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1.
The structure of supercritical western boundary currents is investigated using a quasi-geostrophic numerical model. The basic flow is of meridional Munk balance, and the input boundary is perturbed by the most unstable wave solution obtained from linear spatial instability calculations. Self-preserving (or equilibrium) solutions are obtained for the model runs at Re=30, 60, 90, and 120, and their energy and vorticity budgets are analyzed. In an analogy with the laboratory turbulence of wall boundary layers, the western boundary layer is divided into inner and outer layers. In the inner layer, the mean energy is dissipated via direct viscous dissipation, while in the outer layer it is converted to the eddy energy via turbulence production. The main scenario is that the mean energy is produced in the inner layer via ageostrophic pressure work divergence, and it is partly removed due to viscous action within a narrow region near the wall, defined here as viscous sub-layer. The remaining portion is converted to the eddy energy via turbulence production in the outer layer, which is in turn transported to the inner layer, then again to the viscous sub-layer where it is ultimately dissipated. In the near-wall side, the vorticity balance of the mean flow is maintained by viscous effect and Reynolds flux divergence, while in the offshore side it is maintained by beta effect and Reynolds flux divergence. The length scale of the supercritical boundary current is roughly
, where LM is the Munk length, as observed from a dimensional analysis. 相似文献
2.
A smoke visualization experiment has beenperformed in the first 3,m ofneutral and unstable atmospheric boundary layersat very large Reynolds number(Re > 106). Under neutral atmosphericconditions mean wind profiles agreewell with those in the canonical flatplate zero-pressure-gradient turbulentboundary layer. The experiment was designedto minimize the temperaturedifference between the passive marker (smoke)and the air to ensure that anyobserved structures were due to vortical, ratherthan buoyant, motions. Imagesacquired in the streamwise–wall-normal planeusing a planar laser light-sheetare strikingly similar to those observed inlaboratory experiments at low to moderate Reynolds numbers. They reveal large-scaleramp-like structures withdownstream inclination of 3°–35°.This inclination isinterpreted as the hairpin packet growthangle following the hairpin vortexpacket model ofAdrian, Meinhart, and Tomkins.The distribution of this characteristicangle agrees with the results of experiments at far lower Reynolds numbers,suggesting a similarity in structures among low, moderate, and high Reynoldsnumber boundary layers at vastly different scales. These results indicate thatthe hairpin vortex packet model extends over a large range of scales. Theeffect of vertical heat transport in an unstable atmosphere on wall structuresis investigated in terms of the hairpin vortex packet model. 相似文献
3.
During spring and autumn, many lakes in temperate latitudes experience intensive convective mixing in the vertical, which leads to almost isothermal conditions with depth. Thus the regime of turbulence appears to be similar with that characteristic of convective boundary layers in the atmosphere. In the present paper a simple analytical approach, based on boundary-layer theory, is applied to convective conditions in lakes. The aims of the paper are firstly to analyze in detail the temperature distribution during these periods, and secondly to investigate the current system, created by the horizontal temperature gradient and wind action. For these purposes, simple analytical solutions for the current velocities are derived under the assumption of depth-constant temperatures. The density-induced current velocities are shown to be small, in the order of a few mm/sec. The analytical model of wind-driven currents is compared with field data. The solution is in good qualitative agreement with observed current velocities under the condition that the wind field is steady for a relatively long time and that residual effects from former wind events are negligible.The effect of the current system on an approximately depth-constant temperature distribution is then checked by using the obtained current velocity fields in the heat transfer equation and deriving an analytical solution for the corrected temperature field. These temperature corrections are shown to be small, which indicates that it is reasonable to describe the temperature distribution with vertical isotherms.Notation
T
temperature
-
t
time
-
x, y, z
cartesian coordinates
-
molecular viscosity
-
h
,
v
horizontal and vertical turbulent viscosity
-
K
h
,K
v
horizontal and vertical turbulent conductivity
-
Q
heat flux through the water surface
-
D
depth
-
u, v, w
average current velocity components inx, y andz directions
-
f
Coriolis parameter
-
p
pressure
-
density
-
g
gravity acceleration
-
a
constant in the freshwater state equation
-
h
s
deviation from the average water surface elevation
-
L
*,H
*
length and depth scale
-
U
*,W
*
horizontal and vertical velocity scale
- T
temperature difference scale
-
bottom slope
-
u
*
friction velocity at the water surface
-
von Karman constant
-
L
Monin-Obukhov length scale
-
buoyancy parameter
-
l
turbulence length scale
-
C
1,C
2,C
3
dimensionless constants in the expressions for the vertical turbulent viscosity
- ,
dimensionless vertical coordinate and dimensionless local depth
-
angle between surface stress direction andx-axis
-
T
bx
,T
by
bottom stress components
-
c
bottom drag coefficient 相似文献
4.
