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91.
A.M. Reynolds 《Boundary-Layer Meteorology》1998,89(1):161-170
Recently Du ( Boundary-Layer Meteorology 83, 207–219, 1997) estimated the value of the Lagrangian velocity structure constant, C0, in the inertial subrange by comparing experimental diffusion data and simulation results obtained with the one-dimensional form of Thomson's model ( J. Fluid Mech. 180, 529–556, 1987). Du reported that for several different flows (grid turbulence, a wind-tunnel boundary layer and the atmospheric surface layer under neutral stratification) the value of C0 is 3.0±0.5. Here, it is shown that optimal model agreement with experimental diffusion data for the wind-tunnel boundary layer is, in fact, obtained when C0=5.0 ± 0.5. It is also shown that accounting for the skewness of velocity statistics and finite Reynolds number effects does not significantly change this estimate for the value of C0. It is suggested that one-dimensional Lagrangian stochastic models are inconsistent with the supposed universality of C0. 相似文献
92.
Turbulent Statistics of Neutrally Stratified Flow Within and Above a Sparse Forest from Large-Eddy Simulation and Field Observations 总被引:17,自引:13,他引:4
Hong-Bing Su Roger H. Shaw Kyaw Tha Paw Chin-Hoh Moeng Peter P. Sullivan 《Boundary-Layer Meteorology》1998,88(3):363-397
Turbulent statistics of neutrally stratified shear-driven flow within and above a sparse forest canopy are presented from a large-eddy simulation (LES) and compared with those from observations within and above a deciduous forest with similar height and foliage density. First- and second-order moments from the LES agree with observations quite well. Third-order moments from the LES have the same sign and similar vertical patterns as those from the observations, but the LES yields smaller magnitudes of such higher-order moments. Turbulent spectra and cospectra from the LES agree well with observations above the forest. However, at the highest frequencies, the LES spectra have steeper slopes than observations. Quadrant and conditional analyses of the LES resolved-scale flow fields also agree with observations. For example, both LES and observation find that sweeps are more important than ejections for the transport of momentum within the forest, while inward and outward interaction contributions are both small, except near the forest floor. The intermittency of the transport of momentum and scalar increases with depth into the forest. Finally, ramp structures in the time series of a passive scalar at multiple levels within and above the forest show similar features to those measured from field towers. Two-dimensional (height-time cross-section) contours of the passive scalar and wind vectors show sweeps and ejections, and the characteristics of the static pressure perturbation near the ground resemble those deduced from field tower-based measurements. In spite of the limited grid resolution (2 m × 2 m × 2 m) and domain size (192 m × 192 m × 60 m) used in this LES, we demonstrate that the LES is capable of resolving the most important characteristics of the turbulent flow within and above a forest canopy. 相似文献
93.
94.
Armin Freundt 《Bulletin of Volcanology》1998,59(6):414-435
High-grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature
or even solidus temperature. Corresponding pyroclastic-flow particles range from plastic to partially liquid and are able
to aggregate or coalesce. This contrasts with particles in pyroclastic flows producing unwelded ignimbrite, which are capable
of elastic grain interactions. The low aspect ratio and great areal extent of high-grade ignimbrites requires transport in
a particulate state either by (a) high-concentration mass flow facilitated by fluidizing gas reducing internal friction, or
by (b) expanded turbulent flow of low but downward increasing concentration. This paper presents experiments designed to investigate
the effects of plastic to liquid particles on these two contrasting transport mechanisms. Gas fluidization experiments using
polyethyleneglycole (PEG) powders heated above minimum sintering (Tms) and melting (Tm) temperatures cover a wide range of fluidization velocities (Umf>Ua>0.6·Ut) but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash, where particle volume concentration
outside the bubbles is high (≈10–1). When the powders reach a critical temperature Tm≥T≥Tms, defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized
beds as well as in experiments using mixtures of high- and low-Tm (≥30 wt.%) PEG powders, when T≥Tms of the lower-Tm powder. This indicates that extended particulate transport at T≥Tms is not possible at such high particle concentrations. In the turbulent flow experiments, liquid sprays of molten PEG or water,
vertically injected into a high-Re (>104) horizontal air flow, form a low-concentration (10–5 to 10–4) turbulent suspension current. Proximal formation of partially coalesced aggregates, which settle faster than individual
particles, causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid-particle
sedimentation from turbulent suspensions. As particles become finer downstream and coalescence efficiency decreases in response
to cooling, more distally formed aggregates become too small and rare to modify sedimentation-rate decay from that of suspension
flows containing solid particles. The key difference between the two transport systems is particle concentration, C. Since
particle collision rate Rcoll∝C2, collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency (1≥Ecoal≥0) is larger than 10-3. Low-concentration suspensions, on the other hand, require much higher values of Ecoal for significant aggregation to occur. Dilute pyroclastic flows will have higher particle volume fractions (≈10–3) than the experimental currents, but then viscous pyroclasts should have lower coalescence efficiencies than PEG droplets.
Experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high-grade
ignimbrite sheets.
Received: 28 August 1996 / Accepted: 3 December 1997 相似文献
95.
HUANG Suiliang Associate Professor Dr. Water Environment Simulation Laboratory State Key Joint Laboratory of Environmental Simulation Pollution Control Beijing Normal University Beijing WAN Zhaohui Professor Dr. Department of Sedime 《国际泥沙研究》1997,(3)
I.INTRODUCTIONThefateofmanytraceheavymetalsinnaturalwatersis,toalargeextent,controlledbysorptionprocessesandthedynamicsoftheparticlesthemselves.Thepotentialinfluencesofenvironmentalvariablessuchastemperature,CaZ+,Na+andCI--,dissolvedorganiccarbonandPHvaluesontraceheavymetalpartitioninghavebeenwidelyinvestigated.Forexample,StumnandMorgan(1981)suggestthattheextentofsorptionvariespositivelywithtemperature.Itfollowsthatdecreasedsorptionmightbeexpectedatlowtemperatures,e.g.duringwintermont… 相似文献
96.
