Evaluation of Coupled Meso−Microscale Simulations of the WFIP 2 Physics Site

Coupling microscale large-eddy simulation (LES) solvers to mesoscale weather models like the Weather Research and Forecasting (WRF) model creates a powerful simulation tool to study complex flow cases in detail under a wide range of weather conditions. Mesoscale-to-microscale coupling (MMC) can for instance be used to investigate wind-plant energy extraction and turbine loading more accurately in real-life flow situations. One of the challenges of MMC is the specification of initial and boundary conditions for the turbulent scales that are resolved at the microscale level but that are being modeled in the mesoscale simulation. Without a proper way to initiate realistic turbulence, it may take several kilometers of fetch for the turbulence to come to a realistic equilibrium state. There are various ways to expedite the transition to fully developed turbulence, all trying to minimize the overhead in computational cost.

In the current study, we evaluate coupled meso-micro scale simulations in moderately complex terrain conditions. The inflow conditions for velocity and temperature at the inlet of the microscale domain thereby consist of a smooth profile derived from the mesoscale model, enriched with turbulent fluctuations. Initial results show that the profiles of the compressible mesoscale solver should be imposed with care in the microscale simulation since the latter solves an incompressible set of equations under the Boussinesq approximation. We investigate whether the compressible flow solution can be rescaled with the density profile to make it compatible with the Boussinesq approximation. Alternatively, we could opt for the anelastic approximation in the microscale solver to better account for density changes with height.

For the fluctuating part of the inflow conditions, we propose to use turbulence information from an auxiliary microscale large-eddy simulation over flat terrain. As there is no horizontal heterogeneity in this precursor simulation, we can use periodic boundary conditions in the horizontal directions, allowing for a rapid generation of realistic turbulent structures through the recycling at the boundaries. The precursor simulation is driven by internal source terms, like large-scale advection, derived from the mesoscale solver. Alternatively, these sources may be computed within the microscale solver such that they drive the planar-averaged solution to match that of the mesoscale solver. In that case, a simple P-controller with height-time varying gains is used to compute the source terms. The idea of the LES-generated turbulence is that the conditions simulated in the auxiliary domain, although over flat terrain with periodic boundary conditions, will be similar to the developed turbulence that the terrain case should include. The fluctuating fields from this mesoscale-informed precursor LES are extracted and superimposed on the mesoscale-model-derived inflow of the inflow/outflow terrain case.

Our study focuses on the complex terrain in the Columbia River Basin. We simulate the wind flow through a 30x30 km region containing the heavily instrumented Wind Forecast Improvement Project 2 (WFIP 2) Physics Site. The mesoscale input data is obtained from WRF simulations. The microscale simulations are performed using the OpenFOAM-based Simulator fOr Wind Farm Applications (SOWFA), a wind-plant computational fluid dynamics tool developed at the National Renewable Energy Laboratory. We compare the results of our LES-generated turbulence approach with other methods that use stochastic perturbations or synthetic turbulence models to initiate resolved-scale turbulence in the microscale domain.

This work was authored by the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy under Contract No. DE-AC36-08GO28308. Funding was provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.