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1.
Results of laboratory and numerical experiments on both homogeneous and density-stratified flow over single, bluff obstacles of various shapes are presented. The obstacle height is in most cases of the same order as the base diameter and the major controlling (flow) parameter is the Froude number, defined here as Fh=U/Nh, where U is the (uniform) upstream velocity, h the obstacle height and N is the buoyancy frequency. Attention is concentrated, firstly, on the case of homogeneous flows over rather weakly tapered obstacles and, secondly, for bodies whose height is similar to their base width, on the case Fh=0.1, representing stratification sufficiently strong that lee-wave motions do not play a significant role in the flow dynamics. For right-circular cones it is shown that the sectional contributions to the total fluctuating side force (lift) show significant phase variations up the height of the obstacle, which are not always reflected in the developed vortex street further downstream. For some obstacle shapes, the vortex lines linking the von Karman eddies at different heights can be significantly tilted, particularly in the upper part of the wake. Vortex convection speeds do not appear generally to vary greatly with height and, as found in previous work, the shedding frequency remains constant with height, despite the strong variation of cross-stream obstacle width. By comparison with the homogeneous results, it is suggested that the stratification enhances the shedding instability, which would otherwise be very weak for squat obstacles, but does not annihilate the ability of the flow at one level to influence that at another. 相似文献
2.
A systematic investigation of the effects of various parametrizations of dissipation, e.g. quadratic and linear frictional drag, harmonic lateral viscosity, and harmonic lateral diffusion on inertial flow over a sill and possible hydraulic control is presented. Rotation effects are ignored and the geometry is assumed to vary only slowly with downstream distance so that the flow may be considered one-dimensional. Results are given both for a single-active layer and for two-active layers with a rigid lid.If the parametrization is only a function of the dependent variables and not of their spatial derivatives, then it may be possible to hydraulically control the flow. A general expression is derived for the possible control point and the two gradients there, which are functions of the slope and possibly of flow rate. Specific energy is irreversibly removed from the flow and non-controlled as well as controlled flows can exhibit significant asymmetry in fluid depth over a sill. The upstream specific energy, and hence depth of the lower layer, of the controlled flow is greater than for an ideal fluid. Frictional effects modify the behaviour of long gravity waves, such that they are dispersive and damped with time. The system will only exhibit hydraulic control if these effects are small.For a viscous single layer of fluid, the gradient in surface elevation is always uniquely defined, so classically defined hydraulic control, as such, cannot exist. However, for values of non-dimensional lateral eddy viscosity coefficient,
, where q is the flow rate, there is a narrow band of specific energies centred around that for the control solution in an ideal fluid, Ecrit, for which the surface elevation, h is very asymmetric over the sill; the solutions resemble the inviscid, hydraulically controlled solutions. Outside this range, either the fluid depth tends to zero, or the surface elevation is almost uniform over the sill. A ‘control’-type solution exists which has the conjugate values of the inviscid equation up- and downstream of the sill, where the gradient in fluid depth, and hence the viscous term, is zero. For larger values of AM, the band of specific energies is much wider, and the upstream specific energy of the ‘control’-type solution is much lower than that for an inviscid fluid. Long gravity waves are dispersive and damped with time. There is a short-wave cut-off, k2 > h/(4AM2), above which waves are stationary in the flow. Longer waves, k2 h/(4AM2), are critical if
, as for an ideal fluid. If these waves can propagate significant distances, then any observed asymmetry in h will be due to inertial and not to viscous effects. The behaviour of unidirectional, two-layer flow is similar. The governing equation for viscous, two-layer exchange flow is singular, and typically excludes the ‘control’-type solutions found for unidirectional flows.Establishing the existence and behaviour of steady inertial flows in the presence of lateral diffusion between layers is more difficult. It significantly alters the single-layer solutions once the non-dimensional coefficient AH is large, i.e.