A Random Displacement Model (RDM) and a Langevin Equation Model (LEM) are used to simulate point releases in a complex flow around a building. The flow field is generated by a three-dimensional finite element model that uses the standardk- model to parameterize the turbulence. The RDM- and LEM-calculated concentration fields are compared, with particular emphasis on the structure in regions with high turbulence and/or recirculation. RDM and LEM results are similar qualitatively, but RDM tends to predict lower concentration levels. In part this is due to the higher early-time diffusion. However, the expected convergence at later times is prevented by the interaction of the diffusion with the strongly inhomogeneous mean flow.Notation
a
i
coefficient in the Langevin equation
-
b
ij
coefficient in the Langevin equation
-
C
0
the universal constant associated with the Lagrangian structure function
-
H
building height (22.5 m)
-
K
eddy viscosity
-
K
k
eddy viscosity used in the definition of the off-diagonal Reynolds stresses
-
k
turbulent kinetic energy
- LEM
Langevin Equation Model
-
p
1
local unit vector in thexy-plane, orthogonal tos
-
p
2
local unit vector, orthogonal to boths andp
1
- RDM
Random Displacement Model
-
s
local unit vector in the streamline direction
-
T
local decorrelation time (Lagrangian time scale)
-
U
magnitude of the local Eulerian mean wind velocity
-
u
s
total velocity in the streamline direction
-
u
1
velocity component in thexy-plane, orthogonal to the streamline direction
-
u
2
velocity component orthogonal to bothu
s andu
1
- i
mean Eulerian wind velocity
-
W
i
stochastic vector-valued Wiener process
-
x
unit vector inx-direction
-
y
unit vector iny-direction
-
z
unit vector inz-direction
-
angle between thexy-plane and the mean wind streamline
-
angle between the projection in thexy-plane of the streamline and thex-axis
- ij
the Kronecker delta function
-
rate of turbulence dissipation
- i/ga
the part ofa
i that contains mean wind and turbulence gradients
- ij
inverse of a Reynolds stress tensor component
- ij
shorthand for a quantity that defines a part of i/ga
- i
shorthand for a quantity that defines a part of i/ga
- ij
Reynolds stress tensor component 相似文献
5.
