There is a significant gap between catchment-scale, calibrated, one-dimensional flood models and river-reach (small-scale) hydrodynamic models in two and three dimensions. The latter are valuable for modeling of water quality, aquatic habitat, sediment transport, and geomorphologic evolution, but cannot be practically up-scaled to an entire river basin. The principal methodology problem for up-scaling multi-dimensional models is that the model grid must be coarsened to fit available computer power. With a coarse model grid, a single grid cell may contain very different bottom types and obstacles, therefore requiring site-specific grid-dependent calibration. As a consequence, up-scaling existing models prevents direct application of the knowledge of small-scale turbulent behavior that has been developed in laboratory and field studies over the last 50 years. Such knowledge is readily used in existing small-scale models to eliminate or reduce calibration requirements.
This project will directly address the gap in our modeling methods by developing and testing a new theory for "Coarse Grid Simulation (CGS)". For this new modeling approach, the Large-Eddy Simulation (LES) formalism is applied to the Reynolds-Averaged Navier-Stokes (RANS) equations to create a set of CGS equations that explicitly separate grid-dependent averaging terms from fundamental turbulence properties that can be empirically characterized. The research hypothesis to be proved/falsified is that fine-scale processes in a fine-grid model can be represented at arbitrary grid scales in a CGS model without ad hoc calibration. Future development and large-scale validation of the method will require field studies that collect detailed velocity and turbulence data, so this project includes a preliminary field investigation with the recently-developed Pulse-Coherent Acoustic Doppler Current Profiler to gain a better understanding of the turbulence and velocity details that can reasonably be collected.
The broader impact of the research on the scientific community is that CGS provides a new practical methodology for catchment-scale hydrodynamic models that can be used to drive water quality, aquatic habitat and geomorphological studies. This project also combines education and outreach by supporting a female Ph.D. student and providing an opportunity for a UT undergraduate student (recruited through the UT Society of Hispanic Professional Engineers or the UT National Society of Black Engineers) to work on the field study in concert with NSF-sponsored REU students from other universities.
The intellectual merit of this project is an entirely new way of framing the turbulence closure problem that separates grid-scale effects from turbulence at coarse grid resolutions. The new approach explicitly treats the effects of a coarse grid scale and allows turbulence generated by unresolved (but empirically known) subgrid-scale features to be integrated over a grid cell.