97.
Turbulent kinetic energy budgets from a large-eddy simulation of airflow above and within a forest canopy 总被引:3,自引:2,他引:3
The output of a large-eddy simulation was used to study the terms ofthe turbulent kinetic energy (TKE) budget for the air layers above andwithin a forest. The computation created a three-dimensional,time-dependent simulation of the airflow, in which the lowest third ofthe domain was occupied by drag elements and heat sources to representthe forest. Shear production was a principal source of TKE in theupper canopy, diminishing gradually above tree-top height and moresharply with depth in the canopy. The transfer of energy to subgridscales (dissipation) was the main sink in the upper part of the domainbut diminished rapidly with depth in the canopy. Removal ofresolved-scale TKE due to canopy drag was extremely important,occurring primarily in the upper half of the forest where the foliagedensity was large. Turbulent transport showed a loss at the canopytop and a gain within the canopy. These general features have beenfound elsewhere but uncertainty remains concerning the effects ofpressure transport. In the present work, pressure was calculateddirectly, allowing us to compute the pressure diffusion term. Wellabove the canopy, pressure transport was smaller than, and opposite insign to, the turbulent transport term. Near the canopy top andbelow, pressure transport acted in concert with turbulent transport toexport TKE from the region immediately above and within the uppercrown, and to provide turbulent energy for the lower parts of theforest. In combination, the transport terms accounted for over half ofthe TKE loss near the canopy top, and in the lowest two-thirds of thecanopy the transport terms were the dominant source terms in thebudget. Moreover, the pressure transport was the largest source ofturbulent kinetic energy in the lowest levels of the canopy, beingparticularly strong under convective conditions. These resultsindicate that pressure transport is important in the plant canopyturbulent kinetic energy budget, especially in the lowest portion ofthe stand, where it acts as the major driving force for turbulentmotions. 相似文献
98.
F. D. EATON J. R. HINES W. H. HATCH R. M. CIONCO J. BYERS D. GARVEY DAVID R. MILLER 《Boundary-Layer Meteorology》1997,83(2):331-346
Boundary-layer meteorologicalmeasurements were made before, during, and after theMay 10, 1994 partial (94%) solar eclipse over thedesert at the Atmospheric Profiler Research Facilityat White Sands Missile Range, New Mexico, U.S.A. A largenumber of sensors were located in the middle of theTularosa Basin to measure mean and turbulentquantities and the refractive index structureparameter (Cn2). This event permitted a rareopportunity to measure, examine, and document theeffects upon the atmospheric boundary layer of asudden cut-off and subsequent turn-on of the sun'sradiant energy. At the peak of the eclipse, whichoccurred for more than three hours, all of the heatexchange parameters were affected, the turbulentprocesses were diminished, and the refractive indexstructure parameter decreased dramatically. A time-heightdisplay from the FM-CW radar shows a Kelvin–Helmholtzwave that developed during theeclipse. The results of several analyses arepresented to document and characterize the eclipse-modifiedboundary layer. 相似文献
99.
LIDAR DETERMINATION OF THE ENTRAINMENT ZONE THICKNESS AT THE TOP OF THE UNSTABLE MARINE ATMOSPHERIC BOUNDARY LAYER 总被引:10,自引:0,他引:10
CYRILLE FLAMANT JACQUES PELON PIERRE H. FLAMANT PIERRE DURAND 《Boundary-Layer Meteorology》1997,83(2):247-284
The thickness of the entrainment zone at the top of the marine atmosphericboundary layer (MABL) has been documented by an airborne lidar on twoconsecutive days during a cold-air outbreak episode over the Mediterranean.In addition to the lidar observations, in situ turbulent flux measurementsat three levels in the MABL were made by a second aircraft. The flights' tracksare broken down in segments 25–30 km long and the data are filtered for theparametrization of turbulent entrainment in the MABL at scales smaller thana few kilometres. The structural parameters of the entrainment zone aredetermined by lidar from the distributions of the instantaneous MABL topheight. The average values Ph0 and Ph2 of the cumulativeprobability distributions are used to define the bottom and top heights of the entrainment zone h0 and h2, respectively. The parameters h0 andh2 are calculated by reference to a linear vertical buoyancy flux profilein the framework of a first-order jump model. The model is constrained by bothlidar and in situ data to determine Ph0 and Ph2 and so h0and h2. In unstable conditions theaverage fraction Ph0 is estimated to be 6.0 ± 1%. It is shown to beslightly sensitive to the presence of cloud at small cloud fractions.The mean value of the ratio of the inversion level buoyancy flux to the surfacebuoyancy flux ARv is found to range from 0.15 to 0.30 depending on the shearin the MABL. The average value is 0.22 ± 0.05. Our resultsare in good agreement with previous analysis at comparable spatial scales.In purely convective conditions, the value of ARv given by theparametrizations fitted to our results is about 0.10–0.12, a value smallerthan the commonly accepted value of 0.2. When compared to previousparametrization results, our proportionality constant for the mechanicalproduction of turbulent kinetic energy is also found to be scaled down, ingood agreement with large-eddy simulation results. It is suggestedthat mesoscale organized motions in the MABL is the source of thisdifference. 相似文献
100.