3D PIV

STEREOSCOPIC DIGITAL PARTICLE IMAGE VELOCIMETRY

Particle Image Velocimetry is now a well established two dimensional measurement technique. In the natural advancement of measurement techniques, two dimensional particle image velocimetry has been extended to the third dimension. Stereoscopy is used to resolve the third component of velocity over an interrogated plane. The interest of the present work relies in the use of video cameras that allow the implementation of Digital Particle Image Velocimetry.

This Stereoscopic Particle Image Velocimetry allows the measurement of the three components of the velocity in a plane rather than in a volume. It only differs from standard two dimensional PIV by the use of two imaging systems rather than one. In the present work two video cameras connected to two digital image recording systems are used. These cameras could either be set at an angle, with the same object field or side by side, in the translation mode; the latter was used. The principle of stereoscopic Particle Velocimetry using translation of imaging systems is presented in Fig. 1.

Figure 1: Principle of translation stereoscopy PIV

Two tracer images defining a velocity vector are seen by both optical systems under a different angle. We therefore obtain two different 2D velocity fields that are combined to obtain the third component.

In the translation mode, the fields of view of the two cameras do not coincide and only a central part is common. To overcome this drawback, the lenses used in the present work have been shifted laterally as shown in Fig. 2. As a result, the two fields of view of the cameras coincide completely. This shift is made possible by the use of lenses designed for 35 mm still cameras. These lenses are designed for a large sensor surface and it is therefore possible to shift the image by a large amount before meeting unacceptable distortions.

Each camera is connected to a computer equipped with a frame grabber. Both computers are inter-connected through a serial line to synchronise the acquisition of the two pictures. The two cameras are also synchronised using one as a master and the other as a slave.

Figure 2: Stereoscopic digital PIV with offset of optical axis

A synchronizer controls the YAG laser and the video cameras. Laser pulses are produced to allow each of them to fire during a different frame of the interlaced video pattern. After recording and digitising, each image is split into its two frames (the odd and the even lines) and a cross-correlation processing is performed. The latter makes use of iterative window displacement. The two resulting velocity fields are then combined using trigonometric relationships that allow to derive the third component of velocity.

To validate the measurement technique, the uniform flow produced in a wind tunnel has been used. A laser sheet illuminated the tracers along an oblique angle through the test section. The velocity vectors are therefore at an angle to the sheet. The Stereoscopic Digital Particle Velocimeter has then been used and the resulting velocity field is shown in Fig. 3. This validation was successful since the resulting vectors are indeed aligned with the axis of the tunnel.

Figure 3: Validation of Stereo DPIV - Velocity field measured in an oblique plane across a uniform flow

This new application of PIV opens the way to 3D velocity measurements with the advantages of Digital PIV, namely direct digital recording and immediate processing.

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