A wind tunnel is a tool used in
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.
Later on, wind tunnel study came into its own: the effects of wind on man made structures or objects needed to be studied when buildings became tall enough to present large surfaces to the wind, and the resulting forces had to be resisted by the building's internal structure. Determining such forces was required before
Still later, wind-tunnel testing was applied to
Wind tunnel testing of sporting equipment has also been prevalent over the years, including golf clubs, golf balls, Olympic bobsleds, Olympic cyclists, and race car helmets. Helmet aerodynamics is particularly important in open cockpit race cars (Indycar, Formula One). Excessive lift forces on the helmet can cause considerable neck strain on the driver, and flow separation on the back side of the helmet can cause turbulent buffeting and thus blurred vision for the driver at high speeds.
The advances in
Air velocity and pressures are measured in several ways in wind tunnels.
Air velocity through the test section is determined by
Aerodynamic forces on the test model are usually measured with
The pressure distributions across the test model have historically been measured by drilling many small holes along the airflow path, and using multi-tube
Pressure distributions on a test model can also be determined by performing a wake survey, in which either a single
The aerodynamic properties of an object can not all remain the same for a scaled model. 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:
In certain particular test cases, other similarity parameters must be satisfied, such as e.g.