Friday, November 20, 2015 - 3:00pm
David Dennis, Lecturer, University of Liverpool
Kellogg Room, Elab II
Abstract: Previous studies of shear-thinning fluids in pipe flow discovered that, although the time-averaged velocity profile was axisymmetric when the flow was laminar or fully turbulent, contrary to expectations it was asymmetric in the laminar-turbulent transition regime. We reveal that in fact the asymmetry is not induced by the laminar-turbulent transition process, but is an instability of the laminar state. Furthermore, the transition process is responsible for returning symmetry to the flow (i.e. the opposite to what was previously believed), which explains why the fully turbulent case is axisymmetric. The experiment was performed using an aqueous solution of xanthan gum (0.15%), an essentially inelastic shear-thinning polymer solution. Stereoscopic particle image velocimetry was used to measure the 3C velocity vectors over the entire circular cross-section of the pipe, 220 pipe diameters downstream of the inlet. The deviation from the axisymmetric laminar state is observed to develop in the form of a supercritical bifurcation with square-root dependence on Reynolds number. The asymmetry is non-hysteretic and reversible, not only having a favoured location, but a preferred route between axisymmetry and asymmetry, which it adheres to regardless of the direction of the transition.
Bio: Dr David JC Dennis is a Lecturer in the fluids engineering research group at the University of Liverpool. His research focuses in the fundamental nature transition and turbulence in wall-bounded flows. The focus of his research in this area is on describing the organisation of turbulence using coherent structures, laminar-turbulent transition and drag reduction techniques. Much of his work involves non-Newtonian fluids, particularly investigating the influence of drag-reducing additives on the organisation of turbulent flows, the effects of elastic turbulence in micro-fluidics and unexplained behaviour such as the asymmetric flow of shear-thinning fluids in transitional pipe flow. He completed an MEng degree in Aeronautical Engineering at Loughborough University in 2005, before joining the fluids group in the Department of Engineering at the University of Cambridge, where he was awarded his PhD on the structural building blocks of wall-bounded turbulent flow in 2009. Before joining Liverpool he worked on combustion instabilities in aero-engine fuel injectors with Rolls-Royce at the University of Cambridge.