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In the development of vehicles such as automobiles, it has recently become increasingly important to consider not only flow dynamic performance but also aerodynamic noises produced while driving. To evaluate and reduce the surrounding aerodynamic noises, it is necessary to use closed-circuit-type 3/4 open-jet wind tunnels, which include a comparatively large open measuring section and are known as 3/4 open-jet aeroacoustic wind tunnels or simply 3/4 open-jet wind tunnels. In these wind tunnels the same wind conditions as those around vehicles being driven can be simulated. Because the open measuring section has a nozzle, a floor and a collector, each of which generates their own aerodynamic noises, their reduction is necessary to measure the aerodynamic noises around a vehicle without interference. On the other hand, because of the pressure loss in the open measuring section, a large and powerful fan is necessary to provide the large flow required for 3/4 open-jet wind tunnels used for actual vehicles. Therefore, the authors have distinguished differences between the techniques of reducing aerodynamic noises in 3/4 open-jet wind tunnels and those in conventional open-jet aeroacoustic wind tunnels and have newly investigated appropriate conditions for reducing both aerodynamic noises and total load in 3/4 open-jet wind tunnels by various techniques, such as the use of pile-fabric materials, entrainment systems and large collectors, on the basis of experimental data obtained from various models of wind tunnels. In this paper, the details of this investigation are described with explanations of the aerodynamic noise phenomena in 3/4 open-jet wind tunnels.

Techniques of Reducing Aerodynamic Noises in 3/4 Open-Jet Wind Tunnels