Wind tunnels are essential tools in aerodynamics, allowing researchers to study airflow around objects. They come in various types, each designed for specific speed ranges and testing needs. From low-speed subsonic tunnels to hypersonic facilities, these devices simulate real-world conditions in controlled environments.

Wind tunnels are classified by their design and operation. Open-circuit tunnels draw air from the atmosphere, while closed-circuit designs recirculate air. Continuous flow tunnels provide steady airflow, whereas intermittent flow tunnels generate short bursts of high-speed air for specialized testing.

Low-speed vs high-speed wind tunnels

• Low-speed and high-speed wind tunnels are designed to test aerodynamic properties of objects at different velocity ranges
• The classification of wind tunnels based on speed helps in understanding the specific flow phenomena and testing requirements associated with each regime

Subsonic wind tunnels

• Operate at speeds below the speed of sound (Mach < 0.8)
• Used for testing aircraft, vehicles, and buildings at low speeds
• Flow in subsonic tunnels is generally incompressible, simplifying analysis and instrumentation
• Examples include low-speed tunnels for studying boundary layer behavior and testing small-scale models

Transonic wind tunnels

• Operate at speeds near the speed of sound (Mach 0.8 to 1.2)
• Used for testing aircraft and missiles in the transonic regime, where compressibility effects become significant
• Transonic tunnels often feature adaptive wall technology to minimize wall interference effects
• Examples include the National Transonic Facility (NTF) at NASA Langley Research Center

Supersonic wind tunnels

• Operate at speeds above the speed of sound (Mach > 1.2)
• Used for testing high-speed aircraft, missiles, and spacecraft components
• Supersonic tunnels require specialized nozzle designs to generate uniform, high-speed flow
• Examples include the 8-Foot High-Speed Tunnel at NASA Langley and the Unitary Plan Wind Tunnel at NASA Ames

Hypersonic wind tunnels

• Operate at very high speeds (Mach > 5)
• Used for testing spacecraft, hypersonic vehicles, and reentry vehicles
• Hypersonic tunnels often use heated, high-pressure air or other test gases to simulate high-enthalpy flows
• Examples include the Hypersonic Wind Tunnel Facility at the Arnold Engineering Development Complex (AEDC)

Open-circuit vs closed-circuit wind tunnels

Open-circuit wind tunnel design

• Air is drawn from the atmosphere, passed through the test section, and exhausted back to the atmosphere
• Simpler and less expensive to construct compared to closed-circuit tunnels
• May require larger buildings or outdoor installations to accommodate the intake and exhaust
• Examples include the 80x120 Foot Wind Tunnel at NASA Ames and the 14x22 Foot Subsonic Tunnel at NASA Langley

Closed-circuit wind tunnel design

• Air circulates continuously within a closed loop, with the test section forming part of the loop
• More complex and expensive to build than open-circuit tunnels
• Offers better control over flow quality and reduced energy consumption for continuous operation
• Examples include the National Full-Scale Aerodynamics Complex (NFAC) at NASA Ames and the European Transonic Windtunnel (ETW) in Germany

Advantages of open-circuit tunnels

• Lower construction and maintenance costs
• Easier to accommodate larger test sections or models
• Suitable for testing engines or other systems that produce exhaust gases

Advantages of closed-circuit tunnels

• Better flow quality control and uniformity
• Reduced energy consumption for continuous operation
• Ability to use heavy gases or pressurized air for specialized testing
• Quieter operation due to enclosed design

Continuous flow vs intermittent flow wind tunnels

Continuous flow wind tunnel operation

• Air flows continuously through the test section at a steady rate
• Suitable for long-duration tests or studies requiring stable flow conditions
• Most common type of wind tunnel for general aerodynamic testing
• Examples include most subsonic, transonic, and supersonic wind tunnels

Intermittent flow wind tunnel operation

• Air flow is generated in short bursts, typically using high-pressure storage tanks or fast-acting valves
• Used for simulating very high-speed flows or studying transient phenomena
• Allows for higher flow velocities and Reynolds numbers compared to continuous flow tunnels of similar size
• Examples include shock tunnels, expansion tubes, and Ludwieg tubes

Shock tubes for intermittent flow

• A type of intermittent flow wind tunnel that generates high-speed flow using a shock wave
• Consists of a high-pressure driver section and a low-pressure driven section separated by a diaphragm
• When the diaphragm is ruptured, a shock wave propagates into the driven section, creating a short-duration, high-speed flow
• Used for studying high-temperature gas dynamics, hypersonic flow, and aerothermodynamics

Atmospheric vs pressurized wind tunnels

Atmospheric wind tunnel testing

• Wind tunnel operates at ambient atmospheric pressure
• Suitable for low-speed testing or when compressibility effects are not significant
• Simpler and less expensive to construct and operate compared to pressurized tunnels
• Examples include most subsonic wind tunnels and some transonic tunnels

Pressurized wind tunnel testing

• Wind tunnel operates at elevated pressures, typically several times atmospheric pressure
• Allows for higher Reynolds numbers and more accurate simulation of full-scale flow conditions
• Required for testing at high transonic and supersonic speeds to avoid liquefaction of air
• Examples include the National Transonic Facility (NTF) and the European Transonic Windtunnel (ETW)