. The flow rate may become zero on the downslope as all the fluid diffuses into the inert, infinitely deep, overlaying layer. The fluid depth is maintained by reverse flow from downstream. In this case, the depth of the active layer tends to zero downstream for all values of specific energy. For two-layer flow, both unidirectional and exchange, the governing equation is such that the lower-layer flow rate and interfacial height return to their upstream values.Motivation for the study is provided by the increasingly fine spatial resolution achievable in large-scale numerical models of the ocean general circulation, and the question of whether they are capable of simulating some form of hydraulic control. Application to modelling oceanic flows over a sill is discussed. 相似文献
3.
In order to investigate flows over topography in an atmospheric context, we have studied experimentally the wake structure of axi-symmetric Gaussian obstacles towed through a linearly stratified fluid. Three dimensionless parameters govern the flow dynamics: F, the Froude number based on the topography height h; Re, the Reynolds number and the aspect ratio r = h/L, where L is the topography horizontal scale. Two-dimensional (2-D), saturated lee wave (SLW) and three-dimensional (3-D) regimes, as defined in Chomaz et al. (1993), are found to be functions of F and r only (Fig. 1) as soon as Re is larger than Rec ≈ 2000. For F < 0.7 the flow goes around the obstacle and the motion in the wake is quasi-two-dimensional. This 2-D layer is topped by a region affected by lee wave motions with amplitude increasing with r and F. For 0.7 < F < 1/r, the flow is entirely dominated by a lee wave of saturated amplitude which suppresses the separation of the boundary layer from the obstacle. Above the critical value 1/r, the lee wave amplitude decreases with F and a recirculating zone appears behind the obstacle. Simultaneously, coherent large-scale vortices start to be shed periodically from the wake at a Strouhal number which decreases as 1/F until it reaches its neutral asymptotic value. 相似文献
4.
Summary ?We have investigated the effects of shear and sharp gradients in static stability and demonstrated how a mountain wave and
its associated surface winds can be strongly influenced. Linear theory for two-dimensional, nonrotating stratified flow over
an isolated mountain ridge with positive shear and constant static stability shows that the horizontal wind speeds on both
the lee and upslope surfaces are suppressed by positive shear. The critical F(=U/Nh where U is the basic wind speed, N the Brunt-Vaisala frequency, and h the mountain height) for the occurrence of wave breaking decreases when the strength of the positive shear increases, while
the location for the wave-induced critical level is higher in cases with larger positive shear. The linear theory is then
verified by a series of systematic nonlinear numerical experiments. Four different flow regimes are found for positive shear
flow over a two-dimensional mountain. The values of critical F which separate the flow regimes are lower when the strength of the positive shear is larger. The location of stagnation aloft
from numerical simulations is found to be quite consistent with those predicted by linear theory.
We calculate the strongest horizontal wind speed on the lee surface (U
max), the smallest horizontal wind speed on the upslope surface (U
min), the reflection (Ref), and the transmission (Tran) coefficients for different combinations of the stability ratio between
the upper and lower layers (i.e. and z
1 (interface height) in a two-layer atmosphere from linear analytical solutions. Both Ref and Tran are found to be functions
of log() but not the interface height (z
1). Ref is larger when is much different from 1, no matter whether it is larger or smaller than 1. However, Tran decreases when log() increases and approaches 0 when log() is large. The magnitude of the largest U
max (smallest U
min) increases (decreases) as the absolute value of log() increases. It is found that the largest U
max occurs when the nondimensional z
1 is near for cases with a less stable upper layer or when z
1 is near for cases with a more stable upper layer. These results are confirmed by nonlinear numerical simulations. We find that linear
theory is very useful in qualitative analysis of the possibility of high-drag state for different stability profiles. The
location of stagnation aloft in a two-layer atmosphere from numerical simulations agrees very well with those predicted by
linear theory.
