Redesign and improvement of an open circuit wind tunnel

National Standards are the realization of units of measurement aimed to be the basis for the national traceability chain, and the connection to International Standards. Depending on the quantity to be measured, they can consist in material standards, primary instruments, or test rigs realizing the quantity to be measured, coupled to a traceable measurement system. The present paper is focused on such a test rig. The Italian airspeed primary standard is generated, for the range between 2 and 25 m/s, by means of an open-circuit, semi-open test chamber wind tunnel, originally built in the thirties; in the nineties, the wind tunnel was reconditioned by replacing the divergent and the engine/propeller group with a fan in a slowly diverging tube. The renewed rig has a satisfactory behaviour in the lower part of its range, but when the velocity is increased, it undergoes strong vibrations induced by the flow. This fact is strongly undesirable since it affects the precision of the measurements in the higher part of the range. Clearly, this configuration is not adequate for a primary standard facility. Therefore, the divergent and engine/propeller parts of the wind tunnel are being reviewed again, using a design inspired to the original one. In the present paper, we will present the new design analysis that was performed on the new divergent and on new fan in order to validate the design concepts and evaluate possible simplifications of the configuration adopted.