Reynolds number effects in wind tunnels

• Reynolds number is a dimensionless parameter that relates inertial forces to viscous forces in fluid flow
• Matching Reynolds number between wind tunnel tests and full-scale conditions is important for ensuring dynamic similarity
• Pressurized wind tunnels allow for higher Reynolds numbers by increasing air density, which reduces the required model scale or flow velocity
• Cryogenic wind tunnels, which use cold, pressurized nitrogen gas, can achieve very high Reynolds numbers for a given model size

Special purpose wind tunnels

Icing wind tunnels

• Designed to simulate atmospheric icing conditions for aircraft and other vehicles
• Equipped with water spray systems and refrigeration units to generate supercooled water droplets
• Used for testing ice protection systems, studying ice accretion physics, and certifying aircraft for flight in icing conditions
• Examples include the Icing Research Tunnel (IRT) at NASA Glenn Research Center and the Altitude Icing Wind Tunnel (AIWT) at NRC Canada

Automotive wind tunnels

• Designed for testing the aerodynamics, acoustics, and thermal management of ground vehicles
• Often feature rolling roads (moving belts) to simulate ground motion and rotating wheels
• May include solar simulation and climatic control systems for environmental testing
• Examples include the Chrysler Aero-Acoustic Wind Tunnel and the Mercedes-Benz Aeroacoustic Wind Tunnel

Vertical wind tunnels for free fall simulation

• Also known as indoor skydiving tunnels
• Generate a vertical airflow to simulate the experience of free fall for human flight
• Used for skydiving training, military parachute training, and entertainment
• Examples include iFLY indoor skydiving facilities and the SkyVenture vertical wind tunnels

Wind tunnels for turbulence research

• Designed to study the fundamental physics of turbulent flows and develop turbulence models
• Often feature highly customizable test sections and advanced flow conditioning systems
• Used for research in fields such as atmospheric boundary layers, wind engineering, and turbulent mixing
• Examples include the Princeton Superpipe facility and the NASA Langley 20-Inch Supersonic Wind Tunnel

Wind tunnel components

Test section design considerations

• The test section is the heart of the wind tunnel, where the model or test article is placed
• Test section size and shape are determined by the intended application and flow regime
• Closed test sections have solid walls and are suitable for most subsonic and transonic testing
• Open test sections have no side walls and are used for testing larger models or reducing wall interference effects
• Slotted-wall test sections feature perforated walls to minimize wall interference in transonic flow

Contraction cone and settling chamber

• The contraction cone is a gradually narrowing section upstream of the test section that accelerates the flow
• Contraction ratios (inlet area to outlet area) typically range from 6:1 to 12:1
• The settling chamber is a wide, low-velocity section upstream of the contraction cone
• Contains flow conditioning elements such as honeycomb and screens to reduce turbulence and improve flow uniformity
• Honeycomb straightens the flow and reduces lateral velocity components
• Screens further reduce turbulence and improve flow uniformity

Diffuser and exhaust sections

• The diffuser is a gradually expanding section downstream of the test section that decelerates the flow
• Diffuser angles are kept small (typically 3-5 degrees) to avoid flow separation and maintain pressure recovery
• The exhaust section connects the diffuser to the atmosphere (in open-circuit tunnels) or the return leg (in closed-circuit tunnels)
• May include silencers or acoustic treatments to reduce noise emissions

Fan and drive systems for wind tunnels

• The fan provides the pressure rise necessary to overcome losses and maintain the desired flow velocity
• Can be located upstream (blow-down configuration) or downstream (suck-down configuration) of the test section
• Drive systems can be electric motors, gas turbines, or hydraulic motors, depending on the power requirements and tunnel size
• Variable-frequency drives (VFDs) are often used to control fan speed and adjust flow velocity
• Fan blade design is optimized for efficiency and low noise generation

Wind tunnel instrumentation

Pressure measurement in wind tunnels

• Pressure measurements are used to determine aerodynamic forces, flow velocities, and pressure distributions
• Static pressure taps on the model surface or tunnel walls are connected to pressure transducers or manometers
• Pitot tubes and Pitot-static probes measure local flow velocity by comparing total and static pressures
• Pressure-sensitive paint (PSP) allows for non-intrusive, full-field pressure measurements on model surfaces

Velocity measurement techniques

• Hot-wire anemometry uses thin, electrically-heated wires to measure local flow velocity and turbulence
• Laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) are optical techniques that measure velocity by tracking seeding particles in the flow
• Doppler global velocimetry (DGV) is a three-dimensional, full-field velocity measurement technique based on the Doppler effect
• Ultrasonic anemometry measures velocity using the time-of-flight of ultrasonic pulses in the flow

Force balance systems for aerodynamic loads

• Force balances measure the aerodynamic forces and moments acting on the model
• Internal balances are mounted inside the model and connect to a sting that supports the model
• External balances are located outside the test section and connect to the model via a support strut
• Strain gauge or piezoelectric sensors are used to measure the forces and moments in multiple axes
• Balance calibration is critical for accurate measurements and accounts for interactions between different load components

Flow visualization methods in wind tunnels

• Flow visualization techniques help in understanding complex flow patterns and identifying regions of interest
• Smoke or fog generators can be used to create visible streamlines or pathlines in the flow
• Tufts or oil films on the model surface reveal surface flow patterns and separation regions
• Schlieren and shadowgraph imaging techniques visualize density gradients in compressible flows
• Laser light sheet illumination can be used with seeding particles to visualize flow structures in a plane