STEREO PIV ANALYSIS AND APPLICATION TO CYLINDER WAKE

Abstract

A theoretical analysis of the spatial resolution of the Stereo Particle Image Velocimetry (PIV) technique has been performed. This has been done in terms of Modulation Transfer Function (MTF) and by applying the technique with and without the correction of the misalignment error between calibration and measurement planes. The results show that some wavelengths of the flow field can be significantly dephased and modulated. The theoretical analysis has been extended to investigate also the effects caused by the laser thickness and the linear dimension of the interrogation window W (i.e. the modulation associated to the Stereo PIV process). As it will be shown in detail in this work, the modulation associated to the last two parameters cannot be corrected, differently from the modulation due to the misalignment. A performance assessment has been conducted with both synthetic and real images and shows a good agreement with the theoretical analysis. The reconstruction of the three-dimensional displacement field is achieved using both methods proposed by Soloff et al. (1997) and by Willert (1997). Finally, the Stereo PIV technique has been used to analyse the vortex shedding caused by both finite and infinite cylinders immersed in a uniform flow field. The Reynolds numbers investigated are within the Shear-Layer Transition Regime, indicated by Williamson (1996) for the infinite cylinder. This regime is characterized by the formation of von Karman vortices. For the finite cylinder, a counter-rotating pair of tip vortices forms at the free end, extends into the wake and interacts in a complex manner with the von Karman vortex shedding. To detect the von Karman vortex shedding a phase averaged method has been chosen. This approach uses the Proper Orthogonal Decomposition (POD) technique and it has been evidenced that it works properly for the infinite cylinder and for some sections near the base of the finite one.