Wind tunnel

A wind tunnel is used to study the aerodynamic and acoustic properties of objects and measured.

The best known are probably the wind tunnel tests of aircraft and cars. Wind-tunnel experiments are used to investigate air resistance, the dynamic lift, or deformations due to aeroelasticity.

Also models of structures like high-rise buildings, smokestacks and bridges are investigated in wind tunnels. For them, the goal is to assess whether they can withstand in original size in storms expected wind forces. In order to simulate the wind flow correctly, sometimes the entire surrounding area must be modeled for it. Only a few objects can be studied without scaling useful in the wind tunnel. Cars are an exception, as they are not too large and the relatively low air velocities sufficiently allow large wind tunnels. For airplanes or buildings to scale down models are used. The shape of the flow around a body depends on its Reynolds number. To obtain realistic results, the investigation in the wind tunnel at the same Reynolds number must be how it corresponds to the flow around the original. This can be achieved by increasing the density of the medium, or a higher speed.

• 2.1 Eiffel channel
• 2.2 Göttingen channel

Construction

Wind Tunnels consist of one or more fans that generate the air flow, rectifier elements to ensure as uniform, non- turbulent flow, a nozzle to accelerate the flow of air and the actual measuring section, in which the investigations are carried out. The flow in the test section is intended to be smooth, parallel, turbulence and low noise. A quantitative assessment of the turbulent velocity fluctuations makes the degree of turbulence of the wind tunnel. The test section, as shown in the graph, to be open, that is, the flow is discharged from the nozzle in an anechoic chamber and collected at the other measurement hall side of a collector or collector, located in the anechoic chamber, a shear layer between the moving and related air builds. However, there are also closed test sections, in which the flow is also performed in the test section through walls, and slotted test sections in which some of these walls are provided with slots.

Also the air guiding of wind tunnels can be open or closed. At the open construction, the air is sucked in from the environment, passing through the measurement path and escapes at the other end to the outside. Closed air ducts are annular. Here, the picked up by the collector according to the measurement section air flow is returned to the blower. Low temperatures and high pressure can be generated in closed wind tunnels ( climatic wind tunnel ). Wind tunnels designed for subsonic speeds can have several meters in diameter, whereas shrinking the size of wind tunnels for the high supersonic range to a few centimeters together.

Wind tunnel experiments are mainly always associated with high costs because of high investment costs in the preparation. Therefore, one tries today increasingly to replace the attempts by computational fluid dynamics (CFD, computational fluid dynamics). The phenomena are mapped quite well already. From vision to replace wind tunnel experiments by numerical simulation, it is still far away in reality. This applies even more to the aeroacoustic simulation (CAA, Computational Aero Acoustics ).

Wind tunnels can be classified according to their mode of operation as follows:

Fan - driven wind tunnels

Storage Wind Tunnels ( Intermittent service )

Fired with unsteady pressure waves wind tunnels

Rationale: A high-pressure storage pipe is separated by a bursting membrane of the low-pressure pipe. With the bursting of the membrane caused transient pressure waves. In the low pressure pipe an unsteady shock wave with a co-rotating wake flow runs. At the same time running in the high pressure tube, a transient wave of expansion, the opposite to the direction of wave travel induces a flow. The so- induced currents are used in pilot plants of different types.

Historical

Eiffel channel

By Gustave Eiffel in 1905 and 1906 at the Eiffel Tower made ​​investigations on the flow resistance of rectangular and oval plates by these were left out of the second platform of the tower out on a wire drop down vertically over the years. Here, the force acting on the plate was recorded on a paper wrapped berußtem metal cylinder. In 1909, Gustave Eiffel built on the Champ de Mars, a laboratory with a wind tunnel, which was relocated in 1912 after Auteuil. The Laboratoire Dynamique Aero Eiffel can be viewed there today. The wind tunnel consisted of a closed measurement chamber, from which on the one hand with the aid of an electric fan, the air was sucked (the " 70 hp" power this way, provided the power center of the Eiffel Tower ). On the opposite side of the room air flowed in from a large storage space through a nozzle into the inside of the measuring section according to. In the air jet thus produced later systematic studies on the behavior of the first airfoil shapes for aircraft were hired by Eiffel's employees. This type of an open channel, which is supplied by external air, therefore, is also called the "Eiffel channel". An Eiffel channel has the disadvantage that the temperature and pressure fluctuations and velocity differences from the environment from which the air is sucked in, continue in the test section, can be such that the flow stopped and set laminar only within certain limits.

Göttingen channel

Partly for this reason it was by Ludwig Prandtl another form of the wind tunnel developed and implemented for the first time in 1908 he founded the model laboratory for aerodynamics in Göttingen. In a wind tunnel in Göttingen type the air is sucked into a collecting funnel by a fan behind the test section, from where it again flows in via a channel of the nozzle from the measuring section. In this way the physical properties of air in the channel can be well controlled. One can, for example, or cool the entire duct and the test section under increased pressure. There are channels that are working with pressures up to 120 bar or temperatures down to -200 ° C. A channel Göttingen type usually has also a higher efficiency, since the kinetic energy of the flowing out of the test section air is used again. Therefore, most wind tunnels have a closed circuit with high power / jet velocity.

This design was inspired by an experimental channel, undertook the Prandtl in Hanover studies with flowing water. Instead of the fan, a water wheel was rotated by a hand crank to rotate at this about 2 m long and 20 cm wide channel, after which the water was aspirated into the measuring section and by a Umströmkanal, who was among these, again on the other side of the measuring section flowed. From above barriers were inserted into the open channel and its flow past can be observed.