and most large aerospace companies, such as Lockheed Martin and BAE Systems. In most cases the wind tunnels are kept busy round the clock - clearly no sign here that CFD has displaced wind tunnels.

Benefits of CFD

* Insight of the design: |
If you have a device or system design which is difficult to test through experimentation, CFD analysis enables you to virtually see inside your design and see how it performs. There are many phenomena that you can witness through CFD, which wouldn't be visible through any other means. CFD gives you a deeper insight into your designs.

* Foresight of the design:
Because CFD is a tool for predicting what will happen under a given set of circumstances, it can quickly answer many 'WHAT IF?' questions. You provide a set of boundary conditions, and the software gives you outcomes.
In a short time, you can predict how your design will perform, and test many variations until you arrive at an optimal result. All of this can be done before physical testing.

* Efficiency:

The foresight you gain from CFD helps you to design better and faster, save money, meet environmental regulations and ensure industry compliance. CFD analysis leads to shorter design cycles and your products get to market faster. In addition, equipment improvements are built and installed with minimal downtime. CFD is a tool for compressing the design and development cycle allowing for rapid prototyping. | * Numerical Lab or Virtual Wind Tunnel: |
CFD results are directly analogous to wind tunnel results obtained in a laboratory – they both represent sets of data for given flow configurations at different Mach numbers, Reynolds numbers, etc. However, unlike a wind tunnel, which is generally a heavy, unwieldy device, a computer program (say in the form of CD) is something you can carry around in your hand. Or a source program in the memory of a given computer can be accessed remotely by people on terminals that can be thousands of miles away from the computer itself. A computer program is, therefore, a readily transportable tool, a “transportable wind tunnel”. Just imagine how many experiments you can do with this wind tunnel and at what cost? A countless number of experiments with negligible cost!!! | * Ability to Simulate Real Conditions: |
Many flow and heat transfer processes cannot be (easily) tested. Imagine a hypersonic vehicle entering into earth's atmosphere with Mach 20. Creating such a high speed flow in a wind tunnel is very difficult or rather impossible. CFD provides the ability to theoretically simulate any physical condition. | * Ability to Simulate Ideal Conditions: |
Can we study effect of viscosity on flow behavior? For example, what are the differences between laminar and turbulent flow over an airfoil for Re = 100,000? If this experiment is done in a wind tunnel, the flow will be viscous always. But CFD allows a great control on physical process, and provides the ability to isolate specific phenomena. For the above case, it’s just a matter of making one run of CFD with turbulence model switched off (laminar flow) and one run of CFD with turbulence model switched on.
There are numerous advantages of such kind and that’s why CFD tool is now-a-days playing major role in the design process of real life engineering applications.

* CFD v/s Wind-tunnel:
CFD holds great promise for replacing the wind-tunnel in coming years as the science behind CFD improves and computers become more powerful. Currently, CFD can provide results almost as accurate as a wind-tunnel that are often more useful due to the sophisticated visualization and domain wide measurements characteristic of CFD. For building services, CFD is an effective tool for simulating wind-tunnel climate to analyze pedestrian comfort, and pollution dispersion. It can also be used to assist engineers with natural ventilation design and building wind-tunnel.

* Current Status:
Wind-tunnel modeling is generally accepted in the scientific and engineering community. Wind-tunnel results have been proven to be representative of real world situations when the modeling correctly accounts for the features of the atmosphere and scaling is exact.

• Computational Fluid Dynamics (CFD) is a well-proven tool that was economically feasible only on mainframe computers until recent advances in computing made it possible to use a desktop PC. However, CFD results may not be as comprehensively comparable to real world results as most wind-tunnels results can be.

• Many CFD validation studies have shown quite comparable results to real world or wind-tunnel studies.

• Many CFD results that are criticized by academics as insufficiently exact are often quite satisfactory for engineering purposes because the degree of error is within reasonable bounds.

• Conservative assumptions can be applied to CFD studies to account for the higher potential degree of error.

The following are the advantages and the disadvantages and their related discussions:

ADVANTAGES | DISCUSSION | * Full domain analysis | Wind-tunnel need instruments to record wind speed at each discrete point. CFD by definition computes these variables throughout the whole study domain. | * Easy alternative analysis | CAD design of buildings in the CFD domain can be altered quickly and remodeling done immediately. Physical models require more time and effort for adjustments, especially if the design changes occur long after the initial wind-tunnel modeling or the wind-tunnel is booked for other projects. | * Cheaper overall | Same or lower cost and quicker turnover times to conduct the modeling in most cases. | * Better visualization of results | Results can be displayed in easy to understand graphical output. Wind-tunnel photographs cannot give nearly as much detail. | * Measuring wind direction, chemical reactions, radiation, etc. is difficult to do in a wind-tunnel. | CFD is generally more flexible at accounting for the unique aspects of each project. | * Proper wind-tunnel facilities are rare. | Wind-tunnel modeling requires large expensive equipment, which is why it is only conducted by several large international firms and universities. CFD modeling can be performed by local firms with better knowledge of local meteorological features. |

