Abstract. Results are presented from the first intercomparison of Large-eddy simulation (LES) models for the stable boundary layer (SBL), as part of the GABLS (Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study) initiative. A moderately stable case is used, based on Arctic observations. All models produce successful simulations, inasmuch as they reflect many of the results from local scaling theory and observations. Simulations performed at 1 m and 2 m resolution show only small changes in the mean profiles compared to coarser resolutions. Also, sensitivity to sub-grid models for individual models highlights their importance in SBL simulation at moderate resolution (6.25 m). Stability functions are derived from the LES using typical mixing lengths used in Numerical Weather Prediction (NWP) and climate models. The functions have smaller values than those used in NWP. There is also support for the use of K-profile similarity in parametrizations. Thus, the results provide improved understanding and motivate future developments of the parametrization of the SBL.
In a previous paper, Germano, et al. (1991) proposed a method for computing coefficients of subgrid-scale eddy viscosity models as a function of space and time. This procedure has the distinct advantage of being self-calibrating and requires no a priori specification of model coefficients or the use of wall damping functions. However, the original formulation contained some mathematical inconsistencies that limited the utility of the model. In particular, the applicability of the model was restricted to flows that are statistically homogeneous in at least one direction. These inconsistencies and limitations are discussed and a new formulation that rectifies them is proposed. The new formulation leads to an integral equation whose solution yields the model coefficient as a function of position and time. The method can be applied to general inhomogeneous flows and does not suffer from the mathematical inconsistencies inherent in the previous formulation. The model has been tested in isotropic turbulence and in the flow over a backward-facing step.
Large-eddy simulation (LES) has been used to study the flow in a planar asymmetric
diffuser. The wide range of spatial and temporal scales, the presence of an adverse
pressure gradient, and the formation of an unsteady separation bubble in the rear
part of the diffuser make this flow a challenging test case for assessing the predictive
capability of LES. Simulation results for mean flow, pressure recovery and skin
friction are in excellent agreement with data from two recent experiments. The inflow
consists of a fully developed turbulent channel flow at a Reynolds number based
on shear velocity, Reτ=500. It is found that accurate representation of the in flow velocity field is critical for accurate prediction of the flow in the diffuser. Although the
simulation in the diffuser is well resolved, the subgrid-scale model plays a significant
role for both mean momentum and turbulent kinetic energy balances. Subgrid-scale
stresses contribute a maximum of 8% to the local value of the total shear stress with
the maximum values found in the inlet duct and along the flat wall where the flow
remains attached. The subgrid-scale model adapts to the enhanced turbulence levels
in the rear part of the diffuser by providing more than 80% of the dissipation rate
for turbulent kinetic energy. The unsteady separation excites large scales of motion
which extend over the major part of the duct cross-section and penetrate deeply
into the core of the flow. Instantaneous flow reversal is observed along both walls
immediately behind the diffuser throat which is far upstream of the location of main
separation. While the mean flow profile changes gradually as the flow enters the
expansion, turbulent stresses undergo rapid changes over a short streamwise distance
along the deflected wall. An explanation is offered which considers the strain field as
well as the influence of geometry changes. The effect of grid resolution and spanwise
domain size on the flow field prediction has been documented and this allows an
assessment of the computational requirements for carrying out such simulations.
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