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In this study, near-neutrally stratified planetary boundary layer (PBL) flows over complex terrain are numerically investigated via a one-way coupling of mesoscale numerical weather prediction (NWP) with unsteady computational fluid dynamics (CFD) codes. First, empirical turbulence model constants are modified to accomplish consistent turbulent viscosities between PBL schemes of mesoscale NWP and the k - ω turbulence closure of microscale CFD. Next, to reconstruct turbulent inflows calculated from coarse NWP analysis, inverse distance weighted (IDW) interpolation is optimized and adopted around the coupling interfaces. This method has been shown to decrease prediction uncertainty, as high-aspect-ratio NWP grids are used. Compared with time-series measurements, the proposed approach presents wind velocities in agreement around both upstream and separated regions in the Askervein Hill case, although the turbulent kinetic energy on the leeward side is still underestimated. Another moderately complex terrain with an enlarged domain around Feng Men Hill also exhibits improved numerical results. Moreover, the unsteady simulations reveal that both wind speed-up and wake are highly correlated with inflow characteristics. The successful integration of the nesting simulation with a general CFD code demonstrates the flexibility and efficiency of this procedure in modeling high-resolution unsteady flows over complex terrain. [ABSTRACT FROM AUTHOR]

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Accurate predictions of wind and dispersion in the atmospheric boundary layer (ABL) can provide essential information to support design and policy decisions for sustainable urban areas. However, computational fluid dynamics (CFD) predictions of the ABL have several sources of uncertainty that can affect the results. An important uncertainty is the definition of the inflow boundary condition, which is influenced by larger scale weather phenomena. In this paper, we propose a method to quantify the effect of uncertainty in the inflow boundary conditions using input from an ensemble of mesoscale simulations. The mesoscale mean velocity and turbulent kinetic energy at the inflow of the CFD domain are used to define probability density functions for the uncertain wind direction and magnitude. A non-intrusive method is used to propagate these uncertainties to the quantities of interest. The methodology is applied to two different cases for which field experimental data are available: the Askervein hill and the Joint Urban 2003 measurements. For the latter case, the results are similar to those of a previous study that characterized the uncertain input parameters based on measurements. Hence, the results show that the proposed mesoscale simulation-based approach provides a valuable alternative in absence of sufficient measurement data. [ABSTRACT FROM AUTHOR]

Large-eddy simulation (LES) models have been used extensively to study atmospheric boundary layer turbulence over flat surfaces; however, LES applications over topography are less common. We evaluate the ability of an existing model – COAMPS®-LES – to simulate flow over terrain using data from the Askervein Hill Project. A new approach is suggested for the treatment of the lateral boundaries using one-way grid nesting. LES wind profile and speed-up are compared with observations at various locations around the hill. The COAMPS-LES model performs generally well. This case could serve as a useful benchmark for evaluating LES models for applications over topography.

Large-eddy simulations of the neutrally stratified flow over the Askervein Hill were performed, to improve the knowledge of the flow obtained from field measurements and numerical simulations with Reynolds averaged Navier-Stokes (RANS) methods. A Lagrangian dynamic subgrid model was used but, to avoid the underdissipative character near the ground, it was merged with a damped Smagorinsky model. Simulations of a flat boundary-layer flow with this subgrid model showed that the turbulent vertical motions and shear stress were better resolved using grids with a stream to spanwise aspect ratio Δ x / Δ y = 2 than with an aspect ratio Δ x / Δ y = 1. Regarding the flow over the Askervein Hill, it was found that large-eddy simulations provide an acceptable solution for the mean-velocity field and better predictions of the turbulent kinetic energy in the upstream side of the hill than the $$k - \varepsilon$$ model. However, as with the $$k - \varepsilon$$ model, grid convergence was not achieved in the lee side and the size of the zone with reversed flow increased with the grid refinement. Nevertheless, the existence of the intermittent separation predicted with unsteady RANS in part one of this work seems unquestionable, due to the deceleration of the flow. In our opinion, a better modelling of the decelerating boundary layer in the lee side is required to improve the results obtained using equilibrium assumptions and achieve grid convergence. [ABSTRACT FROM AUTHOR]

The flow solver “3DWind” is used to explore new aspects of the Askervein hill flow case. Previous work has investigated sensitivities to the grid, the inflow boundary profile, the roughness and the turbulence model. Several different linear and non-linear numerical models have also been validated by means of the Askervein hill case. This analysis focuses on the flow sensitivity to the grid spacing, the incident wind direction and the vertical resolution of topographic data. The horizontal resolution is found to be fine enough to cause only minor differences compared to a grid where every second node is removed. The vertical resolution dependence is mainly attributed to the wall functions. Simulations are performed for wind directions 200°, 205°, 210° and 215° at the reference station. The smallest directional biases compared to experimental values along a line through the hilltop are found for the directions 200° and 205°. There are larger wind direction changes along this line through the hilltop in the 200° case than in the 215° case. Still the simulation results give less veering than found in the experimental results, and this is maybe caused by a slightly stable atmosphere. The sensitivity to the vertical resolution of the topographical data is found to be particularly high close to the ground at the top of the hill; this is where the speed-up is most important. Differences decrease with the height from the ground. At higher levels the speed-ups are smaller and caused by terrain formations with larger scales. [ABSTRACT FROM AUTHOR]

The neutrally stratified flow over the Askervein Hill was simulated using a terrain-following coordinate system and a two-equation (k - ε) turbulence model. Calculations were performed on a wide range of numerical grids to assess, among other things, the importance of spatial discretization and the limitations of the turbulence model. Our results showed that a relatively coarse grid was enough to resolve the flow in the upstream region of the hill; at the hilltop, 10 m above the ground, the speed-up was 10% less than the experimental value. The flow's most prominent feature was a recirculating region in the lee of the hill, which determined the main characteristics of the whole downstream flow. This region had an intermittent nature and could be fully captured only in the case of a time-dependent formulation and a third-order discretization of the advective terms. The reduction of the characteristic roughness near the top of the hill was also taken into account, showing the importance of this parameter, particularly in the flow close to the ground at the summit and in the downstream side of the hill. Calculations involving an enlarged area around the Askervein Hill showed that the presence of the nearby topography affected the flow neither at the top nor downstream of the Askervein Hill. [ABSTRACT FROM AUTHOR]