kth_logo.gif

Licentiate seminar

Hydrodynamic stability and turbulence in fibre suspension flows


Defendant Main Advisor Extra Advisor Date
Mathias Kvick Fredrik Lundell Daniel Söderberg 2012-06-12

Opponent
Srdjan Sasic, Chalmers University of Technology

Evaluation committee

Abstract

Fibres in turbulent flows are studied as well as the effect of fibres on hydrodynamic stability.

The first part deals with orientation and spatial distribution of fibres in a turbulent open channel flow. Experiments were performed for a wide range of flow conditions using fibres with three aspect ratios, rp = 7, 14, 28. The aspect ratio of the fibres were found to have a large impact on the fibre orientation distribution, where the longer fibres mainly aligned in the streamwise direction and the shorter fibres had an orientation close to the spanwise direction.

When a small amount of polyethyleneoxide (PEO) was added to the flow, the orientation distributions for the medium length fibres were found to approach a more isotropic state, while the shorter fibres were not affected.

In most of the experiments performed, the fibres agglomerated into streamwise streaks. A new method was develop in order to quantify the level of agglomeration and the streak width independent of fibre size, orientation and concentration as well as image size and streak width.

The width of the fibre streaks were found to correspond well to the width of the well known low velocity streaks found in turbulent boundary layers. Furthermore, the streakiness, i.e. the tendency for fibres to agglomerate into streaks, was shown decrease with Reynolds number.

In order to increase the knowledge on the effects of fibres on hydrodynamic stability, experiments in a curved rotating channel was performed with water and a nano-fibrillated cellulose (NFC) suspension. Flow visualisations were used to obtain a stability map as a function of rotation rate and flow rate. The addition of NFC was found to stabilise the primary instability. The effect on the secondary instability was inconclusive, however, no destabilisation was observed.

In addition to the experiments, a linear stability analysis was performed with different volume fractions of fibres using two different stationary orientation distributions. The linear stability analysis was found to under predict the increase in critical Reynolds number compared to the experiments.


[Download (6 Mb)]