Three-dimensional reconstruction of helicopter blade–tip vortices using a multi-camera BOS system

Background-oriented schlieren (BOS) techniques

“…In addition to the flight tests described by Bauknecht et al (2015), other examples for this sort of measurements include explosions of gas (Sommersel et al 2008) and C4 (Mizukaki et al 2012). The image subtraction method and the (conventional) BOS correlation method have been found to be suited for this task (Hargather 2013).…”

“…Schairer et al (2013) conducted stereoscopic BOS measurements for the tip vortex localization on a full-scale helicopter model in a large wind tunnel facility using epipolar geometric analysis. Bauknecht et al (2015) used the same method also in combination with photogrammetry for the three-dimensional reconstruction of a large part of the blade tip vortices of a flying helicopter from multi-camera BOS images. The procedure for density gradient localization consists of at least two steps:…”

“…Bauknecht et al (2015), for example, describe a semiautomatic detection algorithm for finding discrete points on vortex lines. Starting from manually selected points close to the vortex filaments, a stepwise cross-correlation between cuts orthogonal to the main vortex axis in combination with a polynomial-based predictor is applied.…”

“…The constraint imposed by epipolar geometry can be written as with the coordinates of the corresponding points in the images A and B and the 3 × 3 fundamental matrix F. This fundamental matrix can be estimated given at least seven corresponding point pairs in the two images. In the study described by Bauknecht et al (2015), the fundamental matrices were estimated for all possible camera combinations in the multi-camera setup.…”

Quantitative Schlieren Measurement of Explosively‐Driven Shock Wave Density, Temperature, and Pressure Profiles

“…Modern high-speed digitali maging has allowed the development of several optical techniques for studyinge xplosive effects, which have expanded the detail to which ab last can be characterized.Many of the imaging studiesv isualize and track shock wave propagation using refractive imagingt echniquesi ncluding schlieren, shadowgraphy,a nd backgroundo riented schlieren (BOS) [5].I no ne of the most ground-breaking studies, Kleine et al used high-speed digital cameras and shadowgraphy to track shockw aves from silver azide charges and demonstrated the spatiald ependence of a" TNT equivalence" measurement [6].S imilar studiese xpanded the analysis using other imaging methods [7,8],explosive materials [9-12],a nd explosive-charges cales [13][14][15].A ll of these approaches have focusedo nm easuring the shock wave propagation opticallyt oe stimate pressure characteristics which are compared to gage measurements.The schlieren and BOS technique are capable of yielding quantitative density information of flow fields [16],b ut few researchers have applied this capabilityt os tudyinge xplosions. Quantitative density measurement from refractive techniques is well-documented for aerodynamics applications [17][18][19],i ncluding shock waves [20],b ut not moving shocks. One exceptioni st he technicalr eport by Biss and McNesby[ 21],w here quantitative schlieren imaging was used to measure the density field near as mall explosive charge.…”