Temporally resolved measurements of heavy, rigid fibre translation and rotation in nearly homogeneous isotropic turbulence

Abstract

A two orthogonal view, holographic cinematography system (volume of$17\times 17\times 17~\text{mm}^{3}$) was used to measure three-dimensional fibre translational velocities, orientations and rotation rates in near homogeneous isotropic air turbulence (HIT). Flow characteristics were determined from temporally resolved particle image velocimetry measurements. Two sets of rigid, nylon fibres having the same nominal length (0.5 mm) but different diameters (13.7 and$19.1~\unicode[STIX]{x03BC}\text{m}$), were released in near HIT at a Taylor microscale Reynolds number of$Re_{\unicode[STIX]{x1D706}}\approx 130$and tracked at more than five times the Kolmogorov frequency. The ratio of fibre length to the Kolmogorov length scale was 2.8 and the two sets were characterized by Stokes numbers of 1.35 and 2.44, respectively. As a result of increased inertia, the probability density functions (PDFs) of the fluctuating fibre translational velocities were narrower than the ones of the air and the fibre velocity autocorrelations decreased at a decreasing rate. While fibre orientations in the cameras’ frame of reference were random as a result of the strong turbulence, it was shown that fibres align with the flow to minimize drag. PDFs of the fibre rotation rates indicated the occurrence of extreme rotation rate events. Furthermore, increasing inertia lowered the normalized, mean squared fibre rotation rates in comparison to results obtained for neutrally buoyant fibres having the same aspect ratio and including the effect of preferential alignment. The present results compare well to direct numerical simulations including the effect of fibre inertia.