Intensity Capping: a simple method to improve cross-correlation PIV results
- 617 Downloads
A common source of error in particle image velocimetry (PIV) is the presence of bright spots within the images. These bright spots are characterized by grayscale intensities much greater than the mean intensity of the image and are typically generated by intense scattering from seed particles. The displacement of bright spots can dominate the cross-correlation calculation within an interrogation window, and may thereby bias the resulting velocity vector. An efficient and easy-to-implement image-enhancement procedure is described to improve PIV results when bright spots are present. The procedure, called Intensity Capping, imposes a user-specified upper limit to the grayscale intensity of the images. The displacement calculation then better represents the displacement of all particles in an interrogation window and the bias due to bright spots is reduced. Four PIV codes and a large set of experimental and simulated images were used to evaluate the performance of Intensity Capping. The results indicate that Intensity Capping can significantly increase the number of valid vectors from experimental image pairs and reduce displacement error in the analysis of simulated images. A comparison with other PIV image-enhancement techniques shows that Intensity Capping offers competitive performance, low computational cost, ease of implementation, and minimal modification to the images.
The authors are grateful for the discussions with J. Rosman, J. Koseff, and S. Monismith and the technical help provided by R. Gurka and R. Rosenzweig. The authors extend special thanks to M. Wernet for implementing his SPOF technique. The authors would like to acknowledge contributions to the Ocean PIV project by J. Jaffe, P. Roberts, F. Simonet, P. Franks, S. Monismith, C. Troy, and A. Horner-Devine. Support for Ocean PIV was provided by NSF grant OCE-0220213. R. Lowe acknowledges support from NSF grant OCE-0453117.
- Gurka R (1999) Dynamics of a flexible tube in the turbulent gas flow of a twin fluid atomizer. M.Sc. thesis, Technion, IsraelGoogle Scholar
- Gurka R, Liberzon A, Hefetz D, Rubinstein D, Shavit U (1999) Computation of Pressure Distribution Using PIV Velocity Data. In: Third international workshop on particle image velocimetry, Santa Barbara, California, pp. 671–6, September 16–18, 1999Google Scholar
- Lowe RJ (2005) The effect of surface waves on mass and momentum transfer processes in shallow coral reef systems. PhD thesis, Stanford UniversityGoogle Scholar
- Raffel M, Willert C, Kompenhans J (1998) Particle image velocimetry: a practical guide. Springer, Berlin Heidelberg New YorkGoogle Scholar
- Rosenzweig R (2005) The macroscopic velocity profile near permeable interfaces: piv measurements, numerical simulations, and an analytical solution of the laminar problem. M.Sc. thesis, Technion, IsraelGoogle Scholar
- Sveen KJ (2004) An introduction to MatPIV v. 1.6.1, eprint series, Dept. of Math. University of Oslo, ``Mechanics and Applied Mathematics", NO. 2 ISSN 0809–4403, August, 2004Google Scholar
- Westerweel J (1993) Digital Particle Image Velocimetry. Theory and Practice, PhD Thesis, Delft University of Technology, The NetherlandsGoogle Scholar