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The High-Speed Video Stroboscope

A novel video stroboscope for periodic and non-periodic repetitive high-speed events has been developed, constructed and tested. These events can be displayed on the monitor in slow motion in real-time. The stroboscope does not utilize a flash light. The object can be illuminated by a standard CW light source such as a halogen lamp or by day light. Hence, self-luminous objects can also be examined.

The system consists of an asynchronously shuttered progressive scan CCD camera, a frequency-independent digital phase shifter and a PCI-bus frame grabber located in an industrial computer. The image of the stroboscopically investigated event is displayed live on the PC monitor. The shutter of the camera is triggered by the event via the phase shifter which can be programmed to change the phase shift slowly. In that case a slow motion effect can be realized in real time. The apparent speed of the displayed movement does not depend on the real event frequency. Both the phase shift and the slow motion period are selectable independent of the real event frequency. Non-periodic but repetitive events can be observed in an alternative time-delay mode of the phase shifter. The camera exposure time can be varied between 1.25 μs and 16 ms. It allows the stroboscopic observation of very fast oscillating (up to 10 kHz) and rotating (above 100,000 rpm) objects.

Sequences of frames can be stored on the hard disc drive for documentation, evaluation and playback.

The applications of the video stroboscope include the macro- and microscopic fluid flows, design and diagnostics of the turbines and engines (fuel injection, combustion), marine propellers (cavitation), and other fast-running machines, human medicine (examination of larynx) and zoology (analysis of insect fly in a wind tunnel).

New Systems

Following systems based on the above described principle have been developed:

• Color video stroboscope,
  (VGA-resolution, CameraLink_Interface)
• Multi-camera video stroboscope
  (simultaneous acquisition of up to 4 cameras, 760 x 582 pixels each) and
• High resolution video stroboscope
  (similar to the multi-camera system, but 1392 x 1040 pixels)

A special version of the high resolution video stroboscope has been successfully applied for the investigation of the flap gaps during the Airbus A340-400 flight tests within the scope of EU-project „Awiator“. This version provides an automatic control of the exposure time and camera gain to handle illumination variations (depending on the time of the day, position of the sun and clouds).

The systems are protected by national and international patents and licensed to manufacturing companies.

Relevant techniques

Besides the qualitative visualization, the systems described provide also quantitative data in the modern techniques as:

RT-PIV	Real-Time Particle Image Velocimetry	Velocity field
RT-BOS	Real-Time Background Oriented Schlieren	Density gradients field
RT-IPCT	Real-Time Pattern Correlation Technique	Deformation
POSI	Optical Position Detection	Position
EDT	Edge Detection Technique	Variation of Position

In this way, the 3-D acquisition of the physical values as velocity, density, position and deformation can be performed at high-speed periodic events. The system application for the wind tunnel experiments as well as the flight tests has contributed to the solution of various aerodynamic problems.

Application examples

1. Injection of Diesel fuel from an injection nozzle. Courtesy Volkswagen AG

Time of the nozzle opening 500 µs, back pressure 50 bar, fuel pressure 1550 bar, temperature of the chamber 450 °C (Click here to enlarge the picture.)

2. An airfoil model with a vibrating trailing-edge flap placed in a wind tunnel. The flow visualisation was performed with a smoke and light sheet technique.

Flow velocity 5 m/s, flap oscillation frequency 30 Hz (Click here to enlarge the picture.)