Determining such forces was required before building codes could specify the required strength of such buildings and these tests continue to be used for large or unusual buildings. Very tall buildings present large surfaces to the wind, and the resulting forces have to be resisted by the building's internal structure or else the building will collapse. The effects of wind on man-made structures need to be studied when buildings became tall enough to be significantly affected by the wind. Other problems are also studied with wind tunnels. Wind tunnel testing was considered of strategic importance during the Cold War for development of aircraft and missiles. Large wind tunnels were built during World War II, and as supersonic aircraft were developed, supersonic wind tunnels were constructed to test them. The development of wind tunnels accompanied the development of the airplane.

In this way, a stationary observer could study the flying object in action, and could measure the aerodynamic forces acting on it. Instead of the air standing still and an aircraft moving, an object would be held still and the air moved around it. The wind tunnel reversed the usual situation. The earliest wind tunnels were invented towards the end of the 19th century, in the early days of aeronautical research, as part of the effort to develop heavier-than-air flying machines. Or, small threads can be attached to specific parts to show the airflow at those points. Smoke can be introduced into the airstream to show the path that air takes around the object. The air pressure at different points can be measured with sensors. The aerodynamic forces on the entire object can be measured, or on individual components of it. Different measurements can be taken from these tests. The object can be an aerodynamic test object such as a cylinder or an airfoil, an individual component of an aircraft, a small model of the vehicle, or, in the largest tunnels, even a full-sized vehicle. Usually, large fans move air through the wind tunnel, while the object being tested is held stationary. Different wind tunnels range in size from less than a foot across, to over 100 feet (30 m), and can have air that moves at speeds from a light breeze to hypersonic velocities. They are used to test the aerodynamic effects of aircraft, rockets, cars, and buildings. Wind tunnels are machines where an object is held stationary inside a tube, and air is blown around it to study the interaction between the object and the moving air. NASA wind tunnel with the scale model of an airplane 16-foot supersonic wind tunnel at Arnold Air Force Base, 1960 A model Cessna with helium-filled bubbles showing pathlines of the wingtip vortices Machine used for studying the effects of air moving around objects

Flume water sensor