Session D33: Environmental Flows I

Flapping line fountains

Results of an experimental study are presented on a turbulent line fountain formed when a dense saline solution is ejected steadily upwards from a slender rectangular slot into a lighter aqueous environment. A combination of flow visualisations and measurements reveal the complex three-dimensional structure of these fountains. The flow is interrogated by simultaneously viewing it from above and from the side. These views reveal that the fountain flaps about the slot in distinct patterns. Insights into the mechanism causing the flapping were acquired by examining the internal structure of the fountain along vertical sections cutting across the slot. Measurements of rise height and lateral excursion of flapping events, alongside their associated spatial and temporal statistics, are discussed to show that a single driving mechanism underlies the motion.   

The fluid dynamics of deep-sea mining

In the next decade, with pressing societal for mineral resources, it is anticipated that deep-sea mining activities will commence. Associated with such activities are numerous challenging fluid dynamics problems. A notable example is the behavior of sediment-laden plumes generated by collector vehicles operating on the sea floor at depths around 4000-5000m. Here we present an overview of the fluid dynamics associated with deep-sea mining activities and results of our studies incorporating classical and numerical modeling, and the results of recent field experiments.    

Buoyancy effects on cross-ventilation

Wind-driven cross-ventilation occurs when there are openings on opposite walls of a room, at equal height. This talk will present the results of laboratory experiments using a water flume and a cross-ventilated model room at 1/10th scale. These experiments examine the effect of an initial indoor-outdoor temperature difference on the ventilation rate and the removal of contaminants from a cross-ventilated room. We present simple mathematical models that explain the observed ventilation rates.   

Entrainment in DNS of Turbulent Thermals

Thermals are the result of a finite release of buoyant fluid. Thermals have been studied extensively in the laboratory, and more recently with large eddy simulations (LES). Here we present the first direct numerical simulations (DNS) of thermals; we study both laminar thermals (Re=630) and turbulent thermals (Re=6300) with Pr=1. Our goal is to quantitatively test the entrainment hypothesis of, e.g., Morton, Taylor, & Turner (1956). We use a thermal tracking algorithm based off techniques proposed in atmospheric science, and calculate the entrainment rate as the rate of change of the thermal volume. In line with previous work, we find that the entrainment rate is inversely proportional to the radius of the thermal, d log V/dz = e/r, where e is the entrainment efficiency. The entrainment efficiency for both laminar thermals (e=0.36) and turbulent thermals (e=0.47) is significantly lower than the reported values in laboratory experiments (e~0.75). We suggest this may be due to the use of salt as a source of density perturbations in laboratory experiments, which have Sc~700.   

On the Effective Buoyancy of Turbulent Thermals

An isolated, buoyant fluid parcel (or thermal) does not accelerate with its Archimedean buoyancy, because of environmental inertia (i.e. virtual mass effects). This reduces its acceleration to an `effective buoyancy’.  Here we utilize a novel correspondence with magnetostatics to derive the first complete, exact analytical expressions for effective buoyancy, for the case of ellipsoidal thermals. Such a geometry is exhibited by real, turbulent thermals in the laboratory (e.g. Scorer 1957) as well as turbulent thermals that we simulate with direct numerical simulation.  The average effective buoyancy of these turbulent thermals is  accurately captured by our analytical expressions. 

The Initial Development of a Thermal and the Application to Downdraughts

Downward moving cold air within thunderstorms, known as downdraughts, can be used to determine the severity of a storm. Therefore an understanding of them is useful for weather forecasting. Typically in weather forecasting these downdraughts are modelled using the theory of a plume from from Morton, Taylor and Turner (1956), which inherently assumes that the plume is long and thin. Downdraughts are generally wider than they are high and hence deviate from the Morton, Taylor and Turner theory.

We perform laboratory experiments using finite releases of dense fluid from cylinders of varying lengths. We use an extension of the dye attenuation technique to take measurements of the edges and center of mass of the release. This enables us to look at how the vertical velocity and shape develops during the descent. In doing so we see the development of the shape factor and spreading rate during the draining and thermal phases of the descent. Using previously developed theory and assumptions of the shape this allows us to analyse the entrainment during both phases and asses the adequacy of the Morton Taylor Turner (1956) entrainment assumption in the context of downdraughts.