BIRS Workshop Lecture Videos
Spatially resolved high-speed imaging of fibre dispersion in a cross-jet Sommerfield, Martin
Talk: Regular Abstract: In most technical applications the particles are not spherical, as usually considered in experiments and numerical calculations, but are non-spherical having either a regular shape (e.g. fibres, cylinders, granulates or disc-like particles) or are even completely irregular such as quartz sand and pulverized coal. With respect of the flow transport of such particles they mostly exhibit larger Reynolds numbers so that a creeping flow or Stokes assumption is not valid for obtaining the flow resistance coefficients (Loth 2008). Numerous experimental studies are available where the drag coefficient of non-spherical particles with a certain orientation is evaluated for higher particle Reynolds numbers based on wind tunnel or sedimentation experiments (see e.g. Haider and Levenspiel 1983). However, when dealing with non-spherical particles in a numerical computation (e.g. by the Euler/Lagrange approach) particle tracking involves determination of their location as well as orientation in the flow. Consequently, drag coefficients for higher particle Re are needed which are depending on particle orientation (Hölzer and Sommerfeld 2008). So far the determination of orientation-dependent resistance coefficients (i.e. drag, lift and torque) was mainly based on DNS (direct numerical simulation) as for example done by Hölzer and Sommerfeld (2009) and Zastawny et al. (2012) for regularly shaped non-spherical particles. Since the resulting dependence of resistance coefficient on orientation are usually rather complex mostly approximations are used in an Euler/Lagrange calculation (see e.g. van Wachem et al. 2015). Therefore, detailed experimental studies are needed on the transport and turbulence response of non-spherical particles for validating such numerical calculations, which are so far very rare. In the present study a fibre laden small jet (di = 5 mm) issuing into a fully developed turbulent cross-flow through a channel of square cross-section (100 mm x 100 mm) was considered. The objective of this study was providing data on the dispersion of fibres in the vortical structures of the cross-jet for different Stokes numbers and also in comparison with spherical particles with the same Stokes numbers (Pasternak and Sommerfeld 2015). The imaging technique applied is based on shadow imaging for avoiding disturbing light scattering effects when using a light sheet. A pulsed LED array was used for illumination with a pulse duration as low as 20 µs for avoiding motion-induced burring. As tracer almost neutrally buoyant 40 µm PMMA particles were used. Images of both phases were collected using a high-speed camera at a framing rate of 3600 Hz and an image size of 768 x 640 pixels. The applied objective yielded an imaging field of 40 mm x 33 mm with a small depth of field providing an effective imaging layer thickness of only 1.5 mm in connection with the adopted image filters. These were a LOG-filter for removing all out-of-focus images and an object-based filter using object size and sphericity to obtain separate images of tracer and fibres. The velocity fields of the tracer were obtained by the MQD (minimum quadratic difference) algorithm (similar to PIV) and the fibre velocities were determined by particle tracking (PIV). In addition fibre orientation and angular velocities were evaluated throughout the flow field. Furthermore, various correlations were determined for characterizing the fibre response in the cross-jet. These detailed data are available for the validation of numerical computations.
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