The above findings are applied to investigate the Boulder severe downslope windstorm of 11 January 1972. We find that the
windstorm cannot develop if the near mountain-top inversion is located at a higher altitude (e.g., km). However, if there exists a less stable layer right below the tropopause, the windstorm can develop in the absence of
a low-level inversion. These results indicate the importance of partial reflection due to the structured atmosphere in influencing
the possibility of severe downslope windstorms, although partial reflection may not be the responsible mechanism for the generation
of windstorms.
Received September 25, 1999/Revised February 9, 2000 相似文献
5.
Martin Claussen 《Boundary-Layer Meteorology》1987,40(1-2):31-86
Modification of a turbulent flow upstream of a change in surface roughness has been studied by means of a stream function-vorticity model.A flow reduction is found upstream of a step change in surface roughness when a fluid flows from a smooth onto a rough surface. Above that layer and above the region of flow reduction downstream of a smooth-rough transition, a flow acceleration is observed. Similar flow modification can be seen at a rough-smooth transition with the exception that flow reduction and flow acceleration are reversed. Within a fetch of –500 < x/z
0< + 500 (z
0 is the maximum roughness length, the roughness transition is located at x/z
0 = 0), flow reduction (flow acceleration) upstream of a roughness transition is one order of magnitude smaller than the flow reduction (flow acceleration) downstream of a smooth-rough (rough-smooth) transition. The flow acceleration (flow reduction) above that layer is two orders of magnitude.The internal boundary layer (IBL) for horizontal mean velocity extends to roughly 300z
0 upstream of a roughness transition, whereas the IBL for turbulent shear stress as well as the distortion of flow equilibrium extend almost twice as far. For the friction velocity, an undershooting (overshooting) with respect to upstream equilibrium is predicted which precedes overshooting (undershooting) over new equilibrium just behind a roughness transition.The flow modification over a finite fetch of modified roughness is weaker than over a corresponding fetch downstream of a single step change in roughness and the flow stays closer to upstream equilibrium. Even in front of the first roughness change of a finite fetch of modified roughness, a distortion of flow equilibrium due to the second, downwind roughness change can be observed. 相似文献
6.
The quasi-geostrophic response of a stratified stream incident upon isolated finite amplitude topography on a f-plane is examined in the limit of a Boussinesq, incompressible, inviscid fluid. Compact solutions are derived subject to the following stipulations: uniform upstream velocity and stratification, a circular obstacle and an entirely isentropic/isopycnic lower surface.It is shown that for a semi-infinite flow domain the criterion for Taylor cap formation (i.e., a region of closed streamlines) is
. However, for the isentropic lower boundary condition the solutions exist (i.e., have physical validity) only if R0F−1 < 0.5. (Here R0 and F refer to the Rossby and Froude numbers defined respectively in terms of the mountain half-width and height.) Also considered are the modifications both to the flow response and to the foregoing existence criterion that are induced by the introduction of an upstream profile comprising two layers of uniform but different stratification. In addition, the relationship of the derived solutions to the results obtained in previous studies is explored, and in particular an outline is given of the impact of adopting the ‘traditional’ simplified lower boundary condition. 相似文献
7.
The Sundqvist parameterization for warm rain production by autoconversion processes as the function of cloud liquid water mixing ratio m is tested by defining a realistic ‘driving’ profile m(z) for a maritime low, warm stratocumulus cloud, and comparing with various recent observations. The results show that the parameterization is acceptable, especially after tuning its rain collection constant C1. It is somewhat sensitive to the vertical resolution of the host model, though. Extending the calculations by considering typical cloud and raindrop size spectra, extra variables such as drizzle amounts and droplet effective radii (forced by the bulk Sundqvist rain rates) could be estimated by numerical integration. Also, these seem to agree fairly well with the available observations. 相似文献
8.
