Laboratory and field data suggest that wave breaking plays a
significant role in supporting turbulent fluxes in the
surface layers of the oceans. The full DNS of these flows is
beyond present computer resources and we seek models that
represent only the essential kinematics and dynamics of
breaking. Here we report on the DNS of a breaker model and
its use as the basic element in a stochastic representation
of breaking in surface boundary layers. Breaking is
simulated by a Froude-scaled body-force in a self-similar
volume of the fluid that evolves in space and time. The DNS
shows good agreement with laboratory data for the
ensemble-averaged kinetic energy and vorticity due to a
single breaker. A comparison of different boundary layers
driven by constant current, constant surface stress, or a
mixture of surface stress plus breaking, elucidates the
importance of breaking. Analysis of the mean velocity
profiles shows that breaking may increase the surface
roughness, z_o, by more than a factor of 30; here
z_o/\lambda \approx 0.04-0.06, where \lambda is the
breaking wavelength. Breaking alters the usual balance of
production \approx dissipation in the TKE budget, with
turbulent and pressure transports and breaker work being
important sources and sinks of TKE.
[FR.002] Optimized Sensor Placement for Urban Flow Measurements
Paritosh Mokhasi, Dietmar Rempfer (Illinois Institute of Technology)
We are interested in modeling the transport of contaminants in the atmospheric boundary layer, in complex geometries as in urban flow problems. One of our goals is to predict contaminant dispersion based on flow and concentration measurements using a minimum number of sensors. Since atmospheric flows are almost always turbulent, a direct approach would require a prohibitive number of sensors to resolve the important scales.
The Proper Orthogonal Decomposition (POD) method provides an
alternative method to describe a turbulent flow using a
minimum amount of information. It allows us to decompose the
flow field into temporal coefficients and spatial functions.
If the temporal coefficients are obtained at a given instant
of time, then, in principle, it is possible to reconstruct
the entire three-dimensional flow field exactly. A method
has been developed that makes it possible to approximately
compute these temporal coefficients from instantaneous
velocity data taken at only a few locations simultaneously.
Using the information of the spatial functions at the
specified locations, a set of simultaneous equations are
developed, which can be solved for the temporal
coefficients. A quantitative comparison of the computed
coefficients and the original coefficients shows that this
method provides an accurate approximation of the temporal
coefficients. Based on this method it will be possible to
develop a practical approach of estimating the
three-dimensional turbulent flow field using velocity
information from a small number of sensors.
[FR.003] A Laboratory Study of Slope Flows Dynamics
ANDREA CAPRIATI, ANTONIO CENEDESE, PAOLO MONTI (DITS. Universita' degli Studi "La Sapienza". Roma. Italia)
Slope flows currents can contribute significantly in the
diurnal circulation and air quality of complex terrain
regions (mountains, valleys, etc.). During the daytime,
solar heating warms the valley sides, causing up-slope (or
anabatic) winds. In contrast, radiative cooling of the
valley sides results in cold down-slope (drainage or
katabatic) flows, characterized by small vertical extensions
(usually 10-200 m) and with the typical features of dense
gravity currents. In this paper, some preliminary results on
slope flows obtained by means of a series of experiments
conducted in the laboratory using a temperature controlled
water tank are shown. Rakes of thermocouples are used to
determine the temperature structure and particle tracking
velocimetry is used for the velocity measurements. A simple
slope consisting of a plate in which the temperature is
forced via a set of Peltier Cells is used. The analysis is
performed considering different slope angles, background
thermal stratifications and surface heat fluxes as well.
Comparisons with theoretical and empirical laws found in
literature are reported.
[FR.004] On the Uniqueness of Lagrangian Stochastic Model in Self-Similar Boundary Layer
Byung-Gu Kim, Changhoon Lee (Yonsei University, Seoul, Korea)
Thomson's well-mixed condition is a well-known criterion to
construct Lagrangian stochastic dispersion models. But it
cannot prescribe a unique model in larger than
one-dimensional problem. Another quantity called spin which
represents rotational property of particle trajectory was
introduced to partially alleviate this nonuniqueness
problem. In order to resolve this we take a totally
different path by adopting a Lagrangian turbulence model
known as the Generalized Langevin Model. Under the
assumption that the Reynolds stresses are homogeneous in
neutral turbulent boundary layer, the model constants are
expressed in terms of the Kolmogorov constant C_0 and
another constant \gamma _5 which controls the spin. Data
from directly simulated channel flow is used to determine
\gamma _5. More importantly, we found that all models
including Thomson's, Borgas', Reynolds', Kurbanmuradov and
Sabelfield's and our models are equivalent to each other
provided that the spin parameter is adjusted properly in
self-similar boundary layer. Some of original versions of
their models have wrong sign of the spin.
[FR.005] Spectral Features of Atmospheric Surface Layer Flow Over an Aligned Array of Obstacles
Matthew Nelson, Eric Pardyjak, Joseph Klewicki (University of Utah)
Three component velocity fluctuation measurements were
acquired with an array of sonic anemometers in and above the
MUST (Mock Urban Setting Test) array. The MUST array
consisted of a 10 by 12 aligned array of rectangular
obstacles each with the dimensions of 12.2 m x 2.42 m x 2.54
m in Utah’s West Desert. To obtain statistically robust
results the data were sorted based on their consistency of
wind direction and measures of statistical convergence. To
reduce the sensitivity to the specific geometry of the
obstacle array, results will focus on oblique wind angles.
Two primary large scale influences were explored. The vortex
shedding off of the regular array of obstacles imposes
length and time scales that are shown to cause a peak in the
turbulent energy spectra. Under thermally stable conditions
the Brunt-Vaisala frequency is evident as an additional peak
in the spectra. Discussion will focus on the relative
location of these spectral peaks as well as possible
interactions between them.
