Effect of artificial length scales in large eddy simulation of a neutral atmospheric boundary layer flow: A simple solution to log-layer mismatch

Abstract

A large eddy simulation (LES) methodology coupled with near-wall modeling has been implemented in the current study for high Re neutral atmospheric boundary layer flows using an exponentially accurate spectral element method in an open-source research code Nek5000. The effect of artificial length scales due to subgrid scale (SGS) and near wall modeling (NWM) on the scaling laws and structure of the inner and outer layer eddies is studied using varying SGS and NWM parameters in the spectral element framework. The study provides an understanding of the various length scales and dynamics of the eddies affected by the LES model and also the fundamental physics behind the inner and outer layer eddies which are responsible for the correct behavior of the mean statistics in accordance with the definition of equilibrium layers by Townsend. An economical and accurate LES model based on capturing the near wall coherent eddies has been designed, which is successful in eliminating the artificial length scale effects like the log-layer mismatch or the secondary peak generation in the streamwise variance.

Exploring the benefits of vertically staggered wind farms: Understanding the power generation mechanisms of turbines operating at different scales

“…A, inner layer, z = 0.075 D L ; B, at hub height of large turbines z = D L ; C, outer layer, z = 2.25 D L . , are corresponding near‐wall and scaling of E u u and E w w spectra in the neutral ABL [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Close to the “wall” at z = 0.075 D L (Figure A), we do not observe any streamwise length scales at which the wall‐normal spectra dominates its streamwise counterpart. This indicates the presence of quasi‐inviscid 2D “inactive” eddy structures, as observed in our previous work . This phenomenon corroborates towards the evidence of vertical turbulent motions from the outer layer towards the “wall” in cases L , L S M , and L I M .…”

“…The details of the Smagorinsky based subgrid scale model and shear‐stress rough‐wall boundary conditions in spectral elements used in our wall‐modeled LES can be found in Chatterjee and Peet. ()…”

“…Each actuator line was discretized using N a = 30 uniformly sized blade elements (ie, per rotor radius) . In addition, a background SEM grid corresponding to neutral ABL was refined to ensure a similar grid resolution of approximately 8 × 8 spectral elements, or 60 × 60 collocation points, in the rotor swept area in the y z plane. An additional refinement was done in the wake of the wind turbines to capture the helical vortices shed by the turbines…”

Regularization modelling for large‐eddy simulation in wall‐bounded turbulence: An explicit filtering‐based approach

“…9,10 The application of weak formulation using spectral element methods (SEMs) to LES is relatively less explored, although several works in this domain published since the last decade must be noted. [11][12][13][14][15][16][17][18] A traditional approach to LES is to generate the differential equations governing the spatio-temporal evolution of large-scale structures, which can be obtained from the Navier-Stokes equations by applying a low-pass filter. The filtering of nonlinear convective product term does not commute with the convection of filtered quantities, which gives rise to an additional term in filtered Navier-Stokes equations called subgrid-scale (SGS) or subfilter-scale term requiring closure.…”