Wind Tunnels

When most people think of wind tunnels, an aerospace tunnel usually comes to mind. This helps designers and engineers understand how aircraft will behave during various flight conditions. Automotive wind tunnels are usually very similar to aerospace tunnels, but they often feature a rolling floor that lets test engineers simulate a car driving on the road at various speeds.



What are Wind Tunnels?

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. Most of the time, large powerful fans blow air through the tube. The object being tested is held securely inside the tunnel so that it remains stationary. The object can be an aerodynamic test object such as a cylinder or an airfoil, an individual component, a small model of the vehicle, or a full-sized vehicle. The air moving around the stationary object shows what would happen if the object was moving through the air.



The motion of the air can be studied in different ways; smoke or dye can be placed in the air and can be seen as it moves around the object. Colored threads can also be attached to the object to show how the air moves around it. Special instruments can often be used to measure the force of the air exerted against the object.

History of Wind Tunnels

The earliest wind tunnels were invented towards the end of the 19th century, in the early days of aeronautic research, when many attempted to develop successful heavier-than-air flying machines. The wind tunnel was envisioned as a means of reversing the usual paradigm: instead of the air standing still and an object moving at speed through it, the same effect would be obtained if the object stood still and the air moved at speed past it. In that way a stationary observer could study the flying object in action, and could measure the aerodynamic forces being imposed on it.

The development of wind tunnels accompanied the development of the airplane. Large wind tunnels were built during World War II. Wind tunnel testing was considered of strategic importance during the Cold War development of supersonic aircraft and missiles.

Automotive Wind Tunnels

In the 1960s, wind tunnel testing was applied to automobiles, not so much to determine aerodynamic forces, but to determine ways to reduce the power required to move the vehicle on roadways at a given speed. In an actual situation the roadway is moving relative to the vehicle but the air is stationary relative to the roadway, but in the wind tunnel the air is moving relative to the roadway, while the roadway is stationary relative to the test vehicle. Some automotive-test wind tunnels have incorporated moving belts under the test vehicle in an effort to approximate the actual condition.

Automotive wind tunnels fall into two categories:

• External flow tunnels are used to study the external flow through the chassis.
• Climatic tunnels are used to evaluate the performance of door systems, braking systems, etc. under various climatic conditions. Most of the leading automobile manufacturers have their own climatic wind tunnels.

Advances in computational fluid dynamics modeling on high-speed digital computers, however, are reducing the demand for wind tunnel testing.

How Does a Wind Tunnel Work?

Air is blown or sucked through a duct equipped with a viewing port and instrumentation. Typically, the air is moved through the tunnel using a stationery turbofan. For very large wind tunnels, a single large fan is not practical, and an array of multiple fans are used in parallel to provide sufficient airflow.



The airflow created by the fans that is entering the tunnel is itself highly turbulent due to the fan blade motion. The air moving through the tunnel needs to be relatively turbulence-free. To correct this problem, closely spaced vertical and horizontal air vanes are used to smooth out the turbulent airflow before reaching the subject of the testing.

A wind tunnel is typically circular rather than square, because there will be greater flow constriction in the corners of a square tunnel that can make the flow turbulent. A circular tunnel provides a smoother flow. The inside facing of the tunnel is typically as smooth as possible, to reduce surface drag and turbulence that could impact the accuracy of the testing. Even smooth walls induce some drag into the airflow, and so the object being tested is usually kept near the center of the tunnel, with an empty buffer zone between the object and the tunnel walls. There are correction factors to relate wind tunnel test results to open-air results.



The lighting is usually embedded into the circular walls of the tunnel and shines in through windows. If the light were mounted on the inside surface of the tunnel in a conventional manner, the light bulb would generate turbulence as the air blows around it. Similarly, observation is usually done through transparent portholes into the tunnel. Rather than simply being flat discs, these lighting and observation windows may be curved to match the cross-section of the tunnel and further reduce turbulence around the window.

Because air is transparent it is difficult to directly observe the air movement itself. Instead, such visual aids as smoke, carbon dioxide injection, tufts, flow cones, evaporating suspensions, oil, tempera paint, fog, sublimation (multiple particle streams released from a grid of many nozzles), color-changing pressure sensitive paint, marker deformations, laser sheets, strobe lights or high-speed cameras may be used.



Videos of Wind Tunnel Tests

Here are three videos of a Challenger, Superbird and Ford GT being tested in a wind tunnel.

Model Dodge Challenger wind tunnel test - YouTube






Aerodynamics and the Design of the Challenger

Here is an informative video (start at 1:47 minute mark) where the designers of the 2008 Challenger discuss how a wind tunnel was used to tweak the retro body style and incorporate front and rear air spoilers to make it stable at high speeds.



A graphic illustration of the aerodynamic problems encountered at high speeds is contained in this video where Legendary Motor Cars races a classic 1970 Super 'Cuda on a circular track at 200 mph.



Also, watch this interesting wind tunnel test demonstrating lift and drag characteristics between a 1969 Charger R/T,1969 Daytona and 2016 Charger Hellcat. Surprisingly, the modern Hellcat performed quite well.