Neil Cagney

Temperature and velocity measurements of a rising thermal plume

The three-dimensional velocity and temperature fields surrounding an isolated thermal plume in a fluid with temperature-dependent viscosity are measured using Particle-Image Velocimetry and thermochromatic liquid crystals, respectively. The experimental conditions are relevant to a plume rising through the mantle. It is shown that while the velocity and the isotherm surrounding the plume can be used to visualize the plume, they do not reveal the finer details of its structure. However, by computing the Finite-Time Lyapunov Exponent fields from the velocity measurements, the material lines of the flow can be found, which clearly identify the shape of the plume head and characterize the behavior of the flow along the plume stem. It is shown that the vast majority of the material in the plume head has undergone significant stretching and originates from a wide region very low in the fluid domain, which is proposed as a contributing factor to the small-scale isotopic variability observed in ocean-island basalt regions. Lastly, the Finite-Time Lyapunov Exponent fields are used to calculate the steady state rise velocity of the thermal plume, which is found to scale linearly with the Rayleigh number, in contrast to some previous work. The possible cause and the significance of these conflicting results are discussed, and it is suggested that the scaling relationship may be affected by the temperature-dependence of the fluid viscosity in the current work.

On multiple manifestations of the second response branch in streamwise vortex-induced vibrations

The structural motion and velocity field in the wake of a cylinder exhibiting vortex-induced vibration (VIV) in the streamwise direction were measured using Particle-Image Velocimetry. The effect of hysteresis on the amplitude response of the cylinder and the existence of multiple wake modes in the region of the second response branch were examined. As the reduced velocity was decreased, there was a reduction in the lock-in range; outside this range the amplitude response was found to be negligible and the A-II mode (which is similar to the von Karman vortex street) was observed in the wake. When the reduced velocity was increased the second branch could be manifested in two forms, depending on whether the wake exhibited the SA or the A-IV mode (in which two and four vortices are shed per wake cycle, respectively). The A-IV mode has been observed in studies in which a cylinder was forced to oscillate in the streamwise direction; however, this represents the first time that it has been recorded in the wake...

Lagrangian structures and mixing in the wake of a streamwise oscillating cylinder

Lagrangian analysis is capable of revealing the underlying structure and complex phenomena in unsteady flows. We present particle-image velocimetry measurements of the wake of a cylinder undergoing streamwise vortex-induced vibrations and calculate the Finite-Time Lyapunov Exponents (FTLE) in backward- and forward-time. The FTLE fields are compared to the phase-averaged vorticity fields for the four different wake modes observed while the cylinder experiences streamwise vortex-induced vibrations. The backward-time FTLE fields characterise the formation of vortices, with the roll up of spiral-shaped ridges coinciding with the roll up of the shear layers to form the vortices. Ridges in the forward-time fields tend to lie perpendicular to the flow direction and separate nearby vortices. The shedding of vortices coincides with a “peel off” process in the forward-time FTLE fields, in which a ridge connected to the cylinder splits into two strips, one of which moves downstream. Particular attention is given to ...

Effects of cell motility and morphology on the rheology of algae suspensions

Algae have been proposed as a source of biofuels and high value chemical products, but if this potential is to be fully realised, it is crucial to understand the factors affecting the suspension rheology. Suspensions of three algae species, Tetraselmis chuii, Chlorella sp. and Phaeodactylum tricornutum, were sheared in a rotational rheometer in order to characterise their rheology and examine the effects of cell concentration, motility and morphology. The volume fraction ranged from 0.05 to 0.2, and the shear rate from 20 to 200 s−1. The rheology measurements are fitted to the Herschel-Bulkley model, and the intrinsic viscosity is estimated using both Einstein’s equation and the Krieger-Dougherty model, which are found to perform well for low concentrations. The intrinsic viscosity of T. chuii suspensions is shown not to be constant, but decreases with strain rate, indicating that the suspension viscosity is less sensitive to the cell concentration at high strain rates. The rate of decline is constant for strain rates below approximately 100 s−1, after which it continues to decline linearly, but at a slower rate. It is speculated that this transition at 100 s−1 is related to the appearance of flocculation at low strain rates. The effect of the cell motility on the rheology of T. chuii suspensions is investigated by comparing the rheology of motile and passive cells. The shear-thinning behaviour is absent and the effective viscosity is considerably lower for the passive cell suspensions, indicating that the motility of the T. chuii cells causes them to align to resist the flow. In contrast, the Chlorella sp. suspensions exhibit shear-thickening behaviour, which has not previously been reported. Finally, the influence of the effective aspect ratio on the cell suspensions is examined by comparing the intrinsic viscosity of all three species. The algal species with the largest aspect ratio, P. tricornutum, has the largest intrinsic viscosity, while the smallest aspect ratio strain, Chlorella sp., has the smallest viscosity. However, it is shown that the increase in viscosity of motile compared to non-motile T. chuii suspensions cannot be attributed to a change in the effective aspect ratio of individual cells due to the motion of the flagella alone.

Mode decomposition and Lagrangian structures of the flow dynamics in orbitally shaken bioreactors

In this study, two mode decomposition techniques were applied and compared to assess the flow dynamics in an orbital shaken bioreactor (OSB) of cylindrical geometry and flat bottom: proper orthogonal decomposition and dynamic mode decomposition. Particle Image Velocimetry (PIV) experiments were carried out for different operating conditions including fluid height, h, and shaker rotational speed, N. A detailed flow analysis is provided for conditions when the fluid and vessel motions are in-phase (Fr = 0.23) and out-of-phase (Fr = 0.47). PIV measurements in vertical and horizontal planes were combined to reconstruct low order models of the full 3D flow and to determine its Finite-Time Lyapunov Exponent (FTLE) within OSBs. The combined results from the mode decomposition and the FTLE fields provide a useful insight into the flow dynamics and Lagrangian coherent structures in OSBs and offer a valuable tool to optimise bioprocess design in terms of mixing and cell suspension.

Development of Vortex Bioreactor Technology for Decentralised Water Treatment

The vortex bioreactor (VBR) is a simple decentralised water treatment system (DeWaTS) that sits at the interface between swirl flow, biotechnology and chemical engineering. The device utilises swirl flow and suspended activated beads to achieve downstream water processing and has been tested for applications including centrifugal-driven separation, pathogen neutralisation and metal absorption. The VBR was optimised for the treatment of faecally contaminated effluents in the developing world, and the design features related to the key challenges faced by the wastewater industry are highlighted here. The VBR has two aspects that can be modified to generate different reactor conditions: the impeller, where the swirl flow is modified through alterations of rotation speed, and impeller geometry and the suspended activated beads, which facilitate mixing and alter the reactor surface area. Data from testing for some of the different applications mentioned above are presented here, and future planned developments for the technology are discussed.