Raymond Hide 《Dynamics of Atmospheres and Oceans》1998,27(1-4)
Previous theoretical and laboratory studies of mechanically driven fluids in general rotation relative to an inertial frame have shown that there is a special class of flows for which the (Eulerian) flow field u(r, t) relative to the rotating frame of reference is unaffected by gyroscopic (Coriolis) forces, and therefore remains the same for all values of the rotation vector Ω. (Here t denotes time and r the position of a general point R in a reference frame attached to the rotating apparatus.) Such flows occur when (a) Ω is independent of time t; (b) u(r, t) is independent of the coordinate z (say) parallel to Ω, (c) the fluid has constant density and is therefore ‘barotropic’ (i.e. no density variations on horizontal surfaces) and (d) the topology of the cross-section of the (cylindrical) container, in planes z = constant, is such that the bounding surfaces can support the concomitant field of (kinematic) pressure P1 satisfying P1 + 2 Ω × U = 0 Condition (d) is equivalent to the requirement that any fluid sources or siks within the system be multipole in character, but not monopole. In the present study the ‘baroclinic’ case is treated, where buoyancy forces due to the action of gravity (and centripetal forces) on horizontal density variations have to be taken into account. These include investigations of flows due entirely to buoyancy forces, such as thermal convection in fluids in rotating cylindrical containers of various shapes and topological characteristics subject to horizontal temperature gradients. The implications for the impressed temperature field of the mathematical requirements that the fields of kinematic pressure P1 and density
(where
denotes the mean density) be everywhere single-valued are guiding such investigations and facilitating the interpretation of their findings. The investigations include laboratory studies, reported elsewhere, of convection in a rotating fluid annulus with a circular cross-section blocked by a radial barrier, where it is found inter alia that advective heat transfer is virtually independent of |Ω| over a wide range of conditions. They also include (as yet unpublished) studies of thermal convection in rotating systems with topologically triply connected cross-sections which can be rendered doubly or simply connected by the insertation of suitable barriers. 相似文献
9.
Andrey A. Grachev Edgar L Andreas Christopher W. Fairall Peter S. Guest P. Ola G. Persson 《Boundary-Layer Meteorology》2007,124(3):315-333
Measurements of atmospheric turbulence made during the Surface Heat Budget of the Arctic Ocean Experiment (SHEBA) are used
to examine the profile stability functions of momentum, φ
m
, and sensible heat, φ
h
, in the stably stratified boundary layer over the Arctic pack ice. Turbulent fluxes and mean meteorological data that cover
different surface conditions and a wide range of stability conditions were continuously measured and reported hourly at five
levels on a 20-m main tower for 11 months. The comprehensive dataset collected during SHEBA allows studying φ
m
and φ
h
in detail and includes ample data for the very stable case. New parameterizations for φ
m
(ζ) and φ
h
(ζ) in stable conditions are proposed to describe the SHEBA data; these cover the entire range of the stability parameter
ζ = z/L from neutral to very stable conditions, where L is the Obukhov length and z is the measurement height. In the limit of very strong stability, φ
m
follows a ζ 1/3 dependence, whereas φ
h
initially increases with increasing ζ, reaches a maximum at ζ ≈ 10, and then tends to level off with increasing ζ. The effects
of self-correlation, which occur in plots of φ
m
and φ
h
versus ζ, are reduced by using an independent bin-averaging method instead of conventional averaging. 相似文献
10.
The roughness height z
0 and the zero-plane displacement height d
0 were determined for a region of complex terrain in the Pre-Alps of Switzerland. This region is characterized by hills of the order of 100 m above the valley elevations, and by distances between ridges of the order of 1 km; it lies about 20 to 30 km north from the Alps. The experimental data were obtained from radiosonde observations under near neutral conditions. The analysis was based on the assumption of a logarithmic profile for the mean horizontal wind existing over one half of the boundary layer. The resulting (z
0/h) and (d
0/h) (where h is the mean height of the obstacles) were found to be in reasonable agreement with available relationships in terms of placement density and shape factor of the obstacles, which were obtained in previous experiments with h-scales 2 to 4 orders of magnitude smaller than the present ones. 相似文献
11.
