Numerical modeling of turbulent flows in arbitrarily complex natural streams

An efficient and versatile numerical model is developed for carrying out high-resolution simulations of turbulent flows in natural meandering streams with arbitrarily complex, albeit fixed, bathymetry and instream hydraulic structures. The numerical model solves the three-dimensional, unsteady, incompressible Navier-Stokes and continuity equations in generalized curvilinear coordinates. This model can handle the arbitrary geometric complexity of natural streams by using the sharp-interface curvilinear immersed boundary (CURVIB) method. To enable efficient simulations on grids with tens of millions of nodes in long and shallow domains typical of natural streams, the algebraic multigrid method (AMG) is used to solve the Poisson equation for pressure. Free-surface is treated either with the rigid-lid approach or modeled using a two-phase flow approach implemented using level-sets. Depending on the desired level of resolution and available computational resources, the numerical model can either simulate turbulence via direct numerical simulation (DNS), large-eddy simulation (LES) or unsteady Reynolds-averaged Navier-Stokes (URANS) simulation. The numerical model is validated by simulating several test cases for which good quality laboratory data or benchmark simulations are available in the literature. The potential of the model as a powerful tool for simulating energetic coherent structures in turbulent flows in natural river reaches is demonstrated by applying it to carry out LES and URANS simulations in a field scale natural-like meandering stream, Outdoor StreamLab, at resolution sufficiently fine to capture vortex shedding from cm-scale roughness elements on the bed. Comparisons between the simulated mean velocity and turbulence kinetic energy fields with field-scale measurements are reported and show that the numerical model can capture all features of the measured flow with high accuracy. Furthermore, the simulated flowfields are analyzed to elucidate the multi-faceted physics of the flow in a natural stream with pool-riffle sequences and to uncover the underlying physical mechanisms. The simulations provide new insights into the role of large-scale roughness in flow through riffles and elucidate the three-dimensional structure, interactions and governing mechanisms of the inner and outer bank secondary flow cells and recirculation zones in the pools. Moreover, the simulations underscore the role of turbulence anisotropy throughout the stream and suggest important links between stream hydrodynamics and morphodynamics. Calculations are also carried out for the same meandering stream with an instream structure installed along its outer bank to demonstrate the utility of the model as a powerful tool for developing science-based design guidelines for stream restoration.