Powerful Facts About Wind Tunnels used in Aerospace Engineering

All You Need to Know About Wind Tunnels used in Aerospace Engineering

A wind tunnel is a laboratory device that is used for testing the flow dynamics around a body such as turbine blades, aerofoils, and so on by simulating uniform flow conditions in the test section in which the body is kept. Its operation is based on the fact that the forces acting on an airplane moving through the air at a certain speed are the same as those which act on a stationary airplane with the air moving past the airplane at said speed. In this way, the interaction between moving air and the stationary model can be likened to the interaction between an aircraft moving through the air.

Wind tunnels are used by aerodynamicists to test proposed aircraft models. Flow conditions are carefully adjusted and monitored to study how the forces acting on the aircraft are affected. Based on the forces observed, the engineer(s) can then make informed predictions of the forces that would act on the actual aircraft. It is an essential part of the development process of an aircraft or spacecraft.

Of course in real life, some conditions may be different. Nonetheless, wind tunnels give a lot of insight into the possible relationship between surrounding airflow and a potential design. If any issues are discovered, the tests performed will inform the engineers on what design modifications can be made to ensure optimal airflow.

In this article, we will look at:

• Classification of wind tunnels
• Types of wind tunnels and their advantages and disadvantages.
• Guide to conducting wind tunnel experiments :
  • Precautions
  • Apparatus used
  • Boundary layer theory

Classification of Wind Tunnels

Wind tunnels can be classified based on different things such as:

• The speed of the air flowing through it: Generally, they can be classified into low-speed and high-speed wind tunnels, however, more specifically, there are subsonic, transonic, supersonic, and hypersonic wind tunnels. Subsonic speeds are below Mach 0.8, transonic speeds are between Mach 0.8 and 1.2, supersonic speeds are between Mach 1.2 and 5, and hypersonic speeds are above Mach 5.

The Mach number is the ratio of the flow speed to the speed of sound under those conditions. You may know the speed of sound to be a constant between 330-340 m/s, but this speed actually changes with temperature and density. In real life, this plays out as pressure and temperature change with altitude. A Mach number of 1 means the speed of flow is equal to the speed of sound, while a Mach number above 1 means the speed of flow is higher than the speed of sound.

Compressibility effects are mostly negligible for subsonic flow but must be considered in calculations for other flows. For hypersonic flow, the chemical state of the gas must also be considered in addition to the compressibility effect.

• The geometry of the tunnel: The main types based on geometry are open return, closed return and blowdown tunnels. Open return tunnels have both ends open and draw air from the room into the tunnel, while closed return tunnels have both ends closed and recirculate the air through the test section.

As for the blowdown tunnel, around its test section, there is a high-pressure vessel upstream and a low-pressure reservoir downstream. Blowdown tunnels are typically used for high subsonic to high supersonic testing. Hypersonic testing involves the use of a variant of the blowdown tunnel called a shock tube.

• The working fluid in the tunnel: different fluids can be used in the wind tunnel depending on the application. Low-speed wind tunnels make use of air as the working fluid. To better visualize the shock waves for when dealing with submarines/boats, water is passed through the tunnel. Helium or nitrogen has been used in some hypersonic tunnels. Likewise, cryogenic nitrogen has been used in some transonic applications for high Reynold’s number testing.

• .The specific purpose for which it is used: examples are propulsion wind tunnels and flow visualization tunnels. The former is used to study the high-temperature exhaust from rocket or gas turbine engines; such facilities are designed to more accurately simulate the high-temperature effects of flows in such regimes. The latter makes use of various visualization techniques to enable engineers to diagnose flow patterns.

Some of the visualization techniques are smoke, tufts, laser sheet, surface oil flow, and schlieren photography. Tufts and surface oil flow help to provide information about the boundary layer on the surface of the test object. Smoke and laser sheets help to visualize free stream flow, the assumption being that the smoke or seed particles flow in the exact same direction as the air. The schlieren photographic technique is used to study flows with a very significant change in fluid density, and to locate shockwaves. The light rays bend across the density gradients to produce a photographic image that locates the gradients.

Types of Wind Tunnels According to Speed, Advantages, and Disadvantages.

For this part of the article, we will take a deeper look into types of wind tunnels with classification based on speed, and their corresponding advantages and disadvantages.

Subsonic Wind Tunnels:

These may be open returns or closed returns. For an open return tunnel, fresh air is drawn in from the atmosphere after each run. The effuser prepares the flow for entry into the test section. The test section provides uniform flow conditions.

