Development and Evaluation of a Reynolds-Averaged Navier–Stokes Solver in WindNinja for Operational Wildland Fire Applications

An open source computational fluid dynamics (CFD) solver has been incorporated into the WindNinja modeling framework. WindNinja is widely used by wildland fire managers, as well as researchers and practitioners in other fields, such as wind energy, wind erosion, and search and rescue. Here, we describe the CFD solver and evaluate its performance against the WindNinja conservation of mass (COM) solver, and previously published large-eddy simulations (LES), for three field campaigns with varying terrain complexity: Askervein Hill, Bolund Hill, and Big Southern Butte. We also compare the effects of two model settings in the CFD solver, namely the discretization scheme used for the advection term of the momentum equation and the turbulence model, and provide guidance on model sensitivity to these settings. Additionally, we investigate the computational mesh and difficulties regarding terrain representation. Two important findings from this work are: (1) CFD solver predictions are significantly better than COM solver predictions at windward and lee side observation locations, but no difference was found in predicted speed-up at ridgetop locations between the two solvers, and (2) the choice of discretization scheme for advection has a significantly larger effect on the simulated winds than the choice of turbulence model.

Development and Evaluation of a Reynolds-Averaged Navier-Stokes Solver in WindNinja for Operational Wildland Fire Applications

An open source computational fluid dynamics (CFD) solver has been incorporated into the WindNinja modeling framework widely used by wildland fire managers as well as researchers and practitioners in other fields, such as wind energy, wind erosion, and search and rescue. Here we describe incorporation of the CFD solver and evaluate its performance compared to the conservation of mass (COM) solver in WindNinja and previously published large-eddy simulations (LES) for three field campaigns conducted over isolated terrain obstacles of varying terrain complexity: Askervein Hill, Bolund Hill, and Big Southern Butte. We also compare the effects of two important model settings in the CFD solver and provide guidance on model sensitivity to these settings. Additionally, we investigate the computational mesh and difficulties regarding terrain representation. Two important findings from this work are: (1) the choice of discretization scheme for advection has a significantly larger effect on the simulated winds than the choice of turbulence model and (2) CFD solver predictions are significantly better than the COM solver predictions at windward and lee side observation locations, but no difference was found in predicted speed-up at ridgetop locations between the two solvers.

Large-Eddy Simulation over Complex Terrain Using an Improved Immersed Boundary Method in the Weather Research and Forecasting Model

The Weather Research and Forecasting (WRF) Model is increasingly being used for higher-resolution atmospheric simulations over complex terrain. With increased resolution, resolved terrain slopes become steeper, and the native terrain-following coordinates used in WRF result in numerical errors and instability. The immersed boundary method (IBM) uses a nonconformal grid with the terrain surface represented through interpolated forcing terms. Lundquist et al.’s WRF-IBM implementation eliminates the limitations of WRF’s terrain-following coordinate and was previously validated with a no-slip boundary condition for urban simulations and idealized terrain. This paper describes the implementation of a log-law boundary condition into WRF-IBM to extend its applicability to general atmospheric complex terrain simulations. The implementation of the improved WRF-IBM boundary condition is validated for neutral flow over flat terrain and the complex terrain cases of Askervein Hill, Scotland, and Bolund Hill, Denmark. First, comparisons are made to similarity theory and standard WRF results for the flat terrain case. Then, simulations of flow over the moderately sloped Askervein Hill are used to demonstrate agreement between the IBM and terrain-following WRF results, as well as agreement with observations. Finally, Bolund Hill simulations show that WRF-IBM can handle steep topography (standard WRF fails) and compares well to observations. Overall, the new WRF-IBM boundary condition shows improved performance, though the leeside representation of the flow can be potentially further improved.

Least Squares Fitting of Computational Fluid Dynamics Results to Measured Vertical Wind Profiles

Microscale numerical modeling is currently the main tool used in wind industry to assess local wind resources. This paper presents a systematic procedure to adjust computational fluid dynamics (CFD) predicted wind profiles to experimental measurements in order to minimize their differences. It can be applied when wind measurements are available. Data from ten masts with several measurement heights from the well-known Bolund hill experiment provided the observed wind profiles. Simulated profiles were calculated with windsim CFD model for the aforementioned site. Speed-up correction factors were defined through the least squares method to cross-correlate each mast as reference to all the others inside the Bolund hill domain. After, the observed and the adjusted wind profiles at the same position were compared. Moreover, root mean square errors (RMSEs) were used as a metric to evaluate the estimations and the ability of each position to be predicted and predictor. Results have shown that the quality of the adjustment process depends on the flow characteristics at each position related to the incoming wind direction. Most affected positions, i.e., when the airflow overcomes the Bolund hill escarpment, present the less accurate wind profile estimations. The reference mast should be installed upstream of the potential wind turbines' locations and after the main local characteristics of topographical changes.

Investigation of Mean and Turbulent Flow Behaviour Over an Escarpment

Mean and turbulent flow behaviour over a 1:25 scale model of Bolund hill was investigated at Western University’s Wind Engineering, Energy, and Environment Research Institute (WindEEE) using Particle Image Velocimetry (PIV). A range of upstream flow and surface conditions were considered. Results showed almost no Reynolds dependency on the mean flow and weak dependency of Reynolds number on the upstream surface roughness conditions. However, a strong Reynolds number and upstream surface rough dependency is observed on the turbulent flow particularly in the shear layer formed in the immediate downstream region of the escarpment. It is concluded that the consideration of Reynolds number independency must be cautiously used when extrapolating the flow parameters from scaled model testing to full scale in the field.