Fluid Mechanics Seminar: Robert Poole

Previous studies of shear-thinning fluids in pipe flow have shown that, although the time-averaged velocity profile is – as expected – axisymmetric when the flow is laminar or fully turbulent, contrary to expectation it exhibits marked asymmetry in the laminar-turbulent transition regime. Despite this strange effect being observed in different experimental facilities no satisfactory explanation yet exists. The general consensus of these previous experiments is that the location of the peak velocity remains at a fixed point in space (at least during one experimental realisation).

Here we present new experimental data obtained using 2D 3C stereo particle image velocimetry which demonstrates that, in fact, the asymmetry does not stay fixed in space and that it may be related to a linear instability occurring prior to classical transition to turbulence (i.e. the appearance of puffs/slugs). The experiments are performed using aqueous solutions of a xanthan gum (0.15wt %), which exhibits shear-thinning of the shear viscosity by about three-orders of magnitude and is only very weakly elastic in the range of shear rates probed in our pipe. To quantitatively describe the degree of azimuthal flow asymmetry we define an “asymmetry factor”. Our results once again confirm significant departures from axisymmetry in transitional flows of shear-thinning fluids but indicate that the asymmetry may occur slightly before transition. Furthermore at higher flowrates within transition, it can be seen that the asymmetry is not fixed in space but that, although it preferentially arises at certain azimuthal locations, there are short durations when the flow switches back to a quasi-axisymmetric state. Associated with some of these events, the flow can also briefly probe an asymmetric state of a different orientation to the preferred state.

†With Chaofan Wen & David J.C. Dennis.

Bio:

Prof. Rob Poole completed his Bachelors degree at the University of Loughborough, and PhD at the University of Liverpool, where he is currently Head of Department for the Centre for Engineering Dynamics.