The Accuracy of Remapping Irregularly Spaced Velocity Data onto a Regular Grid and the Computation of Vorticity
Abstract
The velocity data obtained from Molecular Tagging Velocimetry (MTV) are typically located on an irregularly spaced measurement grid. In this method of velocimetry, the flowing medium is premixed with a molecular complex that can be turned into a long life-time tracer upon excitation by photons. The velocity vector is determined from the displacement of small regions "tagged" by a pulsed laser that are imaged at two successive times within the lifetime of the tracer. This technique may be viewed as the molecular counterpart of PIV. To take advantage of standard data processing techniques, the MTV data need to be remapped onto a regular grid with a uniform spacing. This study examined the accuracy and noise issues related to the use of various low-order polynomial least-square fits for remapping and the subsequent computation of vorticity from these data. The information obtained has relevance to PIV data processing as well. As noted by Spedding and Rignot (1993), the best estimate of the location of the velocity vector acquired through the use of tracer techniques, such as PIV, is at the midpoint of the displacement vector. Thus, unless special care is taken, PIV data also are initially obtained on an irregular grid. This study considered the use of 2nd, 3rd, or 4th order polynomials to remap the velocity field by performing a local least-squares fit of the irregularly spaced data within region of radius R. Four approaches were assessed for computing the out-of-plane vorticity field from the in-plane velocity measurements: direct differentiation of the polynomial fits used in the remapping process, lst and 2nd order finite difference techniques (2nd and 4th order accurate, respectively), and an 8-point circulation method on the regular data. Results show that the most accurate vorticity results are achieved by either directly differentiating the 3rd order polynomial fit to the original irregular data, or by the 2nd order finite difference technique. (5 refs.)
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 15, 1999
- Accession Number
- ADA410461
Entities
People
- Manoochehr M. Koochesfahani
- Richard K. Cohn
Organizations
- Air Force Research Laboratory