The Variable Density Tunnel (VDT) was the second

wind tunnel Wind tunnels are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft ...

at the National Advisory Committee for Aeronautics (

NACA The National Advisory Committee for Aeronautics (NACA) was a United States federal agency founded on March 3, 1915, to undertake, promote, and institutionalize aeronautical research. On October 1, 1958, the agency was dissolved and its assets ...

)

Langley Research Center The Langley Research Center (LaRC or NASA Langley), located in Hampton, Virginia, United States of America, is the oldest of NASA's field centers. It directly borders Langley Air Force Base and the Back River on the Chesapeake Bay. LaRC has f ...

. Proposed by German aerospace engineer,

Max Munk Max Michael Munk (October 22, 1890 – June 3, 1986) was a German aerospace engineer who worked for the National Advisory Committee for Aeronautics (NACA) in the 1920s and made contributions to the design of airfoils. Education and early career ...

in May, 1921, it was the world's first variable density wind tunnel and allowed for more accurate testing of small-scale models than could be obtained with atmospheric wind tunnels. It was actively used as a wind tunnel from 1923 until its retirement in the 1940s. Langley Research Center historian, James R. Hansen, wrote that the VDT provided results superior to the atmospheric wind tunnels used at the time and was responsible for making

, the precursor to

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...

, "a world leader in aerodynamic research". It is now on display on the Langley grounds, near the old Reid Conference Center and is a

National Historic Landmark A National Historic Landmark (NHL) is a building, district, object, site, or structure that is officially recognized by the United States government for its outstanding historical significance. Only some 2,500 (~3%) of over 90,000 places listed ...

Technical purpose

In order to accurately measure the

aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...

of scale models, the density of the air used in a

must also be scaled to replicate realistic conditions that would affect the full-scale

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. ...

. The Reynolds number is a quantification of the complex behaviors of dynamic fluids and is calculated as the ratio of inertia forces to the viscous forces in the flow. The Reynolds number is defined as :

\mathrm = \frac = \frac

where: * is the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

of the fluid (

SI units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...

: kg/m³) * is the velocity of the fluid with respect to the object (m/s) * is a characteristic linear dimension (m) * is the

dynamic viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

of the

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shea ...

(Pa·s or N·s/m² or kg/m·s) * is the

kinematic viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the intern ...

of the

(m²/s). The wind tunnels that were used before the creation of the VDT could only operate at normal atmospheric pressures. As a result, scale models in early wind tunnels encountered air with Reynolds numbers that were off by a factor equal to the inverse of the model's scale (i.e. a 1:10 model would be off by a factor of 10). Without properly scaling the Reynolds number, the air flowing over a scale model would react very differently with a full-scale aircraft than it would in a wind tunnel. If a full-scale airfoil is to be simulated in a wind tunnel by a small-scale model, the Reynolds number can only be matched by increasing the velocity or its density or by decreasing its viscosity. The Variable Density Tunnel was designed and built to be pressurized; solving this issue by increasing the density of the air to raise the Reynolds number. The VDT was subsequently able to provide more accurate measurements of aerodynamic qualities because it was able to accurately replicate how air would respond at full-scale.

History

Origin

In 1920, the

National Advisory Committee for Aeronautics The National Advisory Committee for Aeronautics (NACA) was a United States federal agency founded on March 3, 1915, to undertake, promote, and institutionalize aeronautical research. On October 1, 1958, the agency was dissolved and its assets ...

brought

, a German aerospace engineer and student of

Ludwig Prandtl Ludwig Prandtl (4 February 1875 – 15 August 1953) was a German fluid dynamicist, physicist and aerospace scientist. He was a pioneer in the development of rigorous systematic mathematical analyses which he used for underlying the science of ...

University of Göttingen The University of Göttingen, officially the Georg August University of Göttingen, (german: Georg-August-Universität Göttingen, known informally as Georgia Augusta) is a public research university in the city of Göttingen, Germany. Founded i ...

, to work for them in America. After the end of the

First World War World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fig ...