Subsonic wind tunnels are used to study the flow characteristics of subsonic aircraft models and aerofoils, especially for educational/research purposes as in a university. Parameters such as lift and pitching moment can be obtained for different conditions and analyzed. They can be used to investigate the boundary layer theory (this is discussed later in the article) or study the pressure distribution around an aerofoil and so on. The pitching moment is the moment produced by the aerodynamic forces acting on aerofoil when they are considered to be acting at the aerodynamic center of the aerofoil, rather than at the center of pressure (CP).

Subsonic wind tunnels could also be used in civil engineering applications, i.e., to test the flow over bridges, cables, and so on.

Advantages of Subsonic Wind Tunnels

• Skin friction losses are not as large since the speeds are low.
• It is the least expensive of the three wind tunnel types
• It is the least complicated of the three wind tunnel types since compressibility effects are negligible.

Disadvantages of Subsonic Wind Tunnels

Subsonic wind tunnels have some flow irregularities which can pose disadvantages. These are:

• The mean velocity not being constant over a cross-section of the test object.
• Variation of flow direction due to the formation of vortices. A common solution to this is the incorporation of honeycombs around the test section.
• Surges in mean velocity are known as low-frequency pulsation. This condition extends the time taken to attain steady conditions.

Supersonic Wind Tunnels:

These are used by engineers more for applied aerodynamics purposes rather than education. Some of its applications include studying the stationary aerodynamic characteristics of models of supersonic aircraft which are usually fighter aircraft, the structure of supersonic flows aided by different possible visualization methods, how shock waves cause interference in flow, the effect of sonic boom on flow, and so on.

Advantages of Supersonic Tunnels

• Test times are much shorter.
• If water is used as a working fluid (as in Dryden’s water tunnel), it is cheaper and allows for great flow visualization, especially since the dye is added to water. However, Dryden’s tunnel is much smaller than typical supersonic wind tunnels.

Disadvantages of Supersonic Tunnels

• At higher speeds, the skin friction in the wind tunnel and losses due to obstacles become more prominent.
• Losses as a result of shock waves may arise.
• For open return tunnels of this type, a compressor with a very high-pressure ratio and power rating is required since a significant amount of energy is continuously thrown into the atmosphere. This can be solved by using a closed return wind tunnel, however, visualization is harder with closed return tunnels and they are usually more expensive to construct.
• They are more expensive to operate.

Transonic Wind Tunnels:

Just like supersonic tunnels, this is used more for applied aerodynamics testing.  Transonic wind tunnels have been used in the testing of models such as the oblique-wing research aircraft which NASA used to test the feasibility of an oblique-wing aircraft. The idea was explored as it was believed to be able to reduce drag in the transonic speed regime.

Transonic wind tunnels also find application in the testing of military aircraft models and some civil aircraft models at cruise speed; typical airliners cruise between 0.8 – 0.9 Mach.

Advantages of Transonic Wind Tunnels

• Test times are shorter
• It is often more important in testing civil/ commercial aircraft models since the majority of their flights are spent on a cruise.

Disadvantages of Transonic Wind Tunnels

• More significant skin friction losses than a subsonic tunnel.
• The use of cryogenic nitrogen as a working fluid is obviously more costly than using air.
• Shock waves may also cause losses here.
• Higher cost of construction and operation than subsonic.

Types of Wind Tunnels According to Geometry, Advantages, and Disadvantages.

For this part of the article, we will take a deeper look into types of wind tunnels with classification based on geometry, and their corresponding advantages and disadvantages.

Open Return Tunnel:

In this tunnel, the air which passes through the test section comes directly from the room in which the tunnel is located. The original tunnel the Wright brothers used was an open return tunnel.

Advantages of Open Return Tunnels

• Low construction cost.
• Its design is the most suitable for propulsion and smoke visualization as there is no accumulation of exhaust products.

Disadvantages of Open Return Tunnels

• The flow quality in the test section may be poor, especially because of its openness to winds or the weather. To improve this, there would need to be extensive screens or flow straighteners.
• It is quite expensive to operate as the fan would have to continually accelerate the flow through the tunnel.
• Noisy operation (due to constant fan action)

Closed Return Tunnel:

In this tunnel, the air is constantly recirculated through the test section. The air passed from the exit of the test section back to the fan by a series of turning vanes. From the fan, it is then returned to the contraction section and back through the test section.

