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ed to test full size aircraft and had six large fans driven by high powered electric motors 15 the chalais meudon wind tunnel was used by onera under the name s1ch until 1976 in the development of e g the caravelle and concorde airplanes today this wind tunnel is preserved as a national monument ludwig prandtl was theodore von kármán s teacher at göttingen university and suggested the construction of a wind tunnel for tests of airships they were designing 16 44 the vortex street of turbulence downstream of a cylinder was tested in the tunnel 16 63 when he later moved to aachen university he recalled use of this facility i remembered the wind tunnel in göttingen was started as a tool for studies of zeppelin behavior but that it had proven to be valuable for everything else from determining the direction of smoke from a ship s stack to whether a given airplane would fly progress at aachen i felt would be virtually impossible without a good wind tunnel 16 76 when von kármán began to consult with caltech he worked with clark millikan and arthur l klein 16 124 he objected to their design and insisted on a return flow making the device independent of the fluctuations of the outside atmosphere it was completed in 1930 and used for northrop alpha testing 16 169 in 1939 general arnold asked what was required to advance the usaf and von kármán answered the first step is to build the right wind tunnel 16 226 on the other hand after the successes of the bell x 2 and prospect of more advanced research he wrote i was in favor of constructing such a plane because i have never believed that you can get all the answers out of a wind tunnel 16 302 03 world war ii edit 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 30 000 kw electric motor large scale aircraft models could be tested at air speeds of 400 mph 640 km h 17 during wwii germany developed different designs of large wind tunnels to further their knowledge of aeronautics for example the wind tunnel at peenemünde was a novel wind tunnel design that allowed for high speed airflow research but brought several design challenges regarding constructing a high speed wind tunnel at scale however it successfully 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 by the end of the war germany had at least three different supersonic wind tunnels with one capable of mach 4 4 heated airflows a large wind tunnel under construction near oetztal austria would have had two fans directly driven by two 50 000 hp 37 000 kw hydraulic turbines the installation was not completed by the end of the war and the dismantled equipment was shipped to modane france in 1946 where it was re erected and is still operated there by the onera with its 26 foot 8 m test section and airspeed up to mach 1 it is the largest transonic wind tunnel facility in the world 18 frank wattendorf reported on this wind tunnel for a us response 19 on 22 june 1942 curtiss wright financed construction of one of the nation s largest subsonic wind tunnels in buffalo new york the first concrete for building was poured on 22 june 1942 on a site that eventually would become calspan where the wind tunnel still operates 20 by the end of world war ii 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 400 km h 21 and a vertical wind tunnel at wright field ohio where the wind stream is upwards for the testing of models in spin situations and the concepts and engineering designs for the first primitive helicopters flown in the us 22 after world war ii edit naca wind tunnel test on a human subject showing the effects of high wind speeds on the human face later research into airflows near or above the speed of sound used a related approach metal pressure chambers were used to store high pressure air which was then accelerated through a nozzle designed to provide supersonic flow the observation or instrumentation chamber test section was then placed at the proper location in the throat or nozzle for the desired airspeed mary jackson with a wind tunnel model at langley research center in the united states concern over the lagging of american research facilities compared to those built by the germans led to the unitary wind tunnel plan act of 1949 which authorized expenditure to construct new wind tunnels at universities and at government sites some german war time wind tunnels were dismantled for shipment to the united states as part of the plan to exploit german technological developments 23 fan blades of langley research center s 16 foot transonic wind tunnel in 1990 before it was retired in 2004 in the united states many wind tunnels have been decommissioned from 1990 to 2010 including some historic facilities pressure is brought to bear on remaining wind tunnels due to declining or erratic usage high electricity costs and in some cases the high value of the real estate upon which the facility sits on the other hand cfd validation still requires wind tunnel data and this is likely to be the case for the foreseeable future studies have been done and others are underway to assess future military and commercial wind tunnel needs but the outcome remains uncertain 24 more recently an increasing use of jet powered instrumented unmanned vehicles or research drones have replaced some of the traditional uses of wind tunnels 25 the world s fastest wind tunnel as of 2019 is the lens x wind tunnel located in buffalo new york 26 measurement of aerodynamic forces and moments edit six element external balance below the kirsten wind tunnel six measurements are required three forces and three moments air speed direction and pressures are measured in several ways in wind tunnels air speed through the test section is determined by bernoulli s principle the direction of airflow around a model is shown by fluttering tufts of yarn attached to the aerodynamic surfaces the direction of airflow approaching and leaving a surface can be seen by mounting tufts in the airflow in front of and behind the model smoke or bubbles of liquid can be introduced into the airflow upstream of the model and their paths around the model recorded using photography see particle image velocimetry aerodynamic forces on the test model are measured with beam balances 27 the pressure distribution on a test model has historically been measured by drilling small holes on the surface and connecting them to manometers to measure the pressure at each hole pressure distributions can be measured more conveniently using pressure sensitive paint 28 in which pressure is indicated by the fluorescence of the paint