The project investigates turbulence and mixing in stably stratified fluid. Mixing is central to a wide range of questions from the heat uptake in the global ocean, the transport and dilution of pollutants in the atmosphere, the efficient cooling of buildings, to the homogenising of products in the food industry. However, the mechanisms that are responsible and their physical and dynamical aspects are largely unknown, and it is not possible to predict mixing rates from a knowledge of the overall flow and density fields.
All fluid bodies in nature (the atmosphere, oceans, lakes, the air within a room) have significant regions that are stably stratified, making the study of fluxes and mixing in stratified turbulence essential for applications to a wide range of environmental problems from climate predictions to the design of low-energy buildings. The central question we propose to answer is what determines mixing in stratified flows.
The overall objectives are
I. Identify the structures responsible for mixing turbulent mixing events in a maintained stratified shear at high Reynolds numbers, in particular their connection to classical flow instabilities.
II. Using highly resolved 3D measurements over a volume, study their dynamics and life-cycles.
III. Determine their contributions to mixing in transitional, intermittent and fully turbulent flow, investigating the differences and similarities between these contributions and the equivalent contributions for nonlinear structures in classical unstable initial value problem studies.
IV. Determine the entrainment and relate it to the prevailing dynamical processes.
The Stratified Inclined Duct (SID) has large reservoirs on either side of a long rectangular duct to sustain a long-lived exchange flow. Importantly, the apparatus can be tilted at a small angle θ to force the denser layer to accelerate as it flows downslope and vice versa. The originality of SID is that the simple, natural forcing of gravity through θ sustains vigorous interfacial stratified turbulence that long seemed out of reach of either laboratory or numerical experiments. SID can be seen as a natural extension of Reynolds classic pipe flow experiment. Pipe flow has a single control parameter the Reynolds number. SID, on the other hand, has two control parameters: Re, which is set by the density difference between the two reservoirs and θ. This additional control parameter provides a wealth of new flow phenomena associated with the stratification and allows us to achieve the objectives above.