In this study, a detailed model of an urban landscape has been re-constructed inthe wind tunnel and the flow structure inside and above the urban canopy has beeninvestigated. Vertical profiles of all three velocity components have been measuredwith a Laser-Doppler velocimeter, and an extensive analysis of the measured meanflow and turbulence profiles carried out. With respect to the flow structure inside thecanopy, two types of velocity profiles can be distinguished. Within street canyons,the mean wind velocities are almost zero or negative below roof level, while closeto intersections or open squares, significantly higher mean velocities are observed.In the latter case, the turbulent velocities inside the canopy also tend to be higherthan at street-canyon locations. For both types, turbulence kinetic energy and shearstress profiles show pronounced maxima in the flow region immediately above rooflevel.Based on the experimental data, a shear-stress parameterization is proposed, inwhich the velocity scale, us, and length scale, zs, are based on the level and magnitude of the shear stress peak value. In order to account for a flow region inside the canopy with negligible momentum transport, a shear stress displacement height, ds, is introduced. The proposed scaling and parameterization perform well for the measured profiles and shear-stress data published in the literature.The length scales derived from the shear-stress parameterization also allowdetermination of appropriate scales for the mean wind profile. The roughnesslength, z0, and displacement height, d0, can both be described as fractions of the distance, zs - ds, between the level of the shear-stress peak and the shear-stress displacement height. This result can be interpreted in such a way that the flow only feels the zone of depth zs - ds as the roughness layer. With respect to the lower part of the canopy (z < ds) the flow behaves as a skimming flow. Correlations between the length scales zs and ds and morphometric parameters are discussed.The mean wind profiles above the urban structure follow a logarithmic windlaw. A combination of morphometric estimation methods for d0 and z0 with wind velocity measurements at a reference height, which allow calculation of the shear-stress velocity, u*, appears to be the most reliable and easiest procedure to determine mean wind profile parameters. Inside the roughnesssublayer, a local scaling approach results in good agreement between measuredand predicted mean wind profiles. 相似文献
6.
Experimental determinations of the local heat transfer by forced convection from model leaves heated by a constant energy flux were made in the laboratory under laminar and turbulent flow conditions.The results are expressed in a logarithmic dimensionless plot of the local Nusselt number, Nu
d
, against the local Reynolds number, Re
d
. For the laminar case, Nu
d
was only a linear function of Re
d
1/2 downwind from the leading edge regions, although this relationship departed from that predicted theoretically due to the finite size and thickness of the model. For the turbulent case, a simple relationship between Nu
d
and Re
d
was found over a wide range of Reynolds numbers. The enhancement of heat transfer in the turbulent case depends primarily on the scale of turbulence rather than on the turbulent intensity.Past workers have discussed their results in relation to a factor , defined as the ratio between the heat transfer predicted by the Polhausen equation, and that measured. The results suggest that is not a unique parameter and may not be useful in describing the overall turbulent transfer process. 相似文献
7.
Summary In this paper, we evaluate the applicability of flux-gradient relationships for momentum and heat for urban boundary layers
within the Monin-Obukhov similarity (MOS) theory framework. Although the theory is widely used for smooth wall boundary layers,
it is not known how well the theory works for urban layers. To address this problem, we measured the vertical profiles of
wind velocity, air temperature, and fluxes of heat and momentum over a residential area and compared the results to theory.
The measurements were done above an urban canopy whose mean height zh is 7.3 m. 3-D sonic anemometers and fine wire thermocouples were installed at 4 heights in the region 1.5zh < z < 4zh. We found the following: (1) The non-dimensional horizontal wind speed has good agreement with the stratified logarithmic
profile predicted using the semi-empirical Monin-Obukov similarity (MOS) function, when it was scaled by the surface friction
velocity that is derived from the shear stress extrapolated to the roof-top level. (2) The scaled gradient of horizontal wind
speed
followed a conventional semi-empirical function for a flat surface at a level (z/zh = 2.9), whereas, in the vicinity of the canopy height
was larger than the commonly-used empirical relationship. (3) The potential temperature profile above the canopy shows dependency
on the atmospheric stability and the scaled gradient of temperature
is in good agreement with a conventional shear function for heat. In the case of heat, the dependency on height was not found.
(4) The flux-gradient relationship for momentum and heat in the region 1.5zh < z < 4zh was rather similar to that for flat surfaces than that for vegetated canopies. 相似文献
8.
