An Algorithm of Stereoscopic Particle Image Velocimetry for Full-Scale Room Airflow Studies

One challenge in indoor air quality studies is the measurement of three-dimensional air velocity profiles in an airspace so that the nature of airflow can be better understood and appropriate ventilation systems can be designed. There is much dispute over a variety of computational fluid dynamics (CFD) models, primarily due to a lack of credible data to validate those models. This study aimed to develop a stereoscopic particle imaging velocimetry (SPIV) system suitable for easurement of full-scale room three-dimensional airflow.
The SPIV method is based on the principle of parallax to extract a third (z-direction) velocity component using two cameras. We have developed a new three-dimensional algorithm
in the particle streak mode (PSM) that only requires two cameras to acquire three velocity components (x, y, z) and flow directions in a three-dimensional volume, rather than in a thin
layer of a plane. The two-camera approach is expected to greatly simplify the image acquisition and data processing and improve the accuracy by eliminating the error caused by the third camera image or some special techniques employed to resolve directional ambiguity. The three-dimensional image volume can contain a full range of three-dimensional velocities.
In this new three-dimensional SPIV setup, two cameras are placed at different angles to view the illuminated field and to capture particle displacement images that contain the influence of the third velocity component. The parallax effect allows us to obtain different two-velocity component vector maps from each camera. The differences between the two images arise from the third velocity component and the geometrical configuration of the two cameras. After image calibration, this third
velocity component can be calculated. The two cameras will be set at different exposure times, thus acquiring different lengths of streaks for the same particle path. The differences of the exposure then can be used to distinguish the flow directions.