Labcal UKAS Accredited ‘Merlin’ Wind Tunnel
The tunnel-named Merlin-is a 'closed loop' design, which means that the air is re-circulated around the self-contained tunnel rather than continuously drawing in ambient air. Completely designed and built in-house the tunnel is constructed primarily of wood, providing the high quality internal finish necessary for a smooth air flow with minimal disturbance.
A boat building technique was utilised to create the polished finish required.
To achieve 90m/s operating wind velocity the tunnel has a 5th order polynomial contraction section with a high operating pressure with the test section operating close to ambient pressure. The acceleration in wind speed is caused by going from high to low pressure caused by the shape of the contraction section.
The variable wind velocity is achieved by having two axial contra-rotating fans. This arrangement helps reduce the swirl effect from the first fan. Each fan has a 22kW full load capacity motor which rotates up to a maximum operational set speed of 1460 rpm, with the fans and motors housed in a metal cylindrical section which is supported on anti-vibration mounts.
After the air passes through the ½ metre square test section, which is 1 metre in length, there is a diffuser section which increases the cross sectional area of the tunnel over a distance. This has the beneficial effect of converting and recovering the kinetic energy within the air flow so that the motors do not have to work so hard to maintain the wind velocities.
With such energy within the wind tunnel system, safe operation is paramount. There are removable access points throughout the tunnel each with a safety interlock switch that immediately cuts the motor power if an access point is open. During normal tunnel operation if an attempt is made to open an access point, it will automatically trip the motor power shutting down both motors simultaneously, bringing the fans to a complete halt within 5 seconds.
Maintain Air Temperature – Unique Feature
To reduce the heat generated by the friction of airflow passing around the tunnel and to maximise the motor efficiency, temperature within the tunnel is maintained at a constant value at any designated set velocity,
providing accurate measurement and calibration results.
To achieve this, a water cooled chiller system has been incorporated to control the air temperature with the recovered heat being dispersed outside the building the tunnel is housed in via a chiller unit. The heat exchanger is located prior to the contraction section, enabling temperatures within the test section to be controlled to an accuracy of +/-0.1°C at 90m/s,
with normal testing being carried out at a nominal temperature of 20°C.
With the tunnel being a closed loop design, the airflow has to turn through 90° at each corner. To assist the flow and minimise any turbulence created, turning vanes are fitted in each corner. In addition, just in front of the contraction section leading into the test section, the airflow is smoothed using various filter screens, with alternative filters being used for different wind velocities.
How it measures
Within the test section itself, there are features incorporated that enable reliable, accurate calibration of anemometers, along with the ability to simulate variable wind velocities. The anemometers can be installed and tested at various heights within the test section and at varying vertical angles in a range of +/-30° in 5° increments with +/-0.5° accuracy.
In addition, the anemometer can be rotated in 1-2° horizontal steps with an accuracy of 0.05°. This is all done remotely as it is not possible to enter the test section whilst the tunnel is in operation. By programming in varying wind velocity profiles that, combined with the position control of the anemometer, permits the re-creation of realistic wind conditions as well as a constant wind velocity.
One of the prime features of the wind tunnel is its ability to maintain high wind velocities indefinitely, making prolonged test programmes possible. The average test time for an instrument is half an hour with a maximum test time of two hours typically.
A typical wind speed profile for a standard test/calibration is 5 minutes to the desired wind speed, 2 minutes at level speed and 5 minutes deceleration. As described earlier, it is possible to create complex wind profiles which coupled with altering the instrument alignment produces realistic simulations of complex wind patterns.
The tunnel has two operating stations. One is positioned next to the test section so that the test can be observed if required. However, with ear defenders required for wind velocities above 20m/s, there is a second control area
adjacent to Merlin which enjoys much lower noise levels.
Designed for an optimised wind speed of 90m/s and for higher wind speeds generally of >65m/s, the Merlin wind tunnel was the leading wind tunnel on completion for size, speed and stability. The unique ability of a large test section with temperature control to achieve a stable laminar airflow of 90m/s over long periods is a significant accomplishment.