Velocity and orientation distributions of fibrous particles in the near-field of a turbulent jet

• Fibres' velocity and orientation are measured in the near-field of the jet.
• The influence of fibres' volume fraction on velocity and orientation in jet flow is presented.
• Fibres' angular velocity distribution in the jet flow is measured.

Measurements of velocity, angular velocity and orientation of nylon fibrous particles of long aspect ratio in the super-dilute regime in the near field of a turbulent air jet with a Reynolds numbers of 70,000 are reported. These measurements were performed using Particle Tracking Velocimetry (PTV) based on the two end-points of each fibre, following a method reported previously. The particles were fed via a hopper into a pipe of 34 diameters in length. The fibres' vertical and horizontal velocity and orientation were calculated to analyse the aerodynamic behaviour of these fibrous particles. The key findings are as follows: 1) fibre orientations at the centre of the jet are distributed over a wide range spanning 30° to 90° and the most probable orientation of the fibres is at 54° to the axial, while few fibres are aligned with the direction of the flow, which contrasts with previous findings in a turbulent pipe flow using water as the working fluid; 2) the axial velocity of the fibres on the jet axis is found to change little with an increase in number density, which contrasts with previous findings in free-falling cases where the fibres' settling velocity increases significantly with the volume fraction; 3) the influence of number density on orientation of the fibres in this turbulent jet within the super dilute regime is much weaker than that for the free-falling case, where the orientations decrease with the volume fraction; 4) the absolute mean angular velocity of the fibres increases significantly with the radial distance from the axis to the location of the greatest mean shear; and 5) at the centre-line of the jet, the fibres' normalised radial velocity is an order of magnitude larger than that of spheres with a similar Stokes number.

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