61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008;
San Antonio, Texas
Session N2: Invited Session: Designing Large-Eddy Simulation of High Reynolds Number Wall-bounded Flows
10:30 AM–11:05 AM,
Tuesday, November 25, 2008
Room: 103B
Chair: Robert Moser, University of Texas at Austin
Abstract ID: BAPS.2008.DFD.N2.1
Abstract: N2.00001 : Designing Large-Eddy Simulation of High Reynolds Number Wall-bounded Flows
10:30 AM–11:05 AM
Preview Abstract
Author:
James Brasseur
(Pennsylvania State University)
Large-eddy simulation (LES) of the high Reynolds number boundary
layer
systematically over-predicts the mean velocity gradient in the
shear-dominated inertial surface layer, destroying the
law-of-the-wall. This
``overshoot'' is especially apparent in LES of high Reynolds
number boundary
layers where the viscous layer either does not exist, such as in the
rough-surface atmospheric boundary layer (ABL), or is not
resolved by the
LES grid. In hybrid LES, where the grid integrates RANS in the
viscous layer
with large-eddy simulation above, the error appears as a
``mismatch'' in the
log profile for mean velocity. The overshoot in mean shear
produces a wide
range of negative effects, including a strongly enhanced stream-wise
coherence and an incorrect eddy structure. In the presence of
buoyancy-driven motions, as in the moderately unstable ABL, the
error
infects the entire thermal structure and affects vertical
transport of heat,
humidity and scalar. Furthermore, the overshoot precludes a
grid-independent
prediction for mean velocity. In this discussion I shall describe
the
fundamental source of the overshoot as arising from the combined
effects of
nonphysical friction in the subfilter-scale (SFS) stress model
and numerical
algorithm, grid aspect ratio, and grid resolution in the
vertical. From this
new understanding we develop a framework in which a large-eddy
simulation
should be formulated in order to both resolve the overshoot and
capture the
law-of-the-wall. The theory indicates that three critical
parameters must be
exceeded, defining a ``high-accuracy zone'' (HAZ) within the
framework. We
verify the theory with well over 100 large-eddy simulations of
the neutral
ABL and show that as the simulation moves into the HAZ, the
overshoot is
resolved, the law-of-the-wall is captured, a grid-independent
prediction is
attained, and the LES becomes more sensitive to the lower boundary
condition. I shall argue that the framework should be used in the
systematic
improvement of LES models. [This work was carried with Dr. Tie
Wei, and was
supported by ARO.]
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2008.DFD.N2.1