Sergej A. Kitaigorodskii Yury A. Volkov Andrey A. Grachev 《Boundary-Layer Meteorology》1995,76(1-2):181-197
The aerodynamic classification of the resistance laws above solid surfaces is based on the use of a so-called Reynolds roughness number Re
s =h
s
u
*/, whereh
s is the effective roughness height, -viscosity,u
*-friction velocity. The recent experimental studies reported by Toba and Ebuchi (1991), demonstrated that the observed variability of the sea roughness cannot be explained only on the basis of the classification of aerodynamic conditions of the sea surface proposed by Kitaigorodskii and Volkov (1965) and Kitaigorodskii (1968) even though the latter approach gains some support from recent experimental studies (see for example Geernaertet al. 1986). In this paper, an attempt is made to explain some of the recently observed features of the variability of surface roughness (Toba and Ebuchi, 1991; Donelanet al., 1993). The fluctuating regime of the sea surface roughness is also described. It is shown that the contribution from the dissipation subrange to the variability of the sea surface can be very important and by itself can explain Charnock's (1955) regime. 相似文献
9.
The present study explores the extent to which the logarithmic region of the adiabatic atmospheric boundary layer can be modeled using a three-dimensional large eddy simulation. A value of the von Kármán constant (LES) is obtained by determining the slope of a logarithmic portion of the velocity profile. Its numerical value is found to be dependent on the value of the Smagorinsky-Model Reynolds number, ReSM: the value of LES increases with ReSM. Results indicate that LES approaches a value of 0.35 as ReSM reaches about 7.75 × 105 for the largest domain. The sensitivity of LES to the profile region over which it is evaluated has been tested. Results show that LES is not sensitive to the depth of this evaluation region when we employ five grids above the sub-grid buffer layer where sub-grid-scale effects dominate. The maximum LES is obtained when the lower boundary of the evaluation region is just above the top of the sub-grid-scale buffer layer. This result is consistent with modelled mean speed and resolved-scale shear stress profiles. 相似文献
10.
Vertical profiles of wind speed, temperature and humidity were used to estimate the roughness lengths for momentum (z
0), heat (z
H
) and moisture (z
Q) over smooth ice and snow surfaces. The profile-measurements were performed in the vicinity of a blue ice field in Queen Maud Land, East Antarctica. The values ofz
0 over ice (3·10–6 m) seem to be the smallest ever obtained over permanent, natural surfaces. The settling of snow on the ice and the loss of momentum at saltating snow particles serve as momentum dissipating processes during snow-drift events, expressed as a strong dependence ofz
0 on u#.The scalar roughness lengths and surface temperature can be evaluated from the temperature and humidity profile measurements if the ratioz
H
/z
Q
is specified. This new method circumvents the difficult measurement of surface temperature. The scalar roughness lengths seem to be approximately equal toz0 for a large range of low roughness Reynolds numbers, despite the frequent occurrence of drifting snow. Possible reasons for this agreement with theory of non-saltating flow are discussed. 相似文献
11.
Based on the momentum flux–wind profile relationship of the Monin–Obukhov Similarity (MOS) theory, the observational data from the urban boundary layer field campaign in Nanjing are used to calculate the friction velocity ( $ {u_*} $ ) at the top of the urban canopy and the calculated results are evaluated. The urban surface roughness parameters (the roughness length z 0 and zero-plane displacement height z d) are estimated with the Ba method (Bottema’s morphological method). Two different regimes are employed for the calculations. In the homogeneous approach, z 0 and z d are averagely derived from the surface elements in the whole study area; while in the heterogeneous approach, z 0 and z d are locally derived from the surface elements in the corresponding upwind fetches (or source areas). The calculated friction velocities are compared to the measurement data. The results show that the calculated friction velocities from the heterogeneous approach are in better agreement with the observed values than those from the homogeneous approach are. This study implies that the local roughness parameters can properly represent the dynamical heterogeneity of urban surface, and its application can significantly improve the performance of parameterizations based on the MOS theory in the urban roughness sublayer. 相似文献
12.
