A fast data reduction algorithm for molecular tagging velocimetry: the decoupled spatial correlation technique

Molecular tagging velocimetry (MTV) involves intensive data reduction that extracts flow velocity information from the Lagrangian tracking of phosphor escing fluid material. A computationally efficient algorithm for the data r eduction is thus of practical interest for processing large MTV data sets. We were motivated by this consideration into developing a simplified versio n of the spatial correlation technique, the decoupled spatial correlation t echnique, in an effort to seek a balance between accuracy and efficiency. B y Taylor series analysis it is shown that, if the Lagrangian displacement v ector can be roughly pre-determined, the two components in the displacement vectors that have to be solved simultaneously using the spatial correlatio n technique can now be determined independently in two orthogonal direction s. This decoupling results in about an order of magnitude decrease in the C PU time required. An accuracy estimate based on artificial images that foll ow the motion of a line vortex indicates that the technique can determine d isplacements to within 0.08 pixel. This technique was also used to process MTV images acquired in a cross stream plane of the transverse jet. This how is characterized by a large scale counter-rotating vortex pair (CVP). The velocity fields obtained clearly show the existence of this CVP, which prov ides further verification of this technique.