Digital Particle Image Velocimetry advances flow velocity studies

17 Jun 2002

Kai Herrmann, Niclas Tylli; ETH Zürich

Application: Digital Particle Image Velocimetry (DPIV)

Experimental studies of flow velocity fields have been greatly advanced by the development of optical techniques

It has been recognized that multipoint measurements can be achieved by recording the motion of particles that have been added as tracers to the fluid. A broader group of whole field velocimetry techniques, Pulsed Light Velocimetry (PLV), allow the capture of the quantitative flow velocity field instantaneously. One of these methods is Particle Image Velocimetry (PIV) using a statistical analysis of the recordings, which can provide a high spatial resolution with reliable accuracy.

At the Institute of Energy Technology at the ETH in Zurich, Switzerland, a Digital Particle Image Velocimetry (DPIV) system was constructed to study the so-called backward-facing step flow (BSF). This pattern occurs when fluids flow from one channel to a markedly wider channel over a step discontinuity. Despite the relatively simple geometry, the backward-facing step in flow physics is quite complicated, and a highly adaptable experimental setup with respect to the position of the observation area has been built. The test rig consists of water flowing in a glass channel with a sudden expansion. The light source is a continuous wave laser. It is very important in this setup, to eliminate as much of the background noise as possible.

Due to the very time-consuming image analysis of photographic recordings, one is also interested in an implementation of digital image processing. A new digital CCD-camera system from PCO Computer Optics GmbH in Germany consisting of a SensiCam (ultrafast sequence of 200ns and shorter, capturing of two separate images for PIV, 12-bit digitization, 1280 x 1024 pixels) and an interface cards for the PC, has been installed, offering the advantages of online observation, calibration of setup parameters, very fast image recording and processing, and accurate measurements.

The advantage of digital PIV as compared to classical photographic PIV lies in fast and convenient interrogation of the images as well as in the possibility to record the scattered light images of the two pulses in separated frames. This advantage gives the opportunity to obtain directionally resolved analysis by interrogating the images with cross-correlation techniques, which results in a precise determination of the flow direction and in a higher dynamic velocity range. Furthermore, series of double pulsed image pairs are recordable, so that for low Reynolds numbers a certain time resolution can also be achieved. This has not been possible in classic PIV systems and is a drawback of the technique.