Abstract. The present work analyzes the quality and reliability of an important class of general-purpose, second-order accurate finite-volume (FV) solvers for the large-eddy simulation of a neutrally stratified atmospheric boundary layer (ABL) flow. The analysis is carried out within the OpenFOAM® framework, which is based on a colocated grid arrangement. A series of open-channel flow simulations are carried out using a static Smagorinsky model for subgrid scale momentum fluxes in combination with an algebraic equilibrium wall-layer model. The sensitivity of the solution to variations in numerical parameters such as grid resolution (up to 1603 control volumes), numerical solvers, and interpolation schemes for the discretization of nonlinear terms is evaluated and results are contrasted against those from a well-established mixed pseudospectral–finite-difference code. Considered flow statistics include mean streamwise velocity, resolved Reynolds stresses, velocity skewness and kurtosis, velocity spectra, and two-point autocorrelations. A quadrant analysis along with the examination of the conditionally averaged flow field are performed to investigate the mechanisms responsible for momentum transfer in the flow. It is found that at the selected grid resolutions, the considered class of FV-based solvers yields a poorly correlated flow field and is not able to accurately capture the dominant mechanisms responsible for momentum transport in the ABL. Specifically, the predicted flow field lacks the well-known sweep and ejection pairs organized side by side along the cross-stream direction, which are representative of a streamwise roll mode. This is especially true when using linear interpolation schemes for the discretization of nonlinear terms. This shortcoming leads to a misprediction of flow statistics that are relevant for ABL flow applications and to an enhanced sensitivity of the solution to variations in grid resolution, thus calling for future research aimed at reducing the impact of modeling and discretization errors.

Abstract. In the present work, the quality and reliability of a general-purpose second-order-accurate Finite-Volume-based (FV) solver are assessed in wall-modeled large-eddy simulations of a neutrally-stratified Atmospheric-Boundary-Layer (ABL) flow with no Coriolis effects. The sensitivity of the solution to parameters such as grid resolution and aspect ratio is analyzed, and results are contrasted against those from a well-proven mixed Pseudo-Spectral and Finite-Difference (PSFD) code. Considered flow statistics include mean streamwise velocity, resolved Reynolds stress, turbulence intensities, skewness, kurtosis, spectra and spatial autocorrelations. It is found that first- and second-order velocity statistics are sensitive to the grid resolution and to the details of the near-wall numerical treatment, and a general improvement is observed with horizontal grid refinement. Higher-order statistics, spectra and autocorrelations of the streamwise velocity, on the contrary, are consistently mispredicted, regardless of the grid resolution. Skewness and kurtosis of the streamwise velocity, for instance, are overpredicted in the surface layer, whereas one-dimensional spectra feature a strong sensitivity to the grid resolution in the production range and a rapid decay of energy density at higher wavenumber. In addition, the typical signatures of Large-Scale Motions (LSMs) are absent in the premultiplied streamwise velocity spectra, the spatial autocorrelation functions rapidly decay along both the streamwise and spanwise coordinate directions, and instantaneous snapshots of the velocity field are populated by relatively short and thin streaks, confirming that the flow lacks LSMs. Further, the dominant mechanism supporting the tangential Reynolds stress in ABL flow – spanwise-paired sweeps and ejections– is much weaker than what commonly observed in ABL flows, ejections are severely underpredicted, and sweeps account for most of the tangential Reynolds stress in the surface layer, which is at odds with available measurements and with corresponing results from the PSFD-based solver. The inability of the solver to correctly capture the spatially-localized and relatively strong ejection events, in the authors’ opinion, is the root-canse of many of the observed mismatches and sensitivity of flow statistics to grid resolution. The present findings show that truncation errors have an overwhelming impact on the predictive capabilities of second-order-accurate FV-based solvers, introducing a degree of uncertainty in model results that may be difficult to quantify across applications involving boundary-layer flows. Although mean flow and second-order statistics become acceptable provided sufficient grid resolution, the use of said solvers might prove problematic for studies requiring accurate higher-order statistics, velocity spectra and turbulence topology.

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