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Detention ponds are important units of drainage system for managing excess rainfall and runoff quality. To design detention ponds the outflow characteristic must be accurately known. Perforated riser pipes are a popular outlet control structure for stormwater detention basins. This study examines the discharge characteristics of a circular orifice cut into a vertical riser pipe as an outflow mechanism for the detention pond. In particular, the effects of the head above the orifice, the size of the orifice and the size of the riser pipe on the discharge coefficient are evaluated. A physical model is built and various size riser pipes are installed in the tank to simulate a detention basin. The discharge through the orifice is determined by measuring the rate of change of the water level in the tank against time. A water level against volume drained calibration is used to find the rate of change of volume over time, and hence the discharge coefficient. The study shows that the discharge coefficient increases as the head and the ratio of the orifice diameter to pipe diameter decreases. It is found that the hydrostatic pressure variation across the orifice at the small head has a minimum impact on the discharge. A generalised function is developed to estimate discharge under a varying head over the orifice and for different ratios of orifice diameter to riser pipe diameter, which can be used for the design of detention ponds.

Numerical simulations were conducted to model the transient flow development in a naturally ventilated space containing a centrally located localized source of heat. The simulations were compared with a series of small-scale laboratory experiments and existing theoretical models. The aim of the work was to benchmark CFD models for time-dependent buoyancy-driven natural ventilation against previously published experimental results and theoretical models. The simulations agree well with experimental results during the initial development of the room stratification. The CFD results accurately predict the maximum depth of the hot buoyant layer at the top of the room as well as the steady-state interface height which separates the warm upper buoyant layer from the cooler air below. The simulations also predict well the time taken for the buoyant upper layer to reach its maximum depth. However, at longer times the results diverge. This may be due to thermal diffusion and mixing at the interface between the upper and lower layers due to the inflow from the floor level vents.

This paper reviews recent developments in the theoretical modelling of Boussinesq turbulent plumes in both stratified and unstratified quiescent environments. The review focuses primarily on extensions to the classic entrainment model of Morton et al. (1956; MTT). Recent analytic solutions for the rise height in a stratified environment, for the rise height of turbulent fountains, and for the near‐source flow in large area source plumes are reviewed. Extensions to the theory of MTT are then reviewed, including models for plumes with a buoyancy flux that varies with height and for plumes with a buoyancy flux that varies with time. Recent experimental results are also reviewed and compared to these theoretical developments. Some open questions in turbulent plume theory are then discussed. It is shown that MTT are unable to model the rise of weak fountains satisfactorily and the significance of this result for the rise height of plumes in a stratified environment and, more generally, turbulent mixin...

We consider the filling of a room with smoke from a small, centrally located floor fire. We present theoretical arguments for the behaviour of the filling time relative to the idealised ‘filling box time’ [Baines WD, Turner JS. Turbulent buoyant convection from a source in a confined region. J Fluid Mech 1969;37:51–80] as a function of the room height to width aspect ratio. Initially, the rate at which the smoke layer deepens is shown to be more rapid for relatively wide rooms (large aspect ratio). However, at larger times, relatively tall rooms (small aspect ratio) fill more rapidly due to large scale overturning and engulfing of ambient fluid. A series of experiments were performed to verify these results and showed good qualitative agreement with our theoretical predictions. The experiments were also used to evaluate the extent of deviation of the actual smoke front position from the idealised filling box model as a function of the aspect ratio.

We compare natural ventilation flows established by a range of heat source distributions at floor level. Both evenly distributed and highly localised line and point source distributions are considered. We demonstrate that modelling the ventilation flow driven by a uniformly distributed heat source is equivalent to the flow driven by a large number of localised sources. A model is developed for the transient flow development in a room with a uniform heat distribution and is compared with existing models for localised buoyancy inputs. For large vent areas the flow driven by localised heat sources reaches a steady state more rapidly than the uniformly distributed case. For small vent areas there is little difference in the transient development times. Our transient model is then extended to consider the time taken to flush a neutrally buoyant pollutant from a naturally ventilated room. Again comparisons are drawn between uniform and localised (point and line) heat source geometries. It is demonstrated that for large vent areas a uniform heat distribution provides the fastest flushing. However, for smaller vent areas, localised heat sources produce the fastest flushing. These results are used to suggest a definition for the term ‘natural ventilation efficiency’, and a model is developed to estimate this efficiency as a function of the room and heat source geometries.

We examine the time taken to flush pollutants from a naturally ventilated room. A simple theoretical model is developed to predict the time taken for neutrally-buoyant pollutants to be removed from a room by a flow driven by localised heat inputs; both line and point heat sources are considered. We show that the rate of flushing is a function of the room volume, vent areas ( A * ) and the distribution, number (n) and strength (B) of the heat sources. We also show that the entire problem can be reduced to a single parameter ( μ ) that is a measure of the vent areas, and a dimensionless time ( τ ) that is a function of B, V and μ . Small-scale salt-bath experiments were conducted to measure the flushing rates in order to validate our modelling assumptions and predictions. The predicted flushing times show good agreement with the experiments over a wide range of μ . We apply our model to a typical open plan office and lecture theatre and discuss some of the implications of our results.

The interaction of turbulent plumes is examined in the context of building ventilation flows. Recent models for natural ventilation have been based on simplified treatment of the heat sources in a ...

