Natural ventilation of buildings is the flow generated by temperature differences and by the wind. The governing feature of this flow is the exchange between an interior space and the external ambient. Although the wind may often appear to be the dominant driving mechanism, in many circumstances temperature variations play a controlling feature on the ventilation since the directional buoyancy force has a large influence on the flow patterns within the space and on the nature of the exchange with the outside. Two forms of ventilation are discussed: mixing ventilation, in which the interior is at an approximately uniform temperature, and displacement ventilation, where there is strong internal stratification. The dynamics of these buoyancy-driven flows are considered, and the effects of wind on them are examined. The aim behind this work is to give designers rules and intuition on how air moves within a building; the research reveals a fascinating branch of fluid mechanics.

The fluid mechanics of natural ventilation

Hydrodynamic and hydromagnetic stability

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Buoyancy Effects In Fluids

Delta morphology is thought to be controlled by factors such as river discharge, tides and waves. Numerical modelling shows that sediment cohesion also strongly influences the development of a delta’s characteristics. The morphologies of the world’s deltas are thought to be determined by river discharge, tidal range and wave action 1. More recently, sea-level rise 2,3 and human engineering4 have been shown to shape delta evolution. The effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4,5,6. In particular, the role of sediment cohesion, which is controlled by sediment size and type of vegetation, is unclear. Here we use a numerical flow and transport model7,8,9,10 to show that sediment cohesiveness also strongly influences the morphology of deltas. We find that, holding all other factors constant, highly cohesive sediments form bird’s-foot deltas with rugose shorelines and highly complex floodplains, whereas less cohesive sediments result in fan-like deltas with smooth shorelines and flat floodplains. In our simulations, sediment cohesiveness also controls the number of channels that form within the deltas, and the average angle of bifurcation of those channels. As vegetation generally acts as a cohesive agent, we suggest that deltas that formed before the expansion of land plants in the Devonian period should show fan-like characteristics, a finding consistent with the limited data from the sedimentological record11.

Significant effect of sediment cohesion on delta morphology

Restoration of river deltas involves diverting sediment and water from major channels into adjoining drowned areas, where the sediment can build new land and provide a platform for regenerating wetland ecosystems. Except for local engineered structures at the points of diversion, restoration mainly relies on natural delta-building processes. Present understanding of such processes is sufficient to provide a basis for determining the feasibility of restoration projects through quantitative estimates of land-building rates and sustainable wetland area under different scenarios of sediment supply, subsidence, and sea-level rise. We are not yet to the point of being able to predict the evolution of a restored delta in detail. Predictions of delta evolution are based on field studies of active deltas, deltas in mine-tailings ponds, experimental deltas, and countless natural experiments contained in the stratigraphic record. These studies provide input for a variety of mechanistic delta models, ranging from radially averaged formulations to more detailed models that can resolve channels, topography, and ecosystem processes. Especially exciting areas for future research include understanding the mechanisms by which deltaic channel networks self-organize, grow, and distribute sediment and nutrients over the delta surface and coupling these to ecosystem processes, especially the interplay of topography, network geometry, and ecosystem dynamics.

Natural Processes in Delta Restoration: Application to the Mississippi Delta

Experiments for gravity currents generated from an instantaneous buoyancy source propagating on an inclined boundary in the slope angle range are reported. While the flow patterns for gravity currents on are qualitatively different from those on , similarities are observed in the acceleration phase for the flow patterns between and and in the deceleration phase, the patterns for gravity currents on are found similar to those on . Previously, it was known that the front location history in the deceleration phase obeys a power-relationship, which is essentially an asymptotic form of the solution to thermal theory. We showed that this power-relationship applies only in the early stage of the deceleration phase, and when gravity currents propagate into the later stage of the deceleration phase, viscous effects become more important and the front location data deviate from this relationship. When the power-relationship applies, it is found that at , , which changes to at , at , and on a horizontal boundary, where is the front velocity, is the front location measured from the virtual origin, and is the released buoyancy. Our results indicate that in the slope angle range , the asymptotic relationship between the front velocity and front location in the deceleration phase is not sensitive to the variation of slope angle. In the late deceleration phase when the front location data deviate from the power-relationship, we found that the flow patterns for are dramatically different from those for . For high slope angles, i.e. , the edge of the gravity current head experiences a large upheaval and enrolment by ambient fluid towards the end of the deceleration phase, while for low slope angles, i.e. , the gravity current head maintains a more streamlined shape without violent mixing with ambient fluid throughout the course of gravity current propagation. Our findings indicate two plausible routes to the finale of a gravity current event.

