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1.
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
2.
M. J. Manton 《Boundary-Layer Meteorology》1971,1(3):264-276
The Miles theory of sea wave growth implies that, in a coordinate system moving with the wave phase velocity, closed streamline cats'-eyes are formed around the critical height where the mean wind speed equals the wave phase speed. From inspection of the equations of motion (the Rayleigh system), it is shown that these cats'-eyes may lie either over the wave crests or over the wave troughs, depending upon the behaviour of the mean wind profile. The validity of using Miles first approximation of the streamfunction' to approximate the actual streamline pattern is also discussed. 相似文献
3.
L. Kristensen D. H. Lenschow P. Kirkegaard M. Courtney 《Boundary-Layer Meteorology》1989,47(1-4):149-193
We have postulated a simple model for the spectral tensor
ij
(k) of an anisotropic, but homogeneous turbulent velocity field. It is a simple generalization of the spectral tensor
inf
ij
piso(k) for isotropic turbulence and we show how in the limit of isotropy,
ij
(k) becomes equal to
inf
ij
piso(k). Whereas
inf
ij
piso(k) is determined entirely by one scalar function of k = ¦k¦, namely the energy spectrum, we need three independent scalar functions of k to specify
ij
(k). We show how it is possible by means of the three stream-wise velocity component spectra to determine the three scalar functions in
ij
(k) by solving two uncoupled, ordinary linear differential equations of first and second order. The analytic form of the component spectra each has a set of three parameters: the variance and the integral length scale of the velocity component and a dimensionless parameter, which governs the curvature of the spectrum in the transition domain from the inertial subrange towards lower wave numbers. When the three sets of parameters are the same, the three spectra correspond to isotropic turbulence and they are all interrelated and related to the energy spectrum. We show how it is possible to obtain these spectral forms in the neutral surface layer and in the convective boundary layer from data reported in the literature. The spectral tensor is used to predict the lateral coherences for all three velocity components and these predictions are compared with coherences obtained in two experiments, one using three masts at a horizontally homogeneous site in Denmark and one employing two aircraft flying in formation over eastern Colorado. Comparison shows reasonable agreement although with considerable experimental scatter.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
4.
Silke Zank 《Boundary-Layer Meteorology》1984,29(2):139-153
The structure of atmospheric turbulence in the surface layer over the open ocean is examined under conditions of local free convection. The raw data consist of profile and fluctuation measurements of wind and temperature as obtained from a meteorological buoy. For near neutral conditions and for waves running approximately along the wind direction, wave-induced wind fluctuations can be described by a simplified linear theory based on Miles (1957). In this case, the spectrum of wind velocity is given as the sum of two parts; for the turbulent part, the parameterization as obtained by Kaimal et al. (1972) applies, while the wave-induced part is parameterized using a simplification of Miles' linear theory. For cases of local free convection, the measurements of the vertical component of the wind velocity are well described by similarity theory; as expected,
w
/(-uw)1/2 is proportional to (- z/L)1/3. In order to scale the longitudinal wind velocity component, it seems to be reasonable to extend the list of relevant parameters by the height of the mixed layer z
i. We obtain
u
/(- uw)1/2 (z/z
i)1/3(- z/L)1/3 with only a poor correlation coefficient of r = 0.6. Overall, the results of local free convection scaling obtained from direct measurements show good agreement with those obtained from profile measurements. A comparison between direct and indirect determination of turbulent fluxes of momentum shows an unexplained difference of about 20%. This discrepancy is mainly due to a gap in the uw-cospectrum at the swell frequency. 相似文献
5.