We examine the transient buoyancy-driven flow in a ventilated filling box that is subject to a continuous supply of buoyancy. A rectangular box is considered and the buoyancy input is represented as a turbulent plume, or as multiple non-interacting plumes, rising from the floor. Openings in the base and top of the box link the interior environment with a quiescent exterior environment of constant and uniform density. A theoretical model is developed to predict, as functions of time, the density stratification and the volume flow rate through the openings leading to the steady state. Comparisons are made with the results of small-scale analogue laboratory experiments in which saline solutions and fresh water are used to create density differences. Two characteristic timescales are identified: the filling box time ( is determined for both line and point-source plumes, and the sensitivity of the developing flow to the distribution of buoyancy input assessed.

The coalescence of two co-flowing axisymmetric turbulent plumes and the resulting single plume flow is modelled and compared to experiments. The point of coalescence is defined as the location at which only a single peak appears in the horizontal buoyancy profile, and a prediction is made for its height. The model takes into account the drawing together of the two plumes due to their respective entrainment fields. Experiments showed that the model tends to overestimate the coalescence height, though this discrepancy may be partly explained by the sensitivity of the prediction to the entrainment coefficient. A model is then developed to describe the resulting single plume and predict its virtual origin. This prediction and subsequent predictions of flow rate above the merge height compare very well with experimental results.

Results are presented from an experimental study of the stage discharge relationship for a porous pipe buried under loose laid aggregate. A series of steady-state experiments were performed in which the depth of flow over the pipe was measured for a given pipe discharge. The stage-discharge relationship for the entire system is well described by an orifice equation when the water surface in the aggregate trench was horizontal and the pipe was running full at the outlet. An energy equation analysis shows that the dominant energy losses are due to flow through the small side wall orifices into the pipe and from friction losses along the pipe. Friction losses in the aggregate are shown to be negligible, which again supports the orifice equation model. Discharge coefficients are reported for three commonly available porous pipes. The results of this research can be used in the sizing and placement of subsurface drains in aggregate filled storm water infiltration practices such as infiltration trenches...

Results are presented for the intrinsic permeability and form drag coefficient (CF) for a set of nominally identical cylindrical pervious concrete specimens. Specimens were tested at a range of flow rates in a constant head permeameter. The mean and standard deviation of the permeability and CF show substantial variability. This statistical characterization of hydraulic performance was used to model the behavior that would be observed using other standard methods for characterizing hydraulic performance of porous pavement mixtures. Results indicate that falling head, constant head, and infiltration tests have the potential to significantly underestimate the permeability of porous pavement mixtures. Results from modeled falling head tests showed that this test results in permeability estimates up to a factor of ten lower than the actual permeability and are uncorrelated to the actual permeability. For high permeability pavement mixtures, falling head tests will only provide adequate values of permeability if run at heads so low that measurement resolution is a problem. Constant head tests can be used, though model results indicate that, even for very small hydraulic gradients, the head versus flow rate relationship is quadratic. As such, CF must be calculated. Failure to do so leads to significant underestimation of the permeability. For most applications, the permeability of a well-designed and placed pervious concrete is more than adequate and the improvement in accuracy gained by fully characterizing both the permeability and CF of a pavement is not necessary. However, for poorly constructed pavements, or pavements that need to infiltrate large amounts of run-on from adjacent impervious areas, the additional accuracy may be important. The additional level of accuracy is also important for research into improving design and construction of pervious concrete where comprehensive and accurate characterization of test specimens is needed for statistically significant comparisons between different design specimens.

An experimental study is presented to investigate the mechanism of flushing a trapped dense contaminant from a canyon by turbulent boundary layer flow. The results of a series of steady-state experiments are used to parameterize the flushing mechanisms. The steady-state experimental results for a canyon with aspect ratio one indicate that dense fluid is removed from the canyon by two different processes, skimming of dense fluid from the top of the dense layer; and by an interfacial mixing flow that mixes fresh fluid down into the dense lower layer (entrainment) while mixing dense fluid into the flow above the canyon (detrainment). A model is developed for the time varying buoyancy profile within the canyon as a function of the Richardson number which parameterizes both the interfacial mixing and skimming processes observed. The continuous release steady-state experiments allowed for the direct measurement of the skimming and interfacial mixing flow rates for any layer depth and Richardson number. Both the...

The collision of axisymmetric turbulent plumes with buoyancy fluxes of opposite sign is examined experimentally. The total buoyancy flux loss of each plume as a result of the collision is measured. The measurements are made using a new experimental technique for measuring the buoyancy flux of a plume based on the ventilation theory of Linden, Lane-Serff & Smeed (J. Fluid Mech. vol. 212, 1990, p. 309). The experimental results are presented as functions of the buoyancy flux ratio ψ and the ratio of radial to vertical separation σ . For axially aligned plumes we find that the lower-buoyancy-flux plume loses all its buoyancy flux when ψ 0.25. This plume–plume collision is modelled using a modified set of entrainment equations. The model allows for the exchange of buoyancy and deflection of the plumes as they pass by each other. We present predictions of total buoyancy flux loss as a function of both plume strength and separation. The model predictions are compared to the experimental measurements of buoyancy flux loss, and show good agreement.

This paper presents results of a Large Eddy Simulation (LES) of buoyancy-driven natural ventilation in which two unequal heat sources are used to drive the flow. The aim of this work was to assess the performance of LES in modelling turbulent thermal plumes in a naturally ventilated enclosure and to analyse their interaction with each other. The sub-grid scales of the flow have been resolved by using the Smagorinsky sub-grid scale model. It was found that LES results for the interface height agree well with the theoretical predictions of Linden and Kaye (2006). However, instead of a step change in temperature, as suggested by theory, there exists a more diffuse interface. The merging height of the two plumes and the volume flow rates in the far field merged plume agreed closely with theoretical predictions. Analysis of the pressure isosurfaces demonstrated the interaction phenomena of the two plumes, the coherent structures and the behaviour of the interface.