Improved predictions of geohazard risk from catastrophic environmental flows: the critical role of turbulent mixing processes

Gravity currents are ubiquitous in nature, arising over a vast range of length scales and fluid dynamical disciplines. They are primarily horizontal flows driven by a density difference between ambient fluid and the current, typically arising due to salinity, temperature, or suspended matter. Gravity currents are environmental system controls, including: temperature driven weather front dynamics; thermohaline cycling of nutrients in the world’s oceans; and, transport of particulate material in the air, on-land and underwater. These include key catastrophic geohazard risks, from pyroclastic flows, powder snow avalanches, volcanic ash clouds and submarine landslides.

Despite their importance critical questions remain regarding the dynamics of these flows; this limits our ability to predict their behaviour and thus manage and mitigate risks. As gravity currents propagate they mix with ambient fluid (entrainment), growing in volume with distance. Quantifying how fluid is entrained is vital to understanding how far currents may travel: the critical parameter constraining geohazard risk. Moreover, the scale of natural gravity currents necessitates the use of simplified models; these are typically closed by assuming a balance exists between gravitational driving processes and frictional drag, i.e. pseudo-steady flow. Yet real currents are inherently unsteady. The strong violation of the pseudo-steady balance means that the developed closures are unphysical. Recent field, experimental, and numerical data show that mixing at flow-scales is key to flow dynamics. However, our understanding of such mixing processes, their impact on material transport and flow runout is in its infancy.

Therefore, the aim of the project is to improve the estimation of geohazard risk, through study of the fundamental nature of gravity currents and improved understanding of geophysical, atmospheric, and oceanic field processes. To achieve this the PhD will investigate three intricately linked research questions:

  1. What roles do flow-scale mixing processes play on the entrainment and mixing of ambient fluid into a saline current, and how do mechanisms scale with key dimensionless parameters?
  2. For a pseudo-steady current, how is the entrainment linked to the bulk properties of the mean flow?
  3. For a current far from equilibrium, how can classical models of entrainment be adapted to capture the unsteady dynamics of the bulk flow?

Research questions will be addressed using novel, and world-leading, experimental and numerical methodologies. Full training on experimental and numerical methodologies will be provided. Advances in understanding of gravity current mixing and entrainment processes will be enabled by new experimental methods, integrating simultaneous tomographic Particle Image Velocimetry (PIV) and 3D Laser Induced Fluorescence (LIF). These techniques provide high fidelity, and 3D, density and velocity fields measurements: unique quantification to resolve the coupled dynamics of the two (velocity and density) fields controlling mixing processes. The PhD student will investigate the dynamics across a range of Reynolds, Froude, and Richardson numbers, three essential parameters for understanding mixing processes. Identified mixing mechanisms will not only be directly relevant to large scale geophysical flows and geohazards, but will also be applicable to mixing dynamics in a wide range of other oceanic and atmospheric flows.

Experiments will be complemented with numerical studies, adopting high-resolution simulations. Numerical methods will be used to simulate idealized gravity current dynamics and analyse the spatio-temporal coherence of the flow. Learning to implement a range of advanced post-processing methodologies will be supported, enabling the PhD to assess different flow scale mixing processes and their impact on flow dynamics. Here, existing simplified models for gravity currents will be validated and assessed before determining how they can be improved. Simulations will enable a deep understanding of the controls on mixing that determine material transport and thus the geohazard risk that such flows pose.

Finally, the PhD will assess how flow dynamics change under unsteady current propagation. Numerical simulations, or experiments, will be extended to model the gravity current over a discontinuous sloping boundary. This discontinuous sloping boundary provides a more realistic condition to simulate gravity currents, such as pyroclastic flows and powder snow avalanches, over complex topography. Simulations will quantify how mixing dynamics change when pseudo-steady conditions are no longer satisfied. These advances are critical to permit the development of models that are suitable for unsteady currents, with the outcome being to provide advanced methodology to predict, and assess risk from, real-world catastrophic flow events.