A comprehensive model for the prediction of concentration fluctuations in plumes dispersing in the complex and highly disturbed
wind flows in an urban environment is formulated. The mean flow and turbulence fields in the urban area are obtained using
a Reynolds-averaged Navier-Stokes (RANS) flow model, while the standard k-ϵ turbulence model (k is the turbulence kinetic energy and ϵ is the viscous dissipation rate) is used to close the model. The RANS model provides a specification of the velocity statistics
of the highly disturbed wind flow in the urban area, required for the solution of the transport equations for the mean concentration
and concentration variance (both of which are formulated in the Eulerian framework). A physically-based formulation for the scalar dissipation time
scale t
d
, required for the closure of the transport equation for , is presented. This formulation relates t
d
to an inner time scale corresponding to “internal” concentration fluctuation associated with relative dispersion, rather
than an outer time scale associated with the entire portion of the fluctuation spectrum. The two lowest-order moments of concentration
( and ) are used to determine the parameters of a pre-chosen functional form for the concentration probability density function
(clipped-gamma distribution). Results of detailed comparisons between a water-channel experiment of flow and dispersion in
an idealized obstacle array and the model predictions for mean flow, turbulence kinetic energy, mean concentration, concentration
variance, and concentration probability density function are presented. 相似文献
13.
Summary A fixed platform (Fig.3), installed 100 m from the shoreline in 3 m water depth, was instrumented with velocity, temperature and wave-height sensors. 132 data (10 minutes averages) were analysed to calculate the wind stress; from these, 99 data were used to investigate the vertical distribution of the wind stress; all data are presented with Table 1.It was postulated that the total stress,
t
being constant with height, is made up additively of two components, the wave-supporting stress,
w
, and the turbulent stress,
c
; see Eq. 1. The vertical distribution of these two components is shown schematically in Fig. 1.The total stress,
t
, evaluated outside the zone of wave influence, is given in the classical way with Fig. 4. The wave-supporting stress,
w
(z), was evaluated from the data according to a relation proposed by Kitaigorodskii et al. (1984); it is given with Fig. 5. A height-dependency is clearly evident. The turbulent stress
c
(z), was evaluated with data of the velocity gradient; it is given with Fig. 6. A height-dependency is not evident.The field data from the lake of Geneva give evidence that the additive relation of Eq. 1 seems to be justified.With 6 Figures 相似文献
14.
P. Raethjen 《Meteorology and Atmospheric Physics》1950,2(2-3):207-222
Zusammenfassung Wenn in der Atmosphäre das geostrophische bzw. geozyklostrophische Gleichgewicht des (horizontalen) Strom- und Druckfeldes gestört ist, stellt sich dieses Gleichgewicht spontan wieder her, indem beide Felder sich dem Gleichgewichtszustand nähern. Wenn die anfängliche Gleichgewichtsstörung alle Schichten der Atmosphäre betrifft, ändert sich das (gestörte) Stromfeld kaum, das Druckfeld aber sehr stark. Wenn anfänglich nur eine Teilschicht, die den Bruchteilv der gesamten Atmosphärenmasse enthält, gestört ist, verhält sich die Stromfeldänderung zur Druckfeldänderung wie 1– zu . Die Kompensation aufgeprägter Druckfelder und die Wiederherstellung des geostrophischen bzw.-zyklostrophischen Gleichgewichts sind dabei engstens miteinander verbunden; beide wirken gemeinsam darauf hin, daß die Hoch- und Tiefdruckgebiete alle Schichten der Troposphäre und unteren Stratosphäre umfassen (warmes Hoch, kaltes Tief).
Summary If, in the atmosphere, the geostrophic respectively the geocyclostrophic equilibrium of the (horizontal) field of flow and pressure is disturbed, this equilibrium gets spontaneously re-established by approaching of the two fields. The (disturbed) field of flow changes hardly, but strongly the field of pressure, if the initial disturbance of equilibrium concerns all layers of the atmosphere. If originally only a partial layer is disturbed, comprising the fraction of the whole atmosphere, then change of field of flow is to change of field of pressure as 1– is to . The compensation of imposed fields of pressure and the re-establishment of geostrophic or-cyclostrophic equilibrium respectively here are closely connected; both together lead to the effect that anticyclones and depressions include all layers of troposphere and lower stratosphere (warm High, cold Low).
