3D Velocity measurements by Optical Flow PIV based on a simple and accurate camera calibration, postscript, pdf, Allecsandra Rambert, François Lusseyran, Pierre Gougat and Georges M. Quénot, 11th International Symposium on the Applications of Laser Techniques to Fluid Mechanics, paper 24.2, Lisbon, Portugal, 8-11 July, 2002

The aim of this work is to present first results for 3D velocity field measurements obtained by Optical Flow PIV coupled with an original technique of camera calibration . The calibration method search the parameters of an optimal pinhole camera model using only a single image of a dot grid target, the known camera focal length and sensor pixel size. After the target marks have been located, an optimal projective transform is calculated by a least square method between the target plane coordinates and the image plane coordinates. The camera model is recovered via a canonical decomposition of the linear part of the projective transform and an iterative search for the location where the optical axis intersects the image plane. The interaction between a flow and a cavity is a test configuration useful to study the unsteady flows and the development of the 3D instabilities. It also represents a configuration well adapted to studies relating to transport (aerodynamics of vehicles) and environment (air renewal in a cavity). Flow issued from a boundary layer and a cavity interaction was characterised by PIV based on Optical Flow measurements performed inside and out of the cavity for two perpendicular planes which showed that the flow is three dimensional. Therefore we choose this configuration to perform 3D velocity measurements by stereoscopic PIV.
The results presented in this paper were obtained for low velocities, close to the laminar turbulent transition, for a given geometry of cavity characterised by a shape factor H/L=0.5, where H is the height of the cavity and L its length. The Reynolds number based on the height of the cavity and the mean velocity is about 4000.