Sometimes it happens to be commissioned for conducting modal analysis of structures with a mass really important. It was recently asked modal analysis of the structure that constitutes the rotating floor of 13m diameter of the low speed chamber at the wind tunnel of Politecnico di Milano.
The wind tunnel of the Politecnico is constituted by a closed circuit with vertical development that allows to have two test sections: the high speed for typical applications of aerospace engineering and the low speed and large sized, formed in the return circuit for typical applications of wind engineering and environmental. The flow generator consisting of 14 fans, each provided with 12 blades, arranged in two rows by seven housings of 2x2m each, for a total power of 1.4 MW, which allows the achievement of 55 m/s (200 km/h). The test chamber civil-environmental (boundary layer) allows to reach a maximum speed of the order of 14 m/s (about 50 Km/h). The size of the room, with a section of 13.84 x 3.84 m, together with suitable devices for the generation of turbulence (active and passive), allow to correctly reproduce the velocity field characteristic of the boundary layer of the Earth. The room in the boundary layer is equipped with a revolving table of 13 meters of diameter such as to allow aerodynamic tests for different exposures to the wind of models of large scale inclusive orography. The device, based on the use of air cushions, provides full control of the rotation of 360 degrees of models of considerable size and mass (flow 100000N).
An instrumented hummer or a shaker are not sufficient in order to excite a structure of this magnitude. So it was invented an ad hoc exciter using gravitational force. It was installed on the structure of the swivel table a strain gauge triaxial load cell with a range of 50000N. Then through a system of pulleys it has been dropped a 25Kg mass from a height of 400mm to obtain an optimal excitation of the bearing structure . To measure the vibrations induced 18 accelerometers were placed on the steel structure .