Kosovic, Branko, National Center for Atmospheric
Research
Ever since large-eddy
simulation (LES) was developed (Lilly 1967, Deardorff 1970) it has been
extensively used to study canonical atmospheric boundary layers (ABLs). In most
cases these studies focused on flat plate boundary layers under the assumption
of horizontal homogeneity. Carefully designed LES of canonical atmospheric
boundary layers have contributed significantly to development of better
understanding of these flows and their parameterizations. LES were often
carried out using codes specifically designed for simulations of idealized,
horizontally homogeneous flows with periodic lateral boundary conditions.
Multiscale simulations
over heterogeneous boundary layers are of great importance for a wide range of
applications. Recent developments in numerical weather prediction (NWP) codes
enable their nearly seamless use across a wide range of atmospheric scales from
synoptic to turbulent scales in atmospheric boundary layers. However, achieving accurate
multiscale simulations requires extensive code validation under a range of
atmospheric conditions as well as addressing the question of turbulence
modeling in the range of scales between mesoscales and boundary layer scales
(Wyngaard, 2004). We therefore start by focusing on validation of nested LES of
ABLs using the Weather Research and Forecasting (WRF) model and present results
of several test cases including: complex terrain and heterogeneous surface
characteristics. We first carry out validation of WRF-LES for simulations of
flows over complex terrain using data from Askervein Hill (Taylor and
Teunissen, 1985, 1987). WRFÕs nesting capability is employed with a one-way
nested inner domain that includes complex terrain representation while the
coarser outer nest is used to spin up fully developed atmospheric boundary
layer turbulence and thus represent accurately inflow to the inner domain. Our
validation study employs two subgrid models included in WRF, each with or
without a prognostic equation for turbulent kinetic energy. Toward a goal of
developing capability for multiscale simulations for wind energy applications,
we have implemented a generalized actuator disk model in WRF. The generalized
actuator disk model implemented in WRF-LES is capable of representing the drag
and torque effects distributed over the turbine rotor disk. We demonstrated the
use of WRF-LES to simulate flow through an array of wind turbines under
neutrally stratified conditions. Our simulations show that WRF-LES with a
generalized actuator disk model is capable of capturing the characteristics of
wind turbine wakes and their interactions with consecutive rows of wind
turbines.