Résumé Lorsque dans l'atmosphère l'équilibre respectivement géostrophique et géocyclostrophique du champ (horizontal) de mouvement et de pression est rompu, il se rétablit spontanément en ce sens que les deux champs se rapprochent de l'état d'équilibre. Lorsque la rupture primitive de l'équilibre se produit à tous les niveaux, le champ de mouvement change à peine, mais le champ de pression s'altère profondément. Si à l'origine une couche isolée contenant la fraction de l'atmosphère est perturbée, la variation du champ de mouvement est à celle du champ de pression comme 1– est à . La «compensation» de champs de pression imposés et le rétablissement de l'équilibre respectivement géostrophique et-cyclostrophique sont alors intimément liés; l'un et l'autre ont pour effet que les régions de haute et de basse pression intéressent toutes les couches de la troposphère et de la stratosphère inférieure (anticyclones chauds, dépressions froides).相似文献
15.
The wind stress in the marine surface layer under unstable conditions and low wind speed has been studied using a Synthetic Aperture Radar (SAR) image of the sea surface and time series of the horizontal and vertical wind velocities and of the wind stress recorded on board the C.N.R. research platform, in the northern Adriatic Sea, during a SAR overflight.A conditional sampling technique has been used on the wind stress time series and on the SAR image to detect downward (sweep) and upward (ejection) bursts of the momentum flux, as well as the two-dimensional structure of the radar backscatter.From the ensemble average of both the wind stress and the backscatter structures, it has been possible to estimate the mean duration of the upward (11 s) and the downward (15 s) wind stress bursts and the mean size of the bright patches of the SAR image (120 m). The front of the mean backscatter structure, associated with the downward wind stress bursts, has been related to the time length of the mean sweep stress structure to get, after accounting for a threshold of the wind stress for the generation of the sea surface wavelets, the translation velocity Ut of the mean wind stress of sweep, very close to the mean wind speed. The vertical coherence of the wind stress structures has permited to refer the translation velocity to a level very close to the sea surface, but above the viscous sublayer. The variability of Ut with height has been studied through comparison with the mean wind speed at different heights z calculated by a boundary-layer model. Accounting for the results reported in the literature, there is an indication that Ut is constant with height in the range 0.5 m z 15 m.The two-dimensional pattern of the wind stress structures has been derived from the SAR image. The structures appear elongated crosswind, as with microfronts, with an average cross- to down-wind ratio of 4. The area covered by the downward wind stress structures represents 13% of the total area. 相似文献
16.
G. D. Hess 《Boundary-Layer Meteorology》2004,110(3):319-355
The presence of a low-level, capping inversion layer will affect the height and structure of the planetary boundary layer (PBL). Results from models of varying levels of sophistication, including analytical, turbulent kinetic energy (TKE), second-order closure (SOC), large-eddy simulation (LES) and direct numerical simulation (DNS) models, are used to investigate this influence for the neutral, barotropic PBL. Predicted and observed profiles of stress and geostrophic departure components, and integral measures, such as the parameters of Rossby-number similarity theory, are compared for the KONTUR, Marine Stratocumulus, JASIN, Leipzig, Pre-Wangara and Upavon field experiments.Analytical models of the equilibrium value of inversion height zi, which depend on the surface friction velocity u*, and both the Coriolis parameter f and the free-flow Brunt-Väisälä frequency N, are found to give reasonable estimates of the PBL height. They also indicate that only the KONTUR and Marine Stratocumulus experiments were strongly influenced by N. More quantitative comparisons would require larger, more comprehensive datasets. The effects of the presence of a capping inversion on the profile structure were found to be insignificant for h* = |f|zi/u* > 0.15.The simple analytical model performed quite well over all values of h*; it predicted the profiles of the longitudinal stress component (in the direction of the surface stress) better than the lateral component. The more advanced models performed well for small values of h* (for flow over the sea), but systematically underestimated the cross-isobaric angle