For example, moving at twice the speed of sound is Mach 2, moving at half the speed of sound is Mach 0.5. Early work in fluid mechanics, or the study of how fluids and gases behave and their effect on objects in a flow, indicated that the airflow around a scale model would not correspond exactly to the flow around a full-scale aircraft. To ensure the correlation of model data to full-scale aircraft data, researchers also determine the Reynolds number of flow in a wind tunnel. The speed of air flowing through a wind tunnel is usually expressed in terms of the speed of sound, which is approximately 761 miles per hour at sea level. However, the speed of sound through the atmosphere varies with temperature.
Sound travels more slowly through cooler air.
Aircraft usually fly at higher Mach numbers in the upper atmosphere where the air is colder. Reynolds number expresses the relationship of the density of the fluid, velocity, the dimension of an object, and the coefficient of viscosity of the fluid relationship. The Reynolds number would then be the same in the wind tunnel tests and actual full-scale flights. In addition to Mach number and Reynolds number, researchers measure airflow around a model or specific parts of a model, pressure exerted on the model, lift, drag, and engine thrust. There are many techniques that researchers use to obtain these measurements from wind tunnels and models.