Experiments on gravity currents propagating on different bottom slopes

[1] Most of the models of alluvial fan and delta morphodynamics to date have used one of two assumptions concerning the upstream boundary condition: (1) a fixed sediment feed point at the vertex or (2) a feed point corresponding to a bedrock-alluvial transition that, e.g., migrates upstream under conditions of constant base level. Here, however, we present both experimental and numerical results pertaining to a new configuration, i.e., one in which the sediment feed point migrates downstream. The research is motivated by the operation of the tailings pond of a mine in which a delta forms. The mine operators must frequently move the outfalls of their slurry pipelines downstream to avoid their burial under sediment as the delta aggrades. This downstream migration results in a characteristic delta configuration. Under the right conditions, as the tailings delta progrades downstream, it does not have time to fill all the available space in the lateral direction, thus leaving unfilled open water on either side. The result is the formation of an elongated delta. Here we use both experiments and numerical modeling to characterize the control of the feed point migration rate on the lateral extent of the prograding deltas. We use these results to obtain a partial explanation of natural elongated deltas. In the case of such deltas, the downstream progradation of natural levees provides an analog to the downstream migration of the outfall of a slurry pipeline.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008WR007382

Delta progradation driven by an advancing sediment source: Coupled theory and experiment describing the evolution of elongated deltas

Experiments on the non-Boussinesq gravity currents generated from an instantaneous buoyancy source propagating on an inclined boundary in the slope angle range .

Non-Boussinesq gravity currents propagating on different bottom slopes

Gravity currents generated from an instantaneous buoyancy source propagating down a slope in the range of 0∘ ≤ θ < 90∘ have been investigated in the acceleration phase by means of high-resolution two-dimensional simulations of the incompressible Navier-Stokes equations with the Boussinesq approximation. Front velocity history shows that, after the heavy fluid is released from rest, the flow goes through the acceleration phase, reaching a maximum front velocity Uf,max, and followed by the deceleration phase. The existence of a maximum of Uf,max is found near θ = 40∘, which is supported by the improved theory. It is identified for the first time that the time of acceleration decreases as the slope angle increases, when the slope angle is approximately greater than 10∘, and the time of acceleration increases as the slope angle increases for gravity currents on lower slope angles. A fundamental difference in flow patterns, which helps explain the distinct characteristics of gravity currents on high and low sl...

High-resolution simulations of downslope gravity currents in the acceleration phase

Gravity currents from instantaneous sources down a slope were modeled with classic thermal theory, which has formed the basis for many subsequent studies. Considering entrainment of ambient fluid and conservation of total buoyancy, thermal theory predicted the height, length, and velocity of the gravity current head. In this study, the problem with direct numerical simulations was re-investigated, and the results compared with thermal theory. The predictions based on thermal theory are shown to be appropriate only for the acceleration phase, not for the entire gravity current motion. In particular, for the current head forms on a 10° slope produced from an instantaneous buoyancy source, the contained buoyancy in the head is approximately 58% of the total buoyancy at most and is not conserved during the motion as assumed in thermal theory. In the deceleration phase, the height and aspect ratio of the head and the buoyancy contained within it may all decrease with downslope distance. Thermal theory relies o...

Albert Dai

Papers

Experiments on gravity currents propagating on different bottom slopes

Delta progradation driven by an advancing sediment source: Coupled theory and experiment describing the evolution of elongated deltas

Non-Boussinesq gravity currents propagating on different bottom slopes

High-resolution simulations of downslope gravity currents in the acceleration phase

Gravity Currents from Instantaneous Sources Down a Slope