, Munk required two presidential orders to work at NACA, and reportedly had difficulty adjusting to NACA's organizational structure, which was not as strictly defined as those in Germany. Regardless, he proposed his revolutionary Variable Density Tunnel design in the same year. The large, steel pressure tank of the VDT was designed with a working pressure up to 20 atmospheres and was constructed by Newport News Shipbuilding and Dry Dock Company in

Newport News, Virginia Newport News () is an independent city in the U.S. state of Virginia. At the 2020 census, the population was 186,247. Located in the Hampton Roads region, it is the 5th most populous city in Virginia and 140th most populous city in the U ...

. The tank was long and in diameter. The tank's wall was thick. The tank required 85 tons (77.3 tonnes) of steel. The test section was in diameter to match an existing NACA Wind Tunnel No. 1, which was an open-circuit tunnel operating at atmospheric pressure. The variable-density wind tunnel had a closed-circuit design with an annular return flow to minimize the volume of the tank. A fan powered by a 250-hp motor could produce an air speed up to . During a fire in 1927, the internal wooden test section of the VDT was destroyed and was rebuilt with an open-throat design before complications required it be rebuilt again with a closed-throat design. The VDT re-entered service in 1930 and continued to aid the Langley staff in measuring the aerodynamic qualities of airfoils until it was deemed obsolete in the 1940s and converted into a pressure tank for other wind tunnels. The VDT was decommissioned in 1978 and was declared a National Historic Landmark in 1985 because of its historical influence setting the foundation for the beginning of space flight.

Research usage

The tunnel was used for research for over 20 years, up to the 1940s. The VDT was mainly used to test airfoils as wing design was the most pressing problem in early aeronautics. Notably, the VDT produced the data for 78 classical airfoil shapes that were published in 1933 in "The Characteristics of 78 Related Airfoil Sections from Tests in the Variable-Density Wind Tunnel," NACA Technical Report 460. Information on NACA Technical Report 460 This data was used in designing American

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

aircraft such as the

Douglas DC-3 The Douglas DC-3 is a propeller-driven airliner manufactured by Douglas Aircraft Company, which had a lasting effect on the airline industry in the 1930s to 1940s and World War II. It was developed as a larger, improved 14-bed sleeper versi ...

, the

Boeing B-17 Flying Fortress The Boeing B-17 Flying Fortress is a four-engined heavy bomber developed in the 1930s for the United States Army Air Corps (USAAC). Relatively fast and high-flying for a bomber of its era, the B-17 was used primarily in the European Theat ...

, and the

Lockheed P-38 Lightning The Lockheed P-38 Lightning is an American single-seat, twin piston-engined fighter aircraft that was used during World War II. Developed for the United States Army Air Corps by the Lockheed Corporation, the P-38 incorporated a distinctive twi ...

. Additionally, the VDT served in testing thin airfoil designs and low-drag airfoils that were used to design the

P-51 Mustang The North American Aviation P-51 Mustang is an American long-range, single-seat fighter and fighter-bomber used during World War II and the Korean War, among other conflicts. The Mustang was designed in April 1940 by a team headed by James ...

and reduced drag by close to two thirds.

National Historic Landmark Status

On October 3, 1985 the

United States National Park Service The National Park Service (NPS) is an agency of the United States federal government within the U.S. Department of the Interior that manages all national parks, most national monuments, and other natural, historical, and recreational properties ...

recognized the Variable Density Tunnel as a National Historic Landmark. The nomination cited the VDT as responsible for establishing "NACA as a technically competent research organization... ejuvenatingAmerican aerodynamic research which in time led to the best aircraft in the world." The building in which it was originally located was demolished in 2014; the tank is now on display on the Langley grounds.NASA Langley's National Historic Landmarks
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References

External links

Aviation: From Sand Dunes to Sonic Booms, a National Park Service ''Discover Our Shared Heritage'' Travel ItineraryNASA Langley Cultural Resources: Variable Density Tunnel
{{National Register of Historic Places in Virginia Wind tunnels Langley Research Center National Advisory Committee for Aeronautics Buildings and structures in Hampton, Virginia Transportation buildings and structures on the National Register of Historic Places in Virginia National Historic Landmarks in Virginia National Register of Historic Places in Hampton, Virginia 1921 establishments in Virginia Air transportation buildings and structures on the National Register of Historic Places