DISADVANTAGES | DISCUSSION | * CFD results can be erroneous. | Studies have shown that CFD results do not coincide with real world results in certain circumstances. However, the problem areas are well known and the error is often small enough to be accounted for with conservative assumptions for engineering purposes.Common problems are: 1) Overproduction of turbulent kinetic energy in building wake. 2) Over/under prediction of concentrations of pollutants at some locations. 3) Improper handling of vortex shedding with steady state models. | * Only experienced modelersshould use the software. | A recent study demonstrated that results can vary significantly depending on the modeler. Thorough knowledge of the atmospheric initialization and CFD meshing process is required to limit this. | * Projects cannot be too complex. | The size of the project modeled is limited by the computing power and software used. A large wind-tunnel is not so limited in the size & complexity of the model. Advancing computer technology continually expands the potential of CFD. | * Results are often better for lesscomplex projects. | The accuracy of the wind-tunnel results is not dependent on the complexity of the geometry. | * CFD yields steady state solution, transient solution is more time consuming. | Average wind field acceptable for certain applications including pollution dispersion and pedestrian winds (when turbulent kinetic energy is used to estimate gusts). Transient primary wind runs are used to locate time of worst conditions for conservative results. |
DATA AQUISISTION

The program contains a configuration editor for specifying data displayed and recorded, a data acquisition program offering graphical and channel data display, and a data exporter for viewing binary data or creating ASCII output files. The program is menu-driven and highly flexible. The Feildpoint system includes lightweight and compact analog I/O, digital I/O and thermocouple modules connected to a Pentium PC via a network module and RS232 cable. This modular hardware system may be easily expanded. A multichannel analyzer (MCA) PC card is used for data acquisition from the Climet optical particle counter. Data may also be recorded and displayed from a Condensation nuclei (CN) counter, either via serial cable or using a separate PC counter card. Typical data rates are 1 Hz, although 10 Hz is possible for some parameters.

~~~Conclusion~~~

The wind tunnel project was and will continue to be a learning experience for all the participants. During this study project, the team learned that responsibility and teamwork was important for the complete study of wind-tunnel. At first, it was difficult for the four students to learn to compromise. Each had their own ideas and had to learn to listen to other opinions that had equal merit. Eventually, however, they learned to value each member's contribution and worked together well…

~~~Webliography~~~ The following sites were referred for extracting information regarding wind-tunnel: * www.google.com * www.yahoo.com * www.howstuffworks.com * www.nasa-ames.com

~~~Appendix~~~

National Wind-tunnel facility users:

Academic Institutions: IIT Kanpur, IIT Delhi, IIT Bombay ISRO: VSSC
CSIR: NAL DRDO: ADA, ADE, ADRDE, ARDE, DRDL
CIVIL: BHEL, L&T, Gammon India, REL, PCTL, NTPC, DVC, IJT, Alstom India, Lodha Bellissimo, etc Wind Energy: Suzlon
Automobile: Tata Motors, M&M
Railways: RDSO, Contransys Pvt Ltd

Tunnel Utilization

Year | Wind OFFHours | Wind On Hours | Total Hours | Tunnel effort (Rs. In Lac) | Total project amount(Rs in Lac) | | | | | Paid | Notional | | 2006 | 580 | 190 | 770 | 80 | 38 | 130 | 2007 | 520 | 230 | 750 | 90 | 30 | 150 | 2008 | 540 | 298 | 838 | 155 | 20 | 220 |

Tunnel Utilization (in terms of income)

Tunnel Utilization (in terms of occupancy)

Occupancy forecast for next 3 years Sr # | Organizations | Wind OFFOccupancy (hrs) | Wind ON occupancy (hrs) | Total Occupancy (hrs) | 1 | VSSC | 500 | 300 | 800 | 2 | DRDO | 650 | 350 | 1000 | 3 | CSIR | 325 | 175 | 500 | 4 | Civil | 550 | 300 | 850 | 5 | Others | 250 | 150 | 400 | Total | 2275 | 1275 | 3550 |
Projected occupancy = 80% per year for paid projects.
Projected income = Rs 236 lacs/year (at the rate of Rs 20, 000/hr)

~~~Glossary~~~ Computational Fluid Dynamics: (CFD) a mathematical discipline that solves a set of equations governing the fluid flow over any geometric shape Contraction cone: The section of a wind tunnel where a large volume of low-velocity air is reduced to a small volume of high-velocity air by reducing the cross-sectional area of the tunnel Diffuser: The section of a wind tunnel where the velocity of the airflow is reduced by flowing it into a part of the tunnel with increasing cross-sectional area Drag: The force on a wing in the direction of the airflow created by the airflow Drive section: The section of a wind tunnel that provides the force that causes the air to move through the tunnel. Fuselage: The central portion of an aircraft that holds the crew, passengers, and/or cargo Lift: The force on a wing, opposite the force of gravity, created by the airflow Mach or Mach number: The ratio of the speed of an object, with respect to the surrounding air or other fluid, to the speed of sound in that medium. At sea level in the standard atmosphere, the speed of sound is 340.294 meters per second (1,116.45 feet per second). An airplane traveling at 340.294 meters per second at sea level would be said to be traveling at Mach 1. An airplane traveling at 680.588 meters per second at sea level would be said to be traveling at Mach 2. Settling chamber: The section of a wind tunnel where the airflow is straightened and turbulence is reduced, normally prior to entering the contraction cone. Test section: The section of a wind tunnel where the test article and sensors are placed Turbulence: The property of a flow in which the velocity at a given point varies erratically in magnitude and direction; often used to describe irregular atmospheric motion Whirling arm: A device composed of a test article mounted on the end of a long arm and rotated around a fixed point; used in aerodynamic testing prior to the development of the wind tunnel. ------------------------------------------XXX------------------------------------------