they can also be measured with very small electronic pressure sensors mounted on a flexible strip which is attached to the model 29 the aerodynamic properties of an object can vary for a scaled model 30 however by observing certain similarity rules a very satisfactory correspondence between the aerodynamic properties of a scaled model and a full size object can be achieved the choice of similarity parameters depends on the purpose of the test but the most important conditions to satisfy are usually geometric similarity all dimensions of the object must be proportionally scaled mach number the ratio of the airspeed to the speed of sound should be identical for the scaled model and the actual object having identical mach number in a wind tunnel and around the actual object is not equal to having identical airspeeds reynolds number the ratio of inertial forces to viscous forces should be kept this parameter is difficult to satisfy with a scaled model and has led to development of pressurized and cryogenic wind tunnels in which the viscosity of the working fluid can be greatly changed to compensate for the reduced scale of the model in certain particular test cases other similarity parameters must be satisfied such as the froude number force and moment measurements edit a typical lift coefficient versus angle of attack curve the model is mounted on a balance which measures forces and moments lift drag and lateral forces as well as yaw roll and pitching moments are measured over a range of angle of attack common curves such as lift coefficient versus angle of attack are produced the model must be held stationary and these external supports create drag and potential turbulence that will affect the measurements the supporting structures are kept as small as possible and aerodynamically shaped to minimize turbulence flow visualization edit see also list of computational fluid dynamics software because air is transparent it is difficult to directly observe the air movement itself instead multiple methods of both quantitative and qualitative flow visualization methods have been developed for testing in a wind tunnel qualitative methods edit a scale model light aircraft showing helium filled bubbles which follow pathlines of the wingtip vortices smoke carbon dioxide injection tufts mini tufts or flow cones can be applied to a model and remain attached during testing tufts can be used to gauge air flow patterns and flow separation tufts are sometimes made of fluorescent material and are illuminated under black light to aid in visualization tufts are also used on aircraft in flight during flight testing of the experimental rocket plane spaceshipone tufts were attached to the surface of the wings no wind tunnel testing was done on the aircraft 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 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 tempera paint similar to oil it can be applied to the surface of the model by initially applying the paint in spaced out dots after running the wind tunnel the flow direction and separation can be identified an additional strategy in the use of tempera paint is to use blacklights to create a luminous flow pattern with the tempera paint fog usually from water particles is created with an ultrasonic piezoelectric nebulizer the fog is transported inside the wind tunnel preferably of the closed circuit and 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 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 sublimation alternate definition a flow visualization technique is to coat the model in a sublimatable material where once the wind is turned on in regions where the airflow is laminar the material will remain attached to the model while conversely in turbulent areas the material will evaporate off of the model this technique is primarily employed to verify that trip dots placed at the leading edge in order to force a transition are successfully achieving the intended goal 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 quantitative methods edit pressure sensitive paint psp the air pressure on a surface is measured with paint coatings which reacts to variations in pressure by changing color particle image velocimetry piv and laser doppler velocimetry air velocity is measured with lasers model deformation measurement mdm measures how much a model bends and twists 31 classification edit there are many different kinds of wind tunnels they are typically classified by the range of speeds that are achieved in the test section as follows low speed wind tunnel subsonic and transonic wind tunnel supersonic wind tunnel hypersonic wind tunnel high enthalpy wind tunnel wind tunnels are also classified by the orientation of air flow in the test section with respect to gravity typically they are oriented horizontally as happens during level flight a different class of wind tunnels are oriented vertically so that gravity can be balanced by drag instead of lift and these have become a popular form of recreation for simulating sky diving vertical wind tunnel wind tunnels are also classified based on their main use for those used with land vehicles such as cars and trucks the type of floor aerodynamics is also important these vary from stationary floors through to full moving floors with smaller moving floors and some attempt at boundary level control also being important aeronautical wind tunnels edit the main subcategories in the aeronautical wind tunnels are high reynolds number tunnels edit reynolds number is one of the governing similarity parameters for the simulation of flow in a wind tunnel for mach number less than 0 3 it is the primary parameter that governs the flow characteristics there are three main ways to simulate high reynolds number since it is not practical to obtain full scale reynolds number by use of a full scale vehicle citation needed pressurised tunnels test gases are pressurised to increase the reynolds number heavy gas tunnels heavier gases like freon and r 134a are used as test gases the transonic dynamics tunnel at nasa langley is an example of such a tunnel cryogenic tunnels test gas is cooled down to increase the reynolds number the european transonic wind tunnel uses this technique high altitude tunnels these are designed to test the effects of shock waves against various aircraft shapes in near vacuum in 1952 the university of california constructed the first two high altitude wind tunnels one for testing objects at 50 to 70 miles 80 to 113 km above the earth and the second for tests at 80 to 200 miles 130 to 320 km above the e...