This hybrid approach proposed in the present study is a mixture algorithm of the 3D PTV and Tomo-PIV based on a two-camera system, which is able to measure the flow field in a closed system with high refractive without in situ calibration. Knowing the calibration data in the air and refractive indices of different optical media, a simplified multimedia photogrammetry model is established based on the least square method. A new particle matching algorithm using the concept of match probability between the twin image frames has been developed to reconstruct the physical position of the particles. In order to overcome the disadvantage of low particle density, time-resolved PIV is utilized at a sampling rate being 2000 Hz to acquire the instantaneous particle images. Then, the spectrum superposition of the cross-correlation distribution is applied to increase the signal-to-noise ratio in the velocity prediction.
The technique proposed here can be used to overcome the difficulty in conventional calibration method for closed measurement objects. Both the computer simulation and some experiments imaging a calibration target reference field are conducted to show the accuracies of the calibration and reconstruction process. The capability of the technique in real experimental conditions is assessed with the measurement of the flow structure in a closed slinger combustor by hydraulic simulations.