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A numerical study of mesoscale motion in the atmospheric mixed layerRuscher, Paul Harold 20 March 1981 (has links)
The numerical modeling of motions in the atmosphere's
planetary boundary layer (PBL) is a challenging task. In
general, the boundary layer interacts with both the overlying
atmosphere and the underlying land or water surface
in a complex manner. Random turbulence is also present in
the PBL which precludes exact prediction by numerical
models. Nonetheless, expensive three-dimensional numerical
models have been developed which, with several parameterizations
and assumptions, can give a good idea of the PBL
structure in many situations. However, on certain occasions,
there is strong mixing evident in the PBL which may
enable one to describe the structure of the boundary layer
in a much-simplified theoretical model. By eliminating
the vertical dimension from consideration, this two-dimensional
mixed-layer model can be applied to mesoscale phenomena
(horizontal length scale < 100 km) at greatly-reduced
costs.
The equations for motion and mixed-layer height are
derived for such a situation and methods appropriate to the
numerical modeling of the atmospheric mixed layer are discussed.
Using an energy-conserving finite-difference
analog of the model equations, the model is integrated in
time to simulate the motions which were associated with the
atmospheric vortex street observed near Cheju-do, South
Korea on 17 February 1975. Experiments were carried out
which investigated the effects of lateral diffusion, horizontal
resolution, and mixed-layer depth.
It is concluded that, given proper representation of
prognostic variables on a staggered finite-difference grid,
only small, realistic values of eddy diffusivity need be
utilized. It also appears evident from the numerical experiments
and atmospheric observations that the vortex
street will form only when the obstacle which triggers its
formation protrudes above the mixed layer. Although the
wind fields in the simulations sometimes lack clear, fully
rotational cells well downstream of the island, the characteristic
sinusoidal pattern observed in laboratory experiments
and cloud photographs is explicitly resolved by
the model. The simulated vortex street also compares favorably
with the observed in that the dimensionless governing
parameters of the simulated vortex street (the Reynolds
number, Strouhal number, Lin's parameter, the spacing
ratio, and the speed ratio) closely match the observed
values. / Graduation date: 1981
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Penetrative elements at the top of the atmospheric mixed layerPaumier, James 05 March 1981 (has links)
High resolution data of moisture, temperature and wind velocity
collected by aircraft during the 1975 Air Mass Transformation
Experiment (ANTEX '75) provide information for detailed investigations
of the phenomena occurring at the top of a cloud-topped mixed
layer.
Joint frequency diagrams of humidity and temperature reveal that
for parts of the record the coldest temperatures occur in air near
saturation while drier air or air with substantial liquid water is
warmer. This suggests the possible occurrence of cloud-top entrainment
instability (Deardorff, 1980).
Using humidity as an indicator, the flight record is systematically
searched for penetrative mixed layer elements and pockets of
dry air penetrating into cloudy air (wisps). The separate phenomena
are then composited to produce an "average" wisp and penetrating
element. The composites show evidence of net cooling due to evaporation
downstream from the penetrating element and upstream from the
wisps. Sinking motion is associated with the cold temperatures. These
results suggest the interaction of shear, penetrating elements and
wisps, and the existence of cloud-top entrainment instability. / Graduation date: 1981
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Photographic analysis of buoyant stack plumes in a laboratory model of the turbulent mixed layerHukari, Neil F. 30 August 1984 (has links)
Four buoyant plumes were produced within a laboratory convectively
mixed layer from a source height of about z[subscript s] = 0.15 h,
where h is the height of the convectively mixed layer. The
projected images of these plumes in the X-Z plane were analyzed
using a densitometer (photomultiplier tube) to calculate dimensionless
crosswind integrated concentration values. These values were
examined at regular intervals of non-dimensionalized heights and
downwind distances to calculate center-of-mass heights, approximate
plume limits, and touchdown distances. The plume buoyancy values
were expressed in dimensionless terms as the parameter F[subscript *].
The touchdown distances are greatest and the surface integrated
crosswind concentrations are smallest for the three most
buoyant plumes. The highest center-of-mass and plume limit
positions are also associated with the most buoyant plumes. The
surface crosswind integrated concentration values for even the
least buoyant plumes are much smaller than for non-buoyant plumes
from previous studies. Touchdown distances for buoyant plumes from
this data set agree with data from Willis and Deardorff (1983);
however, the centerline and lower plume limits are at greater
heights for this study.
