Introduction

The 3D Particle Tracking Velocimetry (PTV) offers a flexible technique for the determination of velocity fields in flows. It is based on the visualization of a flow with small, neutrally buoyant particles and a stereoscopic recording image sequences of the particles. In the past decade the successful research work performed by the Institute of Geodesy and Photogrammetry at ETH Zurich led to an operational and reliable measurement tool used in hydrodynamics and space applications. In cooperation with the Institute of Hydromechanics and Water Resources Management at ETH Zurich further progress has been achieved in the improvement of the existing hard- and software solutions. Regarding the hardware setup the acquisition system used at the ETH Zurich was upgraded from offline to online image digitization.

Data acquisition

• Seed a flow with tracer particles
• Illuminate a 3-D observation volume inside the flow by a pulsed lightsource
• Image the scene by 2 (or rather 3-4) synchronized
• Length of image sequences depending from imaging rate and storage device

The system used at the ETH Zurich was upgraded from offline to online image digitization. In the previous system, the image sequences were firstly recorded on analogue videotapes and digitized afterwards, while in the new system two frame grabbers (Matrox Genesis) are used to provide online digitization and storage. The length of the recorded digital image sequences is nowadays restricted by the storage device capabilities. The data rate for a 60 Hz full-frame camera with a resolution of 640 x 480 pixels is about 19 MB/sec, and hence in an experiment which lasts for 1 minute four cameras deliver a total amount of about 4.5 GB image data.

The Particle Tracking Velocimetry software performs the following tasks:

• Calibration of the multi-camera system (determination of camera exterior and interior orientations, lens distortion and further disturbances, (e.g. PDF of Willneff and Maas,2000) and the exact geometric modelling ("multimedia geometry" - each beam from a particle to the sensor passes the three optical media water, glass, air with different refractive indices, which leads to a twice broken beam).
• Image preprocessing: perform highpass filtering due to non-uniformities in the background illumination
• Detect particles in the images by a modified thresholding operator, localize particles with subpixel accuracy by a centroid operator
• Establish stereoscopic correspondences
• Determine 3-D particle coordinates
• Storage of all relevant object and image space information
• Perform tracking in 2-D image and 3-D object space

A crucial point is the handling of ambiguities occuring in different steps of image and data processing:

• Particles may overlap in the images. For that reason a modified thresholding/centroid operator was developed searching for local maxima in the images and dividing particle images at local minima under certain conditions.
• Due to the fact that particle images cannot be distinguished by features like size, shape or color, the only criterion for the establishment of stereoscopic correspondences is the epipolar line. Ambiguities occur when multiple candidates are found in a search area defined by the epipolar line. These ambiguities can only be solved if a third (or even a fourth) camera is being used.
• Ambiguities may also occur in the tracking procedure. Criteria like local correlation and smoothness of the velocity field are employed to solve these criteria.

Data Processing

• Image preprocessing: perform highpass filtering due to non-uniformities in the background illumination
• Detect particles in the images by a modified thresholding operator, localize particles with subpixel accuracy by a centroid operator
• Establish stereoscopic correspondences
• Determine 3-D particle coordinates
• Storage of all relevant object and image space information
• Perform tracking in 2-D image and 3-D object space

A crucial point is the handling of ambiguities occuring in different steps of the data processing chain:

• Particles may overlap in the images. For that reason a modified thresholding/centroid operator was developed searching for local maxima in the images and dividing particle images at local minima under certain conditions.
• Due to the fact that particle images cannot be distinguished by features like size, shape or color, the only criterion for the establishment of stereoscopic correspondences is the epipolar line. Ambiguities occur when multiple candidates are found in a search area defined by the epipolar line. These ambiguities can only be solved if a third (or even a fourth) camera is being used.
• Ambiguities may also occur in the tracking procedure. Criteria like local correlation and smoothness of the velocity field are employed to solve these criteria.

Another important issue is an accurate calibration of the system (determination of camera exterior and interior orientations, lens distortion and further disturbances) and the exact geometric modelling ("multimedia geometry" - each beam from a particle to the sensor passes the three optical media water, glass, air with different refractive indices, which leads to a twice broken beam).

Potential

POTENTIAL:

• Truely 3-D technique: all three components of the velocity field are determined in a 3-D observation volume
• Delivers 3-D vector field for Eulerian analysis plus 3-D trajectories for Lagrangian analysis
• A system based on 4 CCD progressive scan cameras (digitized to 640 x 480 pixels) is capable of tracking more than 1000 particles
• The ralative accuracy of the velocity vectors is ~ 1:4000 of the field of view

Projects and applications

• Velocity derivatives in turbulent flow from 3D-PTV measurements
• JET growth motion in aerosols module observed under micro-gravity conditions on the sounding rocket MASER 8
• Anaglyph display of a portion of a velocity field determined by 3-D PTV (use red/blue glasses for stereo-viewing)
• Study on the applicability of 3-D PTV to surface tension driven convection (Marangoni convection)

joined project with ESA-ESTEC and Alenia Aerospazio

• Study on the applicability of 3-D PTV for measurements in liquid columns (joined project with ESA-ESTEC and Alenia Aerospazio)
• 2-D projection of a 3-D trajectory field in a vortex measured with 3-D PTV