Regional roughness of the landes forest and surface shear stress under neutral conditions 总被引：2，自引：1，他引：2
Mean wind velocity profiles were measured by means of radio-windsondes over the Landes region in southwestern France, which consists primarily of pine forests with scattered villages and clearings with various crops. Analysis of neutral profiles indicated the existence of a logarithmic layer between approximately z – d
0 = 67(±18)z
0 and 128(+-32)z
0 (z is the height above the ground, z
0 the surface roughness and d
0 the displacement height). The upper limit can also be given as z – d
0 = 0.33 (±0.18)h, where h is the height of the bottom of the inversion. The profiles showed that the surface roughness of this terrain is around 1.2 m and the displacement height 6.0 m. Shear stresses derived from the profiles were in good agreement with those obtained just above the forest canopy at a nearby location with the eddy correlation method by a team from the Institute of Hydrology (Wallingford, England). 相似文献
12.
Measurements of temperature and velocity microstructure near and downstream of a shallow seamount are used to compare fossil turbulence versus non-fossil turbulence models for the evolution of turbulence microstructure patches in the stratified ocean. According to non-fossil oceanic turbulence models, all overturn length scales LT of the microstructure grow and collapse in constant proportion to each other and to the turbulence energy (Oboukov) scale LO and the inertial buoyancy (Ozmidov) scale
of the patches; that is, with LTrms ≈1.2LR and viscous dissipation rate ≈ 0*. According to the Gibson fossil turbulence model, all microstructure originates from completely active turbulence with 0 ≈ 3LT2N3(≈ 280*) and LT/√6 ≈ LTrms, but this rapidly decays into a more persistent active-fossil state with 0F ≈ 30vN2, where N is the buoyancy frequency and v is the kinematic viscosity and, without further energy supply, finally reaches a completely fossil turbulence hydrodynamic state of internal wave motions, with F. The last turbulence eddies, with ≈ F, vanish at a buoyant-inertial-viscous (fossil Kolmogorov) scale LKF that is much smaller than the remnant overturn scales LT for large 0/F ratios. These density, temperature, and salinity overturns with LT ≈ 0.6 LR0 0.6 LR persist as turbulence fossils (by retaining the memory of o) and collapse very slowly. In the near wake below the summit depth of Ampere seamount, a much larger proportion of completely active turbulence patches was found than is usually found in the ocean interior away from sources. Dissipation rates and turbulence activity coefficients
of microstructure patches were found to decrease downstream, suggesting that the active turbulence indicated by the patches with AT 1 was caused by the presence of the seamount as a turbulence source. Therefore, the turbulence and mixing processes of ocean layers far away from turbulence sources probably have been undersampled by microstructure data sets lacking any AT 1 patches. This is because large fractions of the mixing and viscous dissipation of the patches occur in short-lived active turbulence regimes that are too brief to be detected. Consequently, large underestimates of the true space-time average turbulence fluxes and turbulence and scalar dissipation rates may result if non-fossil turbulence models are assumed in ocean microstructure data interpretation. 相似文献
13.
Low-level katabatic wind profiles, which have shapes similar to those of the low-level jet (LLJ) wind profiles, are often observed during strong winds in the summer period at Mizuho Station, which is located at 70°42 S, 44°20 E in East Antarctica. The profiles may be classified according to the height of the maximum wind speed, z
m
, found below 30 m height. The behavior of z
m and of conditions in the layer above z
mare explained well by the normalized frequency, f
N = Nz/U at 30 m, whose value can be used to predict the existence of a LLJ wind profile. Subsidence and inertial oscillations above z
m are related closely to the height and time variations of z
m. Thus, not only effects emanating upward from surface but also momentum and heat transported downward from above are significant for the evolution of z
m. 相似文献
14.
J. F. Barlow G. G. Rooney S. von Hünerbein S. G. Bradley 《Boundary-Layer Meteorology》2008,127(2):173-191
Profiles of wind and turbulence over an urban area evolve with fetch in response to surface characteristics. Sodar measurements,
taken on 22 April 2002 during the Salford Experiment in the UK (Salfex), are here related to upstream terrain. A logarithmic
layer up to z = 65m was observed in all half-hour averaged profiles. Above this height the profile showed a different vertical gradient,
suggesting a change in surface cover upstream. The drag coefficient varied by a factor of two over only a 20° direction change.