Advantages of Closed Return Tunnels

• The flow quality in the test section is much better.
• It is not as expensive to operate as the open return tunnel since the fan does not constantly have to accelerate the air. Once the air in the tunnel is circulating, the fan and motor only need to move to overcome the velocity losses along the tunnel wall.
• It operates more quietly than the open return tunnel.
• It has longer run times compared to the blowdown tunnel.

Disadvantages of Closed Return Tunnels

• Higher construction cost because of the added vanes and ducting.
• It is larger and more expensive to operate than the closed return tunnel.
• It is less suited for smoke visualization and propulsion.
• Because heat is trapped within the tunnel, there are hotter running conditions than an open tunnel. Heat exchangers or active cooling mechanisms may have to be incorporated.

Blowdown Tunnel:

There are different possible configurations of blowdown wind tunnels. A common one is the completely closed supersonic configuration. In this, the test section sits at the end of a supersonic nozzle. The Mach number of the flow in the test section is determined by the temperature and pressure in the plenum and the area ratio between the test section on the throat of the nozzle.

As the flow expands in the nozzle, the pressure decreases causing any moisture in the tunnel to condense and liquefy in the test section. This condensation is unwanted, so air is brought into the tunnel through a dryer bed. The air is pumped into a closed high-pressure chamber upstream of the plenum, while air is simultaneously pumped out of a closed low-pressure chamber downstream of the test section.

Test times in blowdown tunnels are limited. At the start of the test run, valves are opened both upstream and downstream of the test section. The pressure differential established a supersonic flow in the test section as the air flows from the high-pressure chamber to the low-pressure chamber. As the air leaves the high-pressure chamber, the pressure thereabout decreases, while the pressure in the low-pressure chamber increases as air enters it. Eventually, the pressure differential is eliminated as the pressures equalize. Thus, the flow stops, and the test is completed. A pressure regulator installed in the plenum ensures that constant conditions are provided. A second throat is usually incorporated downstream of the test section to shock down the supersonic flow to the subsonic before entering the low-pressure chamber. “Shock down” means shock waves are introduced to absorb some of the flow’s kinetic energy, thereby reducing its speed.

Some blowdown tunnels do not incorporate a high-pressure chamber, but rather open the plenum chamber to the atmosphere. Such blowdown tunnels are called indraft tunnels and do not require pressure regulators. However, the pressure ratio across the test section is usually low than that of a closed configuration and consequently, the maximum Mach number is lower.

Advantages of Blowdown Tunnels

• Shorter test times. This also translates to smaller loads on the test model during startup.
• It is less expensive to construct and operate.
• Its design is suitable for smoke visualization and propulsion.

Disadvantages of Blowdown Tunnels

• Shorter test times require faster and often more expensive instrumentation.
• It requires the incorporation of pressure regulator valves.
• The noisy operation, typically requires the incorporation of a muffler downstream of the test section.

Guide for Using a Subsonic Wind Tunnel for an Experiment.

In this part of the article, we will go through some precautions, apparatus and principles typically employed for experiments with (subsonic) wind tunnel experiments. This is especially useful for university/college students.

Precautions to Take During a Wind Tunnel Experiment

Some of these precautions are:

• Zeroing the equipment at the start of the experiment.
• Ensuring the electrical supply to the wind tunnel is disconnected before fitting in any equipment. Not doing so could be very dangerous.
• Ensuring the components and supports are firmly locked in before starting the experiment so that nothing falls out during the course of the experiment.
• Ensuring no loose materials are lying around, e.g., the pitot tubes.
• Making sure the components of the wind tunnel are properly aligned.
• Always switch off the wind tunnel as soon as you finish using it.
• For experiments that involve increasing the speed, turn the knob in small increments rather than in one go.

Apparatus Used in Subsonic Wind Tunnel Experiments

Some common apparatus are:

1. (Liquid) Manometer: this device is used to measure the dynamic pressure in terms of the height of water. The water in the tube is slightly colored to allow for better visibility.

• Digital manometer: this is a differential pressure transducer that measures differential pressure or the pressure relative to the atmosphere. It displays its values on its small screen, making it much easier and quicker to read than a liquid manometer.

• Pitot probe: this component allows for the accurate investigation of the boundary layer as it has a fine micrometer adjustment. The probe has an adjustable pitot tube, a device that measures the velocity of a fluid by converting the fluid’s kinetic energy into potential (pressure) energy.

4. Aerofoil model: aerofoil models with specified dimensions are often used to conduct these experiments. “NACA airfoils” were developed by the US’s National Advisory Committee for Aeronautics (NACA). Each aerofoil has a numerical designation – a four or five-digit number that represents the aerofoil’s critical geometric properties. NACA has a catalog of 78 aerofoils with which models for testing as well as aircraft aerofoils are designed. Their coordinates can be obtained from online databases and programmed into design and simulation software used in the development process of aircraft.

