Integrated gray-level gradient method applied for the extraction of three-dimensional velocity fields of sprays in in-line digital holography

Abstract

In this study, an integrated gray-level gradient method is applied to extract the three-dimensional (3D) velocity fields of sprays. This method consists of a conventional edge-sharpness method and a new method, namely, the overall-sharpness method, which is an efficient supplement of the former. And then the synchronization system is designed and assembled to record double-exposure spray holograms in a short time interval. Finally, using the integrated gray-level gradient method and some image processing techniques, the 3D coordinates of droplets are easily obtained, which can be used to evaluate the 3D velocity fields and the size features of spray droplets in different spray injection pressures. It proves that the integrated gray-level gradient method is well applied to measure the characteristics of sprays in in-line digital holography.

Developments in digital in-line holography enable validated measurement of 3D particle field dynamics

“…(4) and apply focus metrics to the numerically refocused images in order to extract the morphology and 3D location of in-focus particles (for examples see [4,5,15,16]). The accuracy of a given method, especially with respect to the z-position, is a strong function of the chosen focus metric and other processing details.…”

“…This method is based on a combination of previously proposed methods [15,16] and is therefore referred to as the hybrid method of particle detection. The method works by searching for regions in 3D space which display a minimum of intensity within the interior of the region and a maximum sharpness along outer edges of the region.…”

Direct particle depth displacement measurement in DHPTV using spatial correlation of focus metric curves

“…Holography, as a real 3D imaging technique, is capable of reviving the 3D information of the tracer particles, and thus has been widely applied to particle field diagnostics [2,3]. Besides the size and 3D position measurement [4][5][6], digital holography has also been extended to measure the 3D velocity in a manner of particle tracking/image velocimetry (PTV/ PIV) [7][8][9][10][11][12][13][15][16][17][18]. Digital holographic PTV (DHPTV) shows great potential in measurements of 3D velocity field.…”

“…In DHPTV, we usually record a pair [8,12,13,16,17] (or a series [10,11]) of holograms or a hologram with multi-exposures [14,15], and then reconstruct the 3D optical particle fields to retrieve the 3D velocity field. The existing algorithms for 3D velocity calculation fall into two categories: cross-correlation (3D [9] or 2D [17]) and tracking particle one by one [7,8,[10][11][12][13][15][16][17].…”

“…The existing algorithms for 3D velocity calculation fall into two categories: cross-correlation (3D [9] or 2D [17]) and tracking particle one by one [7,8,[10][11][12][13][15][16][17]. The 3D crosscorrelation method takes the maximum point position in the 3D cross correlation coefficients of two optical particle volumes as the velocity.…”