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  • Jawaharlal Nehru Technological University Anantapur (JNTU) College of Engineering (CEP), Pulivendula, Pulivendula, Andhra Pradesh, India - JNTUACEP
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UNIT- I introduction to Wind tunnel 2

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Introduction Need of experiments (i)Theory is incomplete and needs to be supplemented. (ii) Information of fundamental nature needed in many areas. Experimental information towards solving aerodynamic problems could be obtained in a number of ways. Flight tests, rocket flights, drop tests water tunnels, ballistic ranges and wind tunnels are some of the ways by which aerodynamic data can be generated. With the help of well performed experiments even information of fundamental nature could be derived. Wind tunnel Majority of experimental data needed in aerodynamics is generated using wind tunnels. Wind Tunnel is a device for producing airflow relative to the body under test. Wind tunnels provide uniform flow conditions in their test section. 1.1 Classification of wind tunnels Wind tunnels may be classified based on any of the following: (a) Speed, Mach no They are classified as of low speed or high speed wind tunnels .In wind tunnel parlance, high speed wind tunnels are those operating at speeds where compressibility effects are important. They are also classified based on the Mach number of operation as subsonic, transonic, supersonic or hypersonic wind tunnels. (b) Mode of operation (Pressure storage, in-draft or Pressure vacuum type.) (c) Kind of test section (T.S) - Open, Closed or Semi enclosed 1.2 Applications of wind tunnels 1. Aerodynamic applications 2. Non-Aero applications in     Civil Engineering   Automobile Engineering   Calibration of instruments3

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  1.3 Model making, Non-dimensional parameters Geometric similarity One of the most important requirements of models is that there should be geometric similarity between the model and the prototype. By geometric similarity it is meant that ratios of corresponding dimensions in the model and the prototype should be the same. Dynamic similarity Equally important as the geometric similarity is the requirement of dynamic similarity. In an actual flight, when the body moves through a medium, forces and moments are generated because of the viscosity of the medium and also due to its inertia, elasticity and gravity. The inertia, viscous, gravity and elastic forces generated on the body in flight can be expressed in terms of fundamental units. The important force ratios can be expressed as non dimensional numbers. For example,  Reynolds number (Re) = Inertia force/Viscous force  Mach number = Inertia force/Elastic force  Froude number = Inertia force/Gravity force The principle of dynamic similarity is that a scale model under same Reynolds number and Mach number will have forces and moments on it that can be scaled directly. The flow patterns on the full scale body and the model will be exactly similar. It is not necessary and may not be possible that all the aforesaid non dimensional numbers be simulated simultaneously in any experiment. Depending on the flow regime or the type of experiments, certain non-dimensional parameters are important. For example, in a low speed flow regime, simulation of Reynolds number in the experiments is important to depict the conditions of actual flight. In a high speed flow, simulation of Mach number is significant. It may even be necessary and significant that more than one non dimensional parameter are simulated. The principle of dynamic similarity is applicable in other fields of engineering too. As examples: Stanton number is simulated in heat transfer experimentation. Stanton no (St) = Heat transferred in to the fluid / Thermal capacity St  h 4

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