5. AFA3 Three component balance: It supports the aerofoil model so it can measure drag, lift, and pitching moment. It calculates the lift and pitching moment with the following equations:

Lift= Fore load cell force+ Aft load cell force

Pitching Moment= AFA3 Moment Arm Length × (Fore load cell force-Aft load cell force)

• Computer systems: wind tunnels have computers integrated into them that automatically record the obtained lift, drag and pitching moment values when calibrated properly. The software which records and displays these values is called VDAS® which stands for Versatile Data Acquisition System.

Theory of Boundary Layer

The boundary layer is one of the major parameters studied when conducting wind tunnel experiments. It is the area between a body’s surface and the region of free-stream velocity. The fluid molecules moving on the surface can be said to have 0 velocities, because the fluid’s viscosity would make it stick to the surface,  while the layers of molecules above the surface have varying velocities. These velocities increase as we move farther from the wing surface. This occurs until they reach a velocity of the air moving around the body called the free-stream velocity. Thus, the velocity will keep increasing until it reaches a fairly constant value which is the free-stream velocity.

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The velocity of the fluid molecules depends on the shape of the body, the fluid’s viscosity, and compressibility. The fluid surrounding the boundary layer moves around it the same way it would around the surface of a body, therefore, the boundary layer can be said to give an ‘effective shape’ to the surrounding flow. This effective shape can be quite similar to the shape of the body or significantly different in the event that there is a boundary layer separation from the body. The former is usually the case, however, if the latter happens, the result is a significant increase in drag and a decrease in the lift which causes the aircraft to stall.

The boundary layer around a body may not always be laminar (smooth); it can also be turbulent (unsteady). The nature of the flow is determined by a dimensionless parameter called Reynold’s number which is the ratio of the inertial forces of a fluid to its viscous forces. Lower Reynold’s numbers have laminar flow and the variation in velocity as we move away from the surface is uniform. Conversely, higher Reynold’s numbers have turbulent flow and the variation in velocity as we move away from the surface is unsteady and characterized by swirling flows within the boundary layer.

In an experiment, pitot probes are used to measure the speed variation in the wind tunnel. When different probe heights are plotted against flow velocity, the graph obtained can help ascertain whether the flow around an object is laminar or turbulent. A laminar flow will have a graph with a fairly constant gradient, while a turbulent flow graph will not show any clear trend since the flow is unsteady. In the graph below, the 20m/s plot is turbulent while the 25 m/s plot is laminar.

References:

Grc.nasa.gov. 2021. Boundary Layer. [online] Available at: <https://www.grc.nasa.gov/www/k-12/airplane/boundlay.html>

2017. Tecquipment catalog. [ebook] Tecquipment.com. Available at: <http://dotek.com.tr/wp-content/uploads/2017/12/tecquipment.pdf>

Banakh, V., 2021. Wind Tunnels and Experimental Fluid Dynamics Research. [ebook] Available at: <https://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/study-of-turbulent-supersonic-flow-based-on-the-optical-and-acoustic-measurements>

Grc.nasa.gov. 2021. Types of Wind Tunnels. [online] Available at: <https://www.grc.nasa.gov/www/k-12/airplane/tuntype.html>

2017. Tecquipment catalog. [ebook] Tecquipment.com. Available at: <http://dotek.com.tr/wp-content/uploads/2017/12/tecquipment.pdf>

Grc.nasa.gov. n.d. Blowdown Wind Tunnel. [online] Available at: <https://www.grc.nasa.gov/www/k-12/airplane/tunblow.html>

Grc.nasa.gov. n.d. Closed return Wind Tunnel. [online] Available at: <https://www.grc.nasa.gov/www/k-12/airplane/tuncret.html>

Grc.nasa.gov. n.d. Open Return Wind Tunnel. [online] Available at: <https://www.grc.nasa.gov/www/k-12/airplane/tunoret.html>

NASA. 2017. NACA Airfoils. [online] Available at: <https://www.nasa.gov/image-feature/langley/100/naca-airfoils>

Grc.nasa.gov. n.d. Flow Visualization. [online] Available at: <https://www.grc.nasa.gov/www/k-12/airplane/tunvis.html#:~:text=Flow%20Visualization&text=Aerodynamicists%20use%20wind%20tunnels%20to,on%20the%20model%20are%20measured.>