The photographic negatives, having a resolution of 300 lines/mm, are digitized using a Nikon LS-3510AF 35 mm film scanner at a scanning resolution of 125 pixels/mm. Each image is saved as a Transferable Image File (TIF) (Figure 1f).
The software PIV3 (Seke, 1993) uses the TIF files as an input and evaluates the distances between particle images to calculate displacement vectors. The software uses a single-frame cross-correlation method involving the application of two successive fast Fourier transforms to a given window of the total image. An average displacement vector is calculated for each window and is assumed to be located at the center of the window. For the present experiments, a window size of 90 * 90 pixels (0.72 mm * 0.72 mm) with 50% overlap in both vertical and horizontal directions was used, resulting in a grid size of 0.36 mm * 0.36 mm on the film. In order to satisfy the high-image-density criterion, each interrogation window contained a minimum of 50 multiply-exposed particle images. The software TRACETIF (Image Alchemy, 1992) and TWOBOUN (Lin, 1996) were used to trace the contours of the free-surface and the cylinder in the TIF file.
The software V3 (Robinson, 1992) is used to display and evaluate the preliminary flow field of displacement vectors. V3 enables the user to identify and remove inappropriate vectors which are the result of shadows, reflections or laser sheet distortions in the flow field.
The software NFILV (Lin, 1994) applies a bilinear interpolation algorithm to the areas of the flow field for which the displacement vectors have been removed. NFILV uses the magnification factor, laser scanning rate, and bias velocity as an input to calculate actual velocity vectors from the particle displacement vectors. The interpolated velocity field is smoothed by a Gaussian weighted average technique described by Landreth and Adrian (1989), with a smoothing parameter of 1.3, which provides minimal distortion of the velocity field. , which provides minimal distortion of the velocity field. The out-of-plane component of vorticity can be calculated from the two-dimensional velocity field using the software SURFER (Golden Software Inc., 1989).