Turbulence intensity (σ
x
) for each wind component (x) decreased with height, but the ratio suggested an underestimate of σ
u
compared to previous results. Mean urban and suburban cover fraction within the source area for each height decreased sharply
between z = 20 and 50m, increasing slightly above. The near-convergence of cover fractions thus occured for source areas of minimum
length ≈ 2,200 m. In comparison, the mean length scale of heterogeneity L
P was calculated from surface cover data to be 1,284 m, and the corresponding mean blending height h
b was 175 m. Finally, the mean streamline angle, α, was negative and the magnitude decreased with height. An exponential fit
to α for z ≤ 65m gave an e-folding height scale of 159 m. A simple relationship between this height scale and L
P was assumed, giving L
P ≈ 1,080 m, which is in reasonable agreement with the estimate from surface cover type. The results suggest that more emphasis
is required on modelling and measuring surface-layer flow over heterogeneous urban canopies. 相似文献
15.
A comparison between various parameterizations for the bulk transfercoefficient for heat and momentum is carried out for a wide range ofatmospheric stability and values of the roughness lengths for momentum and heat,z0m and z0h respectively.It is confirmed that the parameterization of Launiainen compares wellto a numerical iterative solution for the Obukhov length L as function of the gradients of wind speed and temperatureover a limited range of z0m/z0h and stability conditions.For 0m/z0h > 500, an alternativeinterpolation formulation of Holtslag and Ek,in combination with the formulation of Launiainen, provides a better approximation. 相似文献
16.
A spectral approach is applied to shear-induced turbulence in stratified layers. A system of spectral equations for stationary balance of turbulent energy and temperature variances was deduced in the vicinity of the local shear scale LU = (ε/UZ3)1/2. At wavenumbers between the inertial-convective (k−5/3) and wak turbulence (k−3) subranges, additional narrow spectral intervals—‘production’ subranges—may appear (E k−1, ET k−2). The upper boundary of these subranges is determined as LU, and the lower boundaries as LR (ε/UZN2)1/2(χ/TZ2). It is shown that the scale LU is a unique spectral scale that is uniform up to a constant value for every hydrophysical field. It appears that the spectral scale LU is equivalent to the Thorpe scale LTh for the active turbulence model. Therefore, if turbulent patches are generated in a background of permanent mean shear, a linear relation between temperature and mass diffusivities exists. In spectral terms, the fossil turbulence model corresponds to the regime of the Boldgiano-Obukhov buoyancy subrange (E k−11/5, ET k−7/5). During decay the buoyancy subrange is expanded to lower and higher wavenumbers. At lower wavenumbers the buoyancy subrange is bounded by L** = 3(χ1/2/N1/2TZ), which is equivalent to the Thorpe scale LTh. In such a transition regime only, when the viscous dissipation rate is removed from the set of main turbulence parameters, the Thorpe scale does not correlate with the buoyancy scale LN ε1/2/N3/2 and fossil turbulence is realized. Oceanic turbulence measurements in the equatorial Pacific near Baker Island confirm the main ideas of the active and fossil turbulence models. 相似文献
17.
Aloysius Kou-Fang Lo 《Boundary-Layer Meteorology》1996,80(4):403-413
Flux-profile relationships based on surface-layer similarity theory are used to derive relationships between the Monin-Obukhov stability parameter = z/L and the bulk Richardson number Ri
b
. In contrast to previous studies, the roughness length for heat, z
0h
,is assumed unequal to the roughness length for momentum, z
0m
.For the stable case, an analytic expression of in terms of Ri
b
can be derived and in the unstable case, the solution is obtained through a simple iterative process.Errors introduced from the simplification of z
0h
= z
0m
are evaluated and are shown to be very significant in most cases. Thus, this error in many practical applications may invalidate the intended solution. 相似文献
18.
