What is NaysEddy?

In river engineering problems, the flow cannot always be approximated by two-dimensional models. The complexity of flow, especially in terms of flow details near deformed beds and irregular banks, plays an important role in the flow structures, sediment transport and the morphological change of rivers. Therefore, simulation on the three-dimensional structures of the flow is essential in prediction of realistic bed shear stresses that can be used for suitable estimation of sediment transport and bed morphodynamics.

Several depth-integrated two-dimensional models are implemented in iRIC. They demonstrate great success in the simulation of large scale engineering problems. However, they are not able to capture the three-dimensional structures of the flow even in the presence complex geometries. They estimate the flow roughly ignoring the vertical flow structures and approximate the flow under the assumption of hydrostatic pressure. The phenomena such as secondary flow, turbulence eddies, horseshoe vortices, etc. cannot be observed using the hydrostatic (e.g, shallow-water) approximation.

NaysCube provides a fully three-dimensional solution for these problems using the Reynolds Averaged Navier-Stokes (RANS) approach. NaysCube, which is based on a curvilinear coordinate system with a k-ε turbulence closure, can solve a wide range of three-dimensional problems. However, NaysCube is based on RANS, which averages all flow fluctuations and lead to a smooth flow field without turbulent fluctuation. These fluctuations are important in a number of detailed problems, such as flow over bedforms, hence NaysCube fails in simulation of highly complex geometries, in which the turbulence is significantly affected by the geometry and/or where the turbulence plays an important role in understanding sediment transport.

Thus, we introduce NaysEddy as a three-dimensional solver based on large-eddy simulations (LES). It solves the flow in more detail using Cartesian grids with ghost-cell immersed boundary methods. This solver is flexible, accurate and can solve complex problems with a great success. The approach has been tested under extremely complex conditions with moving boundaries, complex bed topography, and flows with relatively high Froude numbers, etc., and the solver shows its capability in the simulation of those problems. This solver applies the full Navier-Stokes equations without approximation and without temporal averaging (Reynolds Averaging).

However, the computational load of this solver is at least one order larger than that of NaysCUBE, and even more time consuming relative to Nays2dh, FaSTMECH, etc. The solver is effective for reproducing local phenomena of rivers for a short time span in a spatially limited area, rather than reproducing a large reach of the river for a long time span.