[FR.006] Structural Stability of Turbulent Jets
Brian Farrell (Harvard University), Petros Ioannou (University of Athens)
Turbulence in fluids is commonly observed to coexist with
relatively large spatial and temporal scale coherent jets.
These jets may be steady, vacillate with a definite period,
or be irregular. A comprehensive theory for this phenomenon
is presented based on the mutual interaction between the
coherent jet and the turbulent eddies. When a sufficient
number of statistically independent realizations of the eddy
field participate in organizing the jet a simplified
asymptotic dynamics emerges with progression, as an order
parameter such as the eddy forcing is increased, from a
stable fixed point associated with a steady symmetric zonal
jet through a pitchfork bifurcation to a stable asymmetric
jet followed by a Hopf bifurcation to a stable limit cycle
associated with a regularly vacillating jet and finally a
transition to chaos. This underlying asymptotic dynamics
emerges when a sufficient number of ensemble members is
retained in the stochastic forcing of the jet but
qualitative different mean jet dynamics is found when a
small number of ensemble members is retained as is
appropriate for many physical systems. Example applications
of this theory are presented including a model of
midlatitude jet vacillation, emergence and maintenance of
multiple jets in turbulent flow, a model of rapid
reorganization of storm tracks as a threshold in radiative
forcing is passed, and a model of the quasi-biennial
oscillation. Because the statistically coupled wave/mean
flow system discussed is generally globally stable this
system also forms the basis for a comprehensive theory for
equilibration of unstable jets in turbulent shear flow.
[FR.007] The generation of a zonal-wind oscillation by nonlinear interactions of internal gravity waves
Lucy Campbell (Carleton University, Ottawa, Canada)
Nonlinear interactions of internal gravity waves give rise
to numerous large-scale phenomena that are observed in the
atmosphere, for example the quasi-biennial oscillation
(QBO). This is an oscillation in zonal wind direction which
is observed in the equatorial stratosphere; it is
characterized by alternating regimes of easterly and
westerly shear that descend with time. In the past few
decades, a number of theories have been developed to explain
the mechanism by which the QBO is generated. These theories
are all based on ``quasi-linear'' representations of
wave-mean-flow interactions. In this presentation, a fully
nonlinear numerical simulation of the QBO is described. A
spectrum of gravity waves over a range of phase speeds is
forced at the lower boundary of the computational domain and
propagates upwards in a density-stratified shear flow. As a
result of the absorption and reflection of the waves at
their critical levels, regions of large shear develop in the
background flow and propagate downwards with time.
[FR.008] Contrail evolution in the atmosphere
Roberto Paoli (Center for Turbulence Research, Stanford University), Karim Shariff, Nagi Mansour (NASA Ames Research Center)
The persistence of condensation trails, "contrails", in the
far-field of an aircraft wake is numerically analyzed.
Contrails are ice clouds formed by condensation of exhaust
water vapor in a cold atmosphere. Their evolution and
persistence are controlled by the wake dynamics, atmospheric
turbulence, as well as by background water vapor content.
Under suitable atmospheric conditions, they may trigger the
formation of cirrus clouds, thus having a potential climate
impact on regional/global scales. The object of the present
study is to reproduce the contrail evolution numerically in
intermediate scales (between aircraft- and atmosphere-
scales) where exhaust mixing and clouds microphysics are
expected to control their dynamics and, eventually, their
transition to cirrus clouds. The simulations are carried out
using a two-phase approach to deal with both gas and ice
phases. Large eddy simulations are used for the gas phase
while a Lagrangian particles tracking approach is used for
ice crystals. Mass transfer between the two phases is used
to account for vapor condensation, by employing available
ice microphysics models. Besides the physical description of
the phenomenon, the results may be useful for "calibrating"
source terms, representative of the environmental impact of
aircraft-generated emissions, in global climate codes.
[FR.009] Decay of turbulence during evening transition
Eric Pardyjak (University of Utah), Harindra Fernando (Arizona State University)
The non-stationary turbulent decay processes that occur
during the evening transition of the convective boundary
layer play an important role in atmospheric dispersion. As
part of the Phoenix Airflow Experiment (PAFEX-I), the basic
forcing mechanisms, influence of increasing atmospheric
stability and turbulent dynamics were studied. Three
component velocity measurements were made in the atmospheric
surface layer using a sonic anemometer in Glendale, Arizaon
along with a host of other standard atmospheric
measurements. The decay of various turbulent quantities such
as turbulent kinetic energy, surface fluxes, variances,
length scales, and time scales were investigated. Based on
these measurements, a simple time dependant turbulent
kinetic energy model was developed that uses a simple linear
function to represent the surface sensible heat flux. The
results show good agreement with the turbulent kinetic
energy decay field measurements and indicate that simple
similarity models that neglect sensible heat flux can be
easily improved.
[FR.010] Numerical Simulations of Stratified Turbulence Dominated by Vortical Motion
Michael Waite, Peter Bartello (McGill University)
Observations in the atmosphere and ocean frequently yield vertical wavenumber energy spectra of the form E(k_z) \propto k_z^-3, which are commonly attributed to saturating internal gravity waves. However, stably stratified fluids also support quasi-horizontal vortical motion with potential vorticity (PV). Does PV-dominated turbulence generate a k_z spectrum with a slope of -3? We will present a series of simulations of vortically forced turbulence at different stratifications which suggest that the answer to this question is no. In each of our simulations, the k_z spectrum of vortical energy is found to be flat out to the buoyancy wavenumber. We argue that such spectra are consistent with the limiting equations describing the asymptotic regime of strongly-stratified, PV-dominated turbulence. Finally, we contrast these findings with simulations of wave-dominated turbulence, for which k_z spectral slopes near -3 are more readily obtained.