Charlotte Gladstone

On buoyancy-driven natural ventilation of a room with a heated floor

The natural ventilation of a room, both with a heated floor and connected to a cold exterior through two openings, is investigated by combining quantitative models with analogue laboratory experiments. The heated floor generates an areal source of buoyancy while the openings allow displacement ventilation to operate. When combined, these produce a steady state in which the air in the room is well-mixed, and the heat provided by the floor equals the heat lost by displacement. We develop a quantitative model describing this process, in which the advective heat transfer through the openings is balanced with the heat flux supplied at the floor. This model is successfully tested with observations from small-scale analogue laboratory experiments. We compare our results with the steady-state flow associated with a point source of buoyancy: for a given applied heat flux, an areal source produces heated air of lower temperature but a greater volume flux of air circulates through the room. We generalize the model to account for the effects of (i) a cooled roof as well as a heated floor, and (ii) an external wind or temperature gradient. In the former case, the direction of the flow through the openings depends on the temperature of the exterior air relative to an averaged roof and floor temperature. In the latter case, the flow is either buoyancy dominated or wind dominated depending on the strength of the pressure associated with the wind. Furthermore, there is an intermediate multiple-solution regime in which either flow regime may develop.

The Significance of Grain-Size Breaks in Turbidites and Pyroclastic Density Current Deposits

Turbidite beds and pyroclastic density current deposits commonly show abrupt internal changes in grain size with stepwise upward fining. Grain-size breaks divide a turbidite bed into a lower sandy zone, a middle silty zone, and an upper muddy zone. Zone boundaries need not correspond with Bouma division boundaries, but the sandy zone usually consists of A, B, and occasionally C divisions, the silty zone is composed of one or more of the B, C, and D divisions, and the muddy zone correlates with division E. Pyroclastic density current deposits likewise display a tripartite zonation with prominent grain-size breaks. Divergence of particle-laden gravity currents into a lower dense body and an upper dilute turbulent wake can explain the formation of the lower grain-size break. Fine particles are elutriated into the wake region by ambient entrainment through the current head, while coarse grains are transported in the body. Particle size and concentration stratification is sustained by the turbulence and velocity structure of gravity currents, with a velocity maximum and turbulence minimum also dividing currents into two regions. Two stages of deposition result. The upper grain-size break may be produced by buoyant lofting, with fine particles convected into the water column or atmosphere as the flow wanes.

On the application of box models to particle-driven gravity currents

New laboratory experiments on different types of lock-exchange particle-driven gravity currents advancing into a flume of fresh water are presented. These include purely saline currents, monodisperse particle-laden gravity currents with both fresh and saline interstitial fluid, and bidisperse particle-laden currents. For each case a simple box model is developed. These agree well with the experimental data. We find that particulate gravity currents with saline interstitial fluid flowing into ambient fresh fluid are best described using a Froude number of 0.52 in the box model (cf. Huppert & Simpson 1980). However, particulate gravity currents with fresh interstitial fluid are best described using a higher Froude number of 0.67. The change in Froude number reflects the different shape and structure associated with the different density of interstitial fluid. For all experiments, box models provide accurate predictions for up to twenty lock-lengths.