01 Wind Tunnels

ExperimentalExperimental AerodynamicsAerodynamics Lecture 1: Introduction G. Dimitriadis Experimental Aerodynamics Introduction •! Experimental aerodynamics can have the following objectives: –!To measure the forces exerted by the air on moving bodies –!To measure the forces exerted by wind on static bodies –!To help develop or validate aerodynamic theories –!To help design moving or static bodies so as to optimize their aerodynamic efficiency Experimental Aerodynamics History (1) •! Aerodynamics means ‘air in motion’. The term was first documented in 1837. •! Humans have known that moving air can exert significant forces on bodies since the dawn of time. •! Aristotle (4th century BC) is recognized as the first to write that air has weight and that bodies moving through fluids are subjected to forces. Experimental Aerodynamics History (2) •! Archimedes (3rd century BC) formulated the theory of hydrostatic pressure •! Leonardo Da Vinci brought about two major advances in aerodynamics: –! He noticed that water in a river moves faster in places where the river is narrow (basics of Bernouli’s theorem) –! He also stated that the aerodynamic results are the same when a body moves through a fluid as when a fluid moves past a static body at the same velocity: The wind tunnel principle Experimental Aerodynamics Aerodynamic Experiments •! Experiments in aerodynamics (and fluid dynamics) can take many forms. •! Observations: –! Water speed in rivers (Da Vinci, 15th century) •! Measurements –! Drag proportional to object’s area (Da Vinci, 15th century) –! Drag proportional to fluid’s density (Galileo, 17th century) –! Drag proportional to velocity squared (Marriotte, 17th century) –! Speed of sound in air (Laplace 18th centiry) Experimental Aerodynamics More Aerodynamic Experiments •! Aerodynamic (and fluid dynamic) experiments were revolutionized Pitot by two inventions: tube –!The Pitot tube to calculate fluid velocity (Henri Pitot, 18th century) –!The Whirling Arm (Benjamin Robbins, 18th century) Whirling arm Experimental Aerodynamics The whirling arm Cayley’s whirling arm •! Robbins’ whirling arm was the first truly controlled aerodynamic experiment. It demonstrated that Newton’s drag theory was wrong •! George Cayley (19th century) used a whirling arm to measure the drag and lift on airfoils. He also used it to design the Cayley’s glider first successful unmanned glider (1804). •! Otto Lillienthal also used whirling arms to design manned gliders (1866-1889) Otto Lillienthal (1895) •! Samuel Langley built the biggest and fastest whirling arm (1890s) Experimental Aerodynamics Frank Wenham The first wind tunnels (1866) •! The whirling arm has a big weakness: the object passes inside its own wake. This weakness was realized towards the end of the 19th century (even by Langley himself. •! An alternative was the wind tunnel, first designed by Frank Wenham (1871) •! More famous was the wind tunnel used by the Wright brothers in 1901 in order to design their Wright Brothers’ Tunnel 1902 Glider and 1903 Flyer. •! The Wright brothers wind tunnel gave the most accurate and comprehensive lift and drag data of wing sections ever obtained to that point. Experimental Aerodynamics The Wind Tunnel •! The wind tunnel quickly became the basis of most experimental aerodynamic efforts •! Even in the present days of computer simulations and numerical Navier Stokes solutions, the wind tunnel is indispensable: –!It can be used to validate numerical solutions –!It can be used to calibrate numerical solutions –!In fact, numerical solutions are only good when we already know the result Experimental Aerodynamics This course •! This course will be based in and around the ULg Wind Tunnel •! Every lecture will feature a theoretical and a practical session •! Every week a different aspect of experimental aerodynamics will be presented and demonstrated Experimental Aerodynamics Wind Tunnel principles •! The loads exerted by static air on a moving body are equal to those exerted by moving air on a static body, as long as the relative velocities between the air and the body are the same in both cases. •! For a truly representative wind tunnel experiment, the body must have its true size and the wind must have the speed that the object would have if it was moving. •! These conditions are not always possible. Several scaling laws can be used in order to render representative experiments where the size or airspeed have been scaled. Experimental Aerodynamics Scale Parameters •! Reynolds Number: Inertial Forces !Vc Re = = ! = Viscous Forces µ Air density V = Airspeed •! Mach Number: c = Characteristic length Inertial Forces V M = = µ = Air viscosity Elastic Forces a a = Speed of sound in air •! Strouhal Number: f = Frequency of unsteady Unsteady Forces fc phenomena Str = = Steady