for flow over land. These models predicted the profiles of the lateral stress component better than the longitudinal component. The profiles of the analytical model agreed with those of the advanced models when the constant eddy viscosity of the outer layer was increased.Agreement with DNS was achieved by increasing the eddyviscosity of the analytical model by a factor of 5.Zilitinkevich and Esau(2002, Boundary-Layer Meteorology, 104, 371–379)suggest that the neutral, barotropic values of A and B of Rossby-numbersimilarity theory are not universal constants, but depend on the ratio N/|f|. The dependence for A and B is calculated using the analytical model and TKE models. Over the sea (h* 0.1; N/|f| 100, where we have used the Zilitinkevich-Esau relation to convert between h* and N/|f|) there is agreement between the model predictions and observations; however over land where the equilibrium boundary-layer height is greater (h* 0.35; N/|f| 10) the inconsistency between the advanced model predictions (TKE, SOC, LES, and DNS) and observations, as noted previously by Hess and Garratt, still exists. We attribute this disagreement to violations of the strict assumptions of steady, horizontally homogeneous, neutral, barotropic conditions implicit in the observations. At small values of zi and a strongly stable background stratification (h* 0.04; N/|f| 1000) both the TKE and analytical models predict that A and B depend significantly on h*, however observations are unavailable to confirm these predictions. Zilitinkevich and Esau call this case the `long-lived near-neutral PBL', and state that it is found in cold weather at high latitudes. 相似文献
17.
Measurements have been made in both a neutral and a stable boundary layer as part of an investigation of the wakes of wind turbines in an offshore environment, in the EnFlo stratified flow wind tunnel. The working section is long enough for the flow to have become very nearly invariant with streamwise distance. In order to be systematic, the flow profile generators of Irwin-type spires and surface roughness were the same for both neutral and stable conditions. Achieving the required profiles by adjusting the flow generators, even for neutral flow, is a highly iterative art, and the present results indicate that it will be no less iterative for a stable flow (as well as there being more conditions to meet), so this was not attempted in the present investigation. The stable-case flow conformed in most respects to Monin–Obukhov similarity in the surface layer. A linear temperature profile was applied at the working section inlet, resulting in a near-linear profile in the developed flow above the boundary layer and ‘strong’ imposed stability, while the condition at the surface was ‘weak’. Aerodynamic roughness length (mean velocity) was not affected by stability even though the roughness Reynolds number ${<}1$ , while the thermal roughness length was much smaller, as is to be expected. The neutral case was Reynolds-number independent, and by inference, the stable case was also Reynolds-number independent. 相似文献
18.
Mesoscale models using a non-local K-scheme for parameterization of boundary-layer processes require an estimate of the planetary boundary layer (PBL) height z
i
at all times. In this paper, two-dimensional sea-breeze experiments are carried out to evaluate three different formulations for the advective contribution in the z
i
prognostic equation of Deardorff (1974).Poor representation of the thermal internal boundary layer in the sea breeze is obtained when z
i
is advected by the wind at level z
i
. However, significantly better results are produced if the mean PBL wind is used for the advecting velocity, or if z
i
is determined simply by checking for the first sufficiently stable layer above the ground.A Lagrangian particle model is used to demonstrate the effect of each formulation on plume dispersion by the sea breeze. 相似文献
19.
When applied to a sea surface, shortcomings are noted for the ordinary classification of drag conditions at rigid underlying surfaces according to the Reynolds roughness number Re
s
. It is shown that in the case of mobile underlying surfaces, it would be more natural to use the dynamical classification of drag conditions according to the order of magnitude of the ratio ( = /) of the momentum flux toward the waves (
w) to the viscous momentum flux through the surface (
w). The relevant estimates of for the main stages of development of the wind waves indicate that the observed values of the drag coefficient of the sea surface correspond to the case of underdeveloped roughness. 相似文献