Gour-Tsyh Yeh 《Boundary-Layer Meteorology》1974,7(1):21-37
The two-layer system of an atmosphere over water bodies is reduced to a single-layer problem. Values of the interfacial quantities, such as the friction velocity, the surface velocity, the angles, and , between the surface shear stress and the geostrophic wind velocity and the surface wind velocity, respectively, and the surface roughness, all of which depend upon external parameters, such as the geostrophic wind and stratifications, are obtained. The geostrophic drag coefficient C
d, the geostrophic wind coefficient C
f, and the angles , and , of the turbulent flow at the sea-air interface are functions of a dimensionless number, mfG/kg, with S
1 and S
2 as two free stratification parameters. The surface roughness is uniquely determined from the geostrophic wind rather than from the wind profile in the boundary layer.Formerly Visiting Research Associate, Applied Physics Branch, Earth Observations Division, NASA-Manned Spacecraft Center, Houston, Texas. 相似文献
6.
Transport of pollution and heatout of streets into the boundary layer above is not currently understood and so fluxes cannot be quantified. Scalar concentration within the street is determined by the flux out of it and so quantifying fluxes for turbulent flow over a rough urban surface is essential. We have developed a naphthalene sublimation technique to measure transfer from a two-dimensional street canyon in a wind tunnel for the case of flow perpendicular to the street. The street was coated with naphthalene, which sublimes at room temperature, so that the vapour represented the scalar source. The transfer velocity wT relates the flux out of the canyon to the concentration within it and is shown to be linearly related to windspeed above the street. The dimensionless transfer coefficient wT/U represents the ventilation efficiency of the canyon (here, wT is a transfer velocity,U is the wind speed at the boundary-layer top). Observed values are between 1.5 and 2.7 ×10-3 and, for the case where H/W0 (ratio of buildingheight to street width), values are in the same range as estimates of transfer from a flat plate, giving confidence that the technique yields accurate values for street canyon scalar transfer. wT/U varies with aspect ratio (H/W), reaching a maximum in the wake interference regime (0.3 < H/W < 0.65). However, when upstream roughness is increased, the maximum in wT/U reduces, suggesting that street ventilation is less sensitive to H/W when the flow is in equilibrium with the urban surface. The results suggest that using naphthalene sublimation with wind-tunnel models of urban surfaces can provide a direct measure of area-averaged scalar fluxes. 相似文献
7.
Roughness effects on urban turbulence parameters 总被引：3，自引：0，他引：3
Urban roughness lengths are estimated from measurements of u and u
*
at one level under neutral conditions, assuming a logarithmic form for the vertical profile of wind velocity. At a given location in the urban area, estimated values show considerable directional variation. The dependence of some turbulence parameters on the urban roughness lengths is experimentally investigated during near-neutral conditions. The ratios
i
/u
*
decrease with roughness whereas the turbulence intensities
i
/u increase with it. The dependence on roughness is not the same for all components. 相似文献
8.
This is a report on the calibration tests and the determination of the response characteristics of a thrust anemometer designed to measure the three-dimensional instantaneous wind vector. Static tests determined the instrument calibration constants and checked the linearity of the system. Dynamic tests performed in a wind tunnel established that the V
2 drag law applies to the sensing system. The wind-tunnel tests also determined the range of angles for which the anemometer was capable of resolving the vector wind. Finally, the amplitude and phase frequency response characteristics were measured. It was found that knowledge of the latter was important in studying turbulence spectra and cross spectra. Some preliminary results from a recent field program are presented which document the need for correcting the amplitude and phase responses. 相似文献
9.
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
10.
An analysis was performed of experimental data obtained at fixed ship stations during AMTEX 1974 and 1975. This allowed the calculation of the bulk transfer relationships for water vapor and sensible heat in the atmospheric boundary layer for different interpretations of the thickness scale of the boundary layer. It was found that scaling based on the observed thickness, which herein was taken as the height of the lowest value in the potential temperature profile under unstable conditions, produces least scatter in the calculations. The results obtained for the similarity function c(
i
) of the bulk heat transfer coefficient are in good agreement with the results of previous studies. As observed earlier (Brutsaert and Mawdsley, 1976; Mawdsley and Brutsaert, 1977), under unstable conditions the similarity functions D() of the bulk water vapor transfer coefficient are smaller than the corresponding C() functions for sensible heat. In the case of inversion height scaling, the results can be represented by d(
i
) = 0.65 c(
i
). 相似文献