Wind Tunnel Measurements
The wind tunnel measurements carried out cover all kinds of model structures and setups. This call for different ways of measuring response and flow parameters, and flexible measuring units are needed. All measuring units are connected to a desktop computer with a data acquisition board and several COM-ports.
The velocity pressure is always measured in the tests by a permanent installed pitot tube connected to a pressure manometer. In order to be able to determine the air density the wind tunnel also has permanent installed sensors for temperature, barometric pressure and humidity.
Since the wind tunnel is also used for calibration of anemometers, where a high precision is needed, all measuring units detecting wind parameters are high precision instruments and are calibrated once a year on external accredited institutions.
In the wind tunnel turbulence intensity and velocity profiles corresponding to full scale environments can be generated. In order to verify the turbulence characteristic, the flow velocity is measured with gold plated hotwires. By this delicate measuring device the flow velocities and turbulence characteristic is detected in all three main directions.
When flow velocity is requested at specific points on the wind tunnel model, small 13 mm coil hotwires are located on the model. This kind of measurements is used in wind environment investigations.
The wind loads on models are measured in several ways. The global wind load on a model located on the tunnel floor is usually measured by a balance. The balance is located under the tunnel floor with a pin extension through a hole in the floor for installation of the model. In this way the wind load on the model is transferred to the balance. The balance registers horizontal shear forces and bending moment in the flow direction and perpendicular to the flow direction and torsion around a vertical axis. The model is as stiff as possible. When dynamic test results are requested, the flexibility are added by springs in the balance.
When local wind loads are requested pressure measurements are carried out. Here a stiff model is constructed with small pressure taps in the model surface, where the wind loads are requested. From the inside of the model the pressure on the surface is transferred by plastic tubes to transducers converting the pressures to an electrical signal. In this way the local wind load at a specific point on the model is found. Global wind loads are determined by combining contemporary measured pressures.
Wind tunnel tests on section models are also carried out. This is a very powerful tool to investigate dynamic sensitive structures like bridges. A relative stiff section model of the structure is suspended in the wind tunnel. The model is connected to the force transducers by stiff rods in static tests or by springs in dynamic tests.