The recirculating water tunnel is around 5000 mm long and 2200 mm high, whereby the test section is 1250 mm long, 333 mm wide and 250 mm high. The side wall and the basement plate of the test section are transparent for optical access. The tunnel is filled with 1.7 m³ water and is driven by a 4 kW alternating current motor with a hydraulic gear. The maximum flow velocity in the test section behind the nozzle with a contraction ratio of 4:1 is 1 m/s and the turbulence level is around 0.8 % at 0.3 m/s flow velocity. The stability of the mean flow velocity is displayed on the right. The measurement was performed with a PIV system. N represents the picture number, where pictures were taken every 125 ms.
The water tunnel is mainly used for qualitative flow visualisation experiments with ink and with tracer particles that are illuminated with an Argon-Ion laser. The facility can be equipped with a PIV system for quantitative examinations. When synchronisation with an external event is required, PIV can be operated with a pulsed Nd:YAG laser, otherwise a continuous wavelength Argon-Ion laser with shutter wheel is available.
The water tunnel is well suited for steady and unsteady flow investigations and also Time-Resolved-PIV investigations can be easily performed with video recording at low flow velocities. However, also fundamental research can be performed in the facility. To examine the flow phenomena around an oscillating airfoil with combined flapping and pitching motions, a special profile mount was developed, which is shown on next page. This device allows heave amplitudes of 20 mm with frequencies up to 10 Hz. In addition it is possible to damp the pitching motion continuously, by varying sliding forces with a helical spring. Note that the profile is assembled with suction side down, directed to laser light sheet from underneath. By using a photodiode it is possible to synchronise the PIV equipment with the motion of the profile for phase-locked investigations. Of special interest are the laminar separation bubbles but also the global flow structure with vortices. Some recent results of the measured velocity field of the heaving profile at various times are shown in the upper right figure. To maximise the signal to noise ratio each plot is an average of 50 measurements. The figures show the development of the vortex due to increasing circulation, its separation at the trailing edge and its behaviour in the wake. Detailed PIV measurements in the laminar separation bubble on the suction side of the airfoil yield the mean and fluctuating flow quantities. The figure on the right displays streamlines and Reynolds shear stress for steady flow conditions with Re = 60000 and the angle of attack, alpha = 8°. The fluctuations start to increase at the maximum thickness of the bubble, where the flow transitions rapidly from laminar to turbulent. Inside the separation bubble a vortex can be detected. The vortex show a strongly unsteady behaviour. However, this is not displayed in the time-averaged result shown.