Forces V Experimental Aerodynamics Scaling •! Two flows are equivalent as long as all the relevant scale parameters are equal. •! In practice it is nearly impossible to enforce all the scale parameters to be equal •! Consider the following examples: –! Air flow over a real bridge deck with width of 30m and over a model of the bridge deck with width 0.3m. –! Air flow over a real fighter plane at M=1.2 at sea level and a 1/32 scale model. •! In very expensive tunnels such problems are sometimes addressed by changing the pressure and density of the air or, even, using a heavy gas instead of air. Experimental Aerodynamics Open circuit wind tunnel •! Eiffel type Experimental Aerodynamics Closed circuit wind tunnel •! Gottingen type Experimental Aerodynamics Open type tunnels •! Advantages: –! Cheaper to build –! Pollutants are purged (e.g. smoke flow visualization or tests on internal combustion engines) •! Disadvantages: –! The size of the tunnel must be compatible to the size of the room: the room is the return path for the air –! Noisy –! More expensive to run than closed type Experimental Aerodynamics Closed type tunnels •! Advantages: –! Cheaper to run: energy is required only to overcome losses. –! Less noisy than open type. –! The quality of the flow can be easily controlled. •! Disadvantages: –! More expensive to build –! Not easy to purge –! Continuous losses of energy in the tunnel heat up the air, so the air may need cooling, especially in the summer Experimental Aerodynamics Special wind tunnels •! Transonic/Supersonic/Hypersonic •! Low turbulence tunnels •! High Reynolds number (pressurized) •! Transonic dynamics tunnels (for aeroelastic problems, e.g. TDT at NASA Langley or T-128 at TsAGi) •! Environmental tunnels (simulate the earth’s atmospheric boundary layer) •! Automobile tunnels (e.g. with moving floor) Experimental Aerodynamics Pictures of wind tunnels The NASA Langley Transonic Dynamics Tunnel The NRC’s 9mx9m tunnel Experimental Aerodynamics Langley full scale wind tunnel X-48 Blended wing body AST -18.3x9.1x17m test section Mercury capsule -Max speed of 36m/s Experimental Aerodynamics Wind tunnel dimensions •! The dimensions of a wind tunnel depend on several factors: –!Cost and space considerations –!Speed range –!Application area (e.g. aerospace, automotive, environmental flows etc) –!Required Reynolds number, Mach number –!Other requirements (e.g. STOL tests) Experimental Aerodynamics Typical low speed aeronautics tunnels •! For such wind tunnels, the Reynolds number must be around 1,500,000 to 2,000,000 for the flow to be fully turbulent and thus simulate the real flow. •! What are the cross-sectional dimensions of such tunnels? Experimental Aerodynamics Typical low speed aeronautics tunnels - Answer •! For an airspeed of 65m/s this leads to a wing chord of 0.33m. •! Many wings have an aspect ratio of 8-9. This leads to a span of around 2.6m •! The span must be between 0.8 and 0.9 of the wind tunnel’s width, leading to a width of around 3m. •! Many low speed wind tunnels around the world have a section of 3mx2m (width x height) and airspeeds up to 100m/s. Experimental Aerodynamics Tunnel sections Third corner Second diffuser Fan section Second corner Fourth corner First corner Contraction Cone Test section First diffuser Experimental Aerodynamics Test Section •! The test (or working) section can have many cross-sectional shapes: –!Round, elliptical, square, hexagonal, octagonal, rectangular, etc •! The shape affects directly the cost of building the tunnel and power required to run it. •! The shape does not affect the aerodynamic losses in the tunnel Experimental Aerodynamics Cross-sectional shape •! The most usual shapes are rectangular and octagonal. •! The octagonal shape is chosen to minimize secondary flow problems in the corners of a rectangular section Secondary flow areas Experimental Aerodynamics Side view •! The test section is not completely straight •! The boundary layer grows in the test section, reducing its effective area, increasing the velocity and decreasing the static pressure Experimental Aerodynamics Side view (2) •! To overcome this problem, most test sections feature a small geometric increase of their cross-sectional area. !" •! There is no magic value for the angle !. It is often chosen as !=0.5o. Experimental Aerodynamics More about the test section •! The length of a test section is usually chosen as one or two times the size of the major dimension of the cross section. E.g. for a 3mx2m cross section, the length would be 3m-6m. •! There are significant losses in the test section so it should be kept as short as possible •! There must be adequate windows in the test section •! There must be good lighting in the test section •! There must be

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