Vertical profiles of crosswind integrated concentration values
indicate that the least buoyant plume has a bimodal distribution
near the stack then becomes uniform at greater distances. The
vertical profiles for the three most buoyant plumes show the highest
concentration values are present in the upper part of the mixed
layer at most downwind distances examined in this study. This
distribution of effluent is also indicated by the vertical center-of-
mass heights being larger than the plume centerline calculated
from the average of the lower and upper plume limits. / Graduation date: 1985
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Boundary-Layer Receptivity to Three-Dimensional Roughness Arrays on a Swept-WingHunt, Lauren Elizabeth 2011 December 1900 (has links)
On-going efforts to reduce aircraft drag through transition delay focus on understanding the process of boundary-layer transition from a physics-based perspective. For swept-wings subject to transition dominated by a stationary crossflow instability, one of the remaining challenges is understanding how freestream disturbances and surface features such as surface roughness create the initial amplitudes for unstable waves. These waves grow, modify the mean flow and create conditions for secondary instabilities to occur, which in turn ultimately lead to transition. Computational methods that model the primary and secondary instability growth can accurately model disturbance evolution as long as appropriate initial conditions are supplied. Additionally, transition delay using discrete roughness arrays that exploit known sensitivities to surface roughness has been demonstrated in flight and wind tunnel testing; however, inconsistencies in performance from the two test platforms indicate further testing is required. This study uses detailed hotwire boundary-layer velocity scans to quantify the relationship between roughness height and initial disturbance amplitude. Naphthalene flow visualization provides insight into how transition changes as a result of roughness height and spacing.
Micron-sized, circular roughness elements were applied near the leading edge of the ASU(67)-0315 model installed at an angle of attack of -2.9 degrees in the Klebanoff-Saric Wind Tunnel. Extensive flow quality measurements show turbulence intensities less than 0.02% over the speed range of interest. A survey of multiple roughness heights for the most unstable and control wavelengths and Reynolds numbers of 2.4 x 10⁶ 2.8 x 10⁶ and 3.2 x 10⁶ was completed for chord locations of 10%, 15% and 20%. When care was taken to measure in the region of linear stability, it was found that the disturbance amplitude varies almost linearly with roughness height. Naphthalene flow visualization indicates that moderate changes in already-low freestream turbulence levels can have a significant impact on transition behavior.
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Roughness-induced Transient Growth: Continuous-spectrum Receptivity and Secondary Instability AnalysisDenissen, Nicholas Allen 2011 May 1900 (has links)
This dissertation analyzes the effect of periodic roughness elements on the stability
of a flat plate boundary layer. Receptivity data is extracted from direct numerical
simulations and experimental data and the results are compared to theoretical predictions.
This analysis shows that flow in the immediate vicinity of roughness elements is
non-linear; however, the evolution of roughness-induced perturbations is a linear phenomena.
New techniques are developed to calculate receptivity information for cases
where direct numerical simulations are not yet possible. Additionally, the stability
behavior of the roughness wake is analyzed. New instability modes are found, and
the effect of boundary layer complexity, perturbation amplitude and other factors are
examined. It is shown that the wake is much less stable than optimal perturbation
theory predicts, and highlights the importance of receptivity studies. The implication
of these results on transition-to-turbulence is discussed, and future work is proposed.
T
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Boundary layer, grid turbulence, and periodic wake effects on turbulent juncture flows /Sabatino, Daniel R., January 2000 (has links)
Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 179-183).
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Unsteady boundary layer separation /Zalutsky, Konstantin E., January 2000 (has links)
Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 186-192).
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Validation of the coupled NCEP mesoscale spectral model and an advanced land surface model over the Hawaiian IslandsZhang, Yongxin. January 2004 (has links)
Thesis (Ph. D.)--University of Hawaii at Manoa, 2004. / Includes bibliographical references (leaves 193-207).
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Variability of refractivity in the suface layer /Mabey, Deborah L. January 2002 (has links) (PDF)
Thesis (M.S.)--Naval Postgraduate School, 2002. / Thesis advisor(s): Kenneth L. Davidson, Peter Guest. Includes bibliographical references (p. 45). Also available online.
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Coastal stratocumulus topped boundary layers and the role of cloud-top entrainment /Eleuterio, Daniel P. January 2004 (has links) (PDF)
Thesis (Doctor of Philosophy in Meteorology)--Naval Postgraduate School, June 2004. / Thesis advisor(s): Qing Wang. Includes bibliographical references (p. 113-119). Also available online.
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