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    Wind Tunnel Experiments

    Wind Tunnel Background Information

    Definition

    A wind tunnel is a research tool used in aerodynamic research. It is used to study the effects of air moving past solid objects.

    Basics

    Wind tunnels were first proposed as a means of studying vehicles (primarily airplanes) in free flight. The wind tunnel was envisioned as a means of reversing the usual paradigm: instead of the air's standing still and the aircraft moving at speed through it, the same effect would be obtained if the aircraft stood still and the air moved at speed past it. In that way a stationary observer could study the aircraft in action, and could measure the aerodynamic forces being imposed on the aircraft.

    Later, wind tunnel study came into its own: the effects of wind on manmade structures or objects needed to be studied, when buildings became tall enough to present large surfaces to the wind, and the resulting forces had to be resisted by the building's internal structure. Determining such forces was required before building codes could specify the required strength of such buildings.

    Still later, wind-tunnel testing was applied to automobiles, not so much to determine aerodynamic forces per se but more to determine ways to reduce the power required to move the vehicle on roadways at a given speed. In these studies, the interaction between the road and the vehicle plays a significant role, and this interaction must be taken into consideration when interpreting the test results. In an actual situation the roadway is moving relative to the vehicle but the air is stationary relative to the roadway, but in the wind tunnel the air is moving relative to the roadway, while the roadway is stationary relative to the test vehicle. Some automotive-test wind tunnels have incorporated moving belts under the test vehicle in an effort to approximate the actual condition.

    The Wright brothers' use of a simple wind tunnel in 1901 to study the effects of airflow over various shapes while developing their Wright Flyer was in some ways revolutionary. It can be seen from the above, however, that they were simply using the accepted technology of the day, though this was not yet a common technology in America.

    Operation: Air is blown or sucked through a duct equipped with a viewing port and instrumentation where models or geometrical shapes are mounted for study. Typically the air is moved through the tunnel using a series of fans. For very large wind tunnels several meters in diameter, a single large fan is not practical, and so instead an array of multiple fans are used in parallel to provide sufficient airflow. Due to the sheer volume and speed of air movement required, the fans may be powered by stationary turbofan engines rather than electric motors.

    The airflow created by the fans that is entering the tunnel is itself highly turbulent due to the fan blade motion (when the fan is blowing air into the test section - when it is sucking 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.

    With the model mounted on a force balance, one can measure lift, drag, lateral forces, yaw, roll, and pitching moments over a range of angle of attack. This allows one to produce common curves such as lift coefficient versus angle of attack.

    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.

    Topics of Interest

    A vertical wind tunnel (VWT) is a wind tunnel which moves air up in a vertical column. It is a recreational wind tunnel, frequently advertised as "indoor skydiving" or "bodyflight". It is also a popular training tool for skydivers.

    Vertical wind tunnels enable human beings to fly in air without planes or parachutes, through the force of wind being generated vertically. Wind moves upwards at approximately 195 km/h (120 mph or 55 m/s), the terminal velocity of a falling human body belly-downwards, although this can vary from person to person. A vertical wind tunnel is frequently called 'indoor skydiving' due to the popularity of vertical wind tunnels among skydivers, who report that the sensation is extremely similar to skydiving. The human body 'floats' in midair in a vertical wind tunnel, and this is called 'bodyflight' or 'body flight'.

    In experimental fluid mechanics, a sting is a fixture on which models are mounted for testing, e.g., in a wind tunnel. A sting is usually a long shaft attaching to the downstream end of the model so that it does not disturb flow over the model.

    The Doriot Climatic Chambers (DCCs) are a pair of very large, highly specialized wind tunnels located at the US Army’s Soldier Systems Center (SSC) in Natick, Massachusetts. They are a unique facility, capable of simulating an extreme range of global weather conditions for the testing of both the physical properties of military equipment and the physiology and adaptations of human subjects.

    Subsonic tunnel Low speed wind tunnels are used for operations at very low mach number, with speeds in the test section up to 400 km/h (~ 100 m/s, M = 0.3). They are of open-return type, or return flow. The air is moved with a propulsion system made of a large axial fan that increases the dynamic pressure to overcome the viscous losses.

    Transonic tunnel: High subsonic wind tunnels (0.4 < M < 0.75) or transonic wind tunnels (0.75 < M < 1.2) are designed on the same principles as the subsonic wind tunnels. Transonic wind tunnels are able to achieve speeds close to the speeds of sound. The highest speed is reached in the test section. The Mach number is approximately one with combined subsonic and supersonic flow regions. Testing at transonic speeds presents additional problems, mainly due to the reflection of the shock waves from the walls of the test section. Therefore, perforated or slotted walls are required to reduce shock reflection from the walls. Since important viscous or inviscid interactions occur (such as shock waves or boundary layer interaction) both Mach and Reynolds number are important and must be properly simulated. Large scale facilities and/or pressurized or cryogenic wind tunnels are used.

    A supersonic wind tunnel is a wind tunnel that produces supersonic speeds (1.2

    A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working section. The speed of these tunnels vary from Mach 5 to 15. As with supersonic wind tunnels, these types of tunnels must run intermittently with very high pressure ratios when initializing. Since the temperature drops with the expanding flow, the air inside has the chance of becoming liquefied. For that reason, preheating is particularly critical (the nozzle may require cooling). High pressure and temperature ratios can be produced with a shock tube.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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