Type III classification- Education vs research wind tunnel The speed of sound in air at room temperature is approximately 343 m/s and for this case Mach number is less than one (M1). If the maximum speed achieved by the wind tunnel is less than the speed of sound in air, it is called a subsonic wind tunnel. Schematic designs of subsonic and supersonic wind tunnels are shown in figure. That ratio is called the Mach number, named after Ernst Mach, the 19th century physicist. It is traditional to use the ratio of the speed of the fluid, or of any other object, and the speed of sound. The criterion for classification is the maximum speed achieved by the wind tunnel. Type II Classification- Subsonic vs Supersonic wind tunnel It is conventional to call that a closed-circuit (closed-return ) wind tunnel. The wind tunnel is said to have a closed-air circuit. In closed-circuit, or closed-return, wind tunnel, the same air is being circulated in such a way that the wind tunnel does neither draw new air from the surrounding, nor returns it into the surroundings. In open-circuit (open-return) wind tunnel, the air is drawn directly from the surroundings into the wind tunnel and rejected back into the surroundings, the wind tunnel is said to have an open-air circuit. Type I classification - Open vs closed circuit wind tunnel Wind tunnels can be classified using four different criteria The approach described here uses a force balance approach to obtain lift and moment data and the wake rake method to obtain drag. Most aerodynamic performance measurement techniques for airfoils rely on using balance systems or pressure systems, or a combination of both. Accurate measurements of low Reynolds number airfoil performance arekey to understanding and improving the efficiency of low Reynolds number systems. Low Reynolds number aerodynamics of airfoils apply to a host of other applications such as wind turbines, motorsports, high altitude aircraft and propellers, natural flyers, and subscale testing of many full scale systems. The test object, often called a wind tunnel model is instrumented with suitable sensors to measure aerodynamic forces, pressure distribution, or other aerodynamic-related characteristics.Airfoil performance at low Reynolds numbers impacts the performance of a wide range of systems. Air is made to move past the object by a powerful fan system or other means. A wind tunnel consists of a tubular passage with the object under test mounted in the middle. A wind tunnel is a tool used in aerodynamic research to study the effects of air moving past solid objects.
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