N. Williamson

Low-Reynolds-number fountain behaviour

Experimental evidence for previously unreported fountain behaviour is presented. It has been found that the first unstable mode of a three-dimensional round fountain is a laminar flapping motion that can grow to a circling or multimodal flapping motion. With increasing Froude and Reynolds numbers, fountain behaviour becomes more disorderly, exhibiting a laminar bobbing motion. The transition between steady behaviour, the initial flapping modes and the laminar bobbing flow can be approximately described by a function FrRe2/3 =C. The transition to turbulence occurs at Re > 120, independent of Froude number, and the flow appears to be fully turbulent at Re ≈2000. For Fr > 10 and Re 120, sinuous shear-driven instabilities have been observed in the rising fluid column. For Re 120 these instabilities cause the fountain to intermittently breakdown into turbulent jet-like flow. For Fr 10 buoyancy forces begin to dominate the flow and pulsing behaviour is observed. A regime map of the fountain behaviour for 0.7Fr 100 and 15Re 1900 is presented and the underlying mechanisms for the observed behaviour are proposed. Movies are available with the online version of the paper.

Forced turbulent fountain flow behaviour

Numerical simulations of turbulent fountain flow are used to investigate the important energy and mass transfer mechanisms present in the forced fountain flow regime, which has been reported to exist at Froude numbers (Fr ) greater than 3. The flow is equivalent to a negatively buoyant jet with three flow streams, the inner upflow (IF), the outer downflow (OF) and the surrounding ambient fluid (AF). Simulation results are presented for Fr = 4 and 7 at Reynolds number Re = 3350. The mean fountain penetration height scales with the previously reported relation Zm/R0 =2 .46Fr , where R0 is the source radius, but the assumptions behind analytical derivations of the relation are not supported by the present results. The results suggest that the OF may be relatively well described by the dynamics of a pure line plume surrounding the IF but with higher entrainment owing to the unsteady pulsing behaviour of the flow entering the OF from the IF. The length scale for a pure plume appears to apply at Fr = 7 in the OF and a degree of self-similarity exists. Comparisons with previous results suggest the IF is not fully developed at Fr = 7 and entrainment into the IF from the OF may not occur until Fr > 15.

Transition behaviour of weak turbulent fountains

Numerical simulations of fully turbulent weak fountain flow are used to provide direct evidence for the scaling behaviour of fountain flow over the Froude number range Fr = 0.1–2.1 and Reynolds number range Re = 20–3494. For very weak flow at Fr < 0.4, the flow mean penetration height, Zm, scales with Zm/R0 = A1Fr2/3 + A2Fr2/3 where R0 is the source radius. A1 and A2 are constants which quantify the separate effects of the radial acceleration of fountain fluid from the source (A1) and the backpressure from the surrounding intrusion, if present, on the upflow (A2). The evidence presented in this work suggests that the mechanisms for the two parts in the scaling of Zm scale with Fr2/3. The intrusion behaviour varies with the Reynolds number (Re) but there is no Re affect on the fountain penetration height. For Re < 250 the radial intrusion flow is subcritical and has different behaviour. For Fr between 0.4 and 2.1 the effect of source momentum flux increases and the flow structure changes to one where there is a coherent upflow and a cap region where the flow stagnates and then reverses. The two regions have separate scaling behaviour such that the overall height, through this transition range of Froude numbers, can be described by Zm/R=C1Fr2/3 + C2Fr2, where C1 and C2 are constants. Over this transition range the effect of source velocity profile is more significant than the Reynolds number effects and the effect of inlet turbulence is minor.

Feasibility of air cycle systems for low-temperature refrigeration applications with heat recovery

Abstract In this paper the viability of an air cycle refrigeration system (Joule cycle) is investigated for a moderately low-temperature cooling system with heat recovery. The goal is to determine the best possible cycle configuration of heat exchangers and turbomachinery components for this particular application, and then to determine whether it can compete with a conventional vapour compression system. Simple models were developed for each cycle configuration, and the results were compared with each other on a consistent basis. The sensitivity of the system to heat exchanger effectiveness and expander and compressor efficiency was determined, and contours were plotted.

Shear driven purging of negatively buoyant fluid from trapezoidal depressions and cavities

Experimental data and large eddy simulation results are analysed to investigate shear driven entrainment of a negatively buoyant fluid from trapezoidal depressions and cavities. This flow is of relevance to a number of environmentally significant applications including purging of saline pools in rivers and pollutant dispersion in cities and towns situated within topographic depressions. New scaling relations for the entrainment rate are developed based on physical arguments. Our scaling relations are shown to agree well with both experiments and numerical simulations of this flow in trapezoidal cavities with aspect ratios ranging between 7 and 17, entry beach angles between 8° and 33°, and an exit beach angle of 33°. For the numerical simulations, a sub-filter scale turbulence model is used that combines the dynamic mixed model of Zang et al. [“A dynamic mixed subgrid-scale model and its application to recirculating flows,” Phys. Fluids A 5, 3186 (1993)]10.1063/1.858675 with the dynamic localization proce...

Survival of cyanobacteria in rivers following their release in water from large headwater reservoirs

Cyanobacterial survival following their release in water from major headwaters reservoirs was compared in five New South Wales rivers. Under low flow conditions, cyanobacterial presence disappeared rapidly with distance downstream in the Cudgegong and Hunter Rivers, whereas the other three rivers were contaminated for at least 300 km. Cyanobacterial survival is likely to be impacted by the geomorphology of each river, especially the extent of gravel riffle reaches (cells striking rocks can destroy them) and by the different turbulent flow conditions it produces within each. Flow conditions at gauging stations were used to estimate the turbulent strain rate experienced by suspended cyanobacteria. These indicate average turbulent strain rates in the Cudgegong and Hunter Rivers can be above 33 and 83 s-1 while for the Murray, Edward and Macquarie Rivers average strain rate was estimated to be less than 30 s-1. These turbulent strain rate estimates are substantially above published thresholds of approximately 2 s-1 for impacts indicated from laboratory tests. Estimates of strain rate were correlated with changes in cyanobacterial biovolume at stations along the rivers. These measurements indicate a weak but significant negative linear relationship between average strain rate and change in cyanobacterial biomass. River management often involves releasing cold deep water with low cyanobacterial presence from these reservoirs, leading to ecological impacts from cold water pollution downstream. The pollution may be avoided if cyanobacteria die off rapidly downstream of the reservoir, allowing surface water to be released instead. However high concentrations of soluble cyanotoxins may remain even after the cyanobacterial cells have been destroyed. The geomorphology of the river (length of riffle reaches) is an important consideration for river management during cyanobacterial blooms in headwater reservoirs.

Natural convection stratification and scaling in a cavity with unsteady sidewall heating

The flow development and behaviour of a fluid in a two-dimensional cavity, heated and cooled periodically from one sidewall, with all other walls adiabatic, is investigated via numerical simulation. The heating and cooling on the sidewall produce alternating direction vertical natural convection boundary layers that entrain fluid from the cavity interior and discharge it at the top and bottom of the cavity. At full development the flow is quasi steady with the natural convection boundary layers maintaining a stable stratification in the cavity interior and the heat content oscillating around a zero mean value. The stratification strength is shown to be strongly dependent on the forcing frequency.