Ph. Drobinski S. Bastin J. Dusek G. Zängl P. H. Flamant 《Meteorology and Atmospheric Physics》2006,92(3-4):285-306
Summary This paper investigates the characteristics of channelled airflow in the vicinity of a junction of three idealized valleys
(one valley carrying the incoming flow and two tributaries carrying the outflow), using a two-dimensional single-layer shallow
water model. Particular attention is given to the flow splitting occurring at the junction. Nondimensionalized, the model
depends on the valley geometry, the Reynolds number, which is related to the eddy viscosity, and on the difference of the
hydrostatic pressure imposed at the exit of the tributaries. At the spatial scale considered in this study, the Rossby number
relating the inertial and Coriolis forces is always larger than 1, implying that the effect of earth rotation can be neglected
to a first approximation.
The analysis of the flow structure within the three valleys as well as the calculation of the split ratio (fraction of the
air flow diverted into one of the two downstream valleys with respect to the total mass flux in the upstream valley) show
that (i) the flow pattern depends strongly on the Reynolds number while the split ratio is comparatively insensitive; (ii)
the valley geometry and the difference between the upstream and downstream hydrostatic pressures affect the flow pattern,
the location of the split point and the split ratio; (iii) the relative contribution of flow deflection by the sidewalls and
the blocking/splitting mechanism differs between the settings of a “Y-shape” valley and a “T-shape” valley.
Quantitative comparison of the present results with numerical simulations of realistic cases and with observations collected
in the region of the Rhine and Seez valleys (Switzerland) (“Y-shape” valley) and in the region of the Inn and Wipp valleys
(Austria) (“T-shape” valley) during the Mesoscale Alpine Programme (MAP) field experiment shows good agreement provided that
the normalized valley depth NΔH/Uu significantly exceeds 1, i.e., when “flow around” is expected. A structural disagreement between the idealized simulations
and the observed wind field is found only when NΔH/Uu ≃ 1, that is, in the “flow over” regime. This shows that the dimensionless valley depth
is indeed a good indicator for flow splitting, implying that the stratification is a key player in reality. 相似文献
19.
Flux Footprints in the Convective Boundary Layer: Large-Eddy Simulation and Lagrangian Stochastic Modelling 总被引：1，自引：1，他引：0
We investigated the flux footprints of receptors at different heights in the convective boundary layer (CBL). The footprints
were derived using a forward Lagrangian stochastic (LS) method coupled with the turbulent fields from a large-eddy simulation
model. Crosswind-integrated flux footprints shown as a function of upstream distances and sensor heights in the CBL were derived
and compared using two LS particle simulation methods: an instantaneous area release and a crosswind linear continuous release.
We found that for almost all sensor heights in the CBL, a major positive flux footprint zone was located close to the sensor
upstream, while a weak negative footprint zone was located further upstream, with the transition band in non-dimensional upwind
distances −X between approximately 1.5 and 2.0. Two-dimensional (2D) flux footprints for a point sensor were also simulated. For a sensor
height of 0.158 z
i, where z
i is the CBL depth, we found that a major positive flux footprint zone followed a weak negative zone in the upstream direction.
Two even weaker positive zones were also present on either side of the footprint axis, where the latter was rotated slightly
from the geostrophic wind direction. Using CBL scaling, the 2D footprint result was normalized to show the source areas and
was applied to real parameters obtained using aircraft-based measurements. With a mean wind speed in the CBL of U = 5.1 m s−1, convective velocity of w
* = 1.37 m s−1, CBL depth of z
i = 1,000 m, and flight track height of 159 m above the surface, the total flux footprint contribution zone was estimated to
range from about 0.1 to 4.5 km upstream, in the case where the wind was perpendicular to the flight track. When the wind was
parallel to the flight track, the total footprint contribution zone covered approximately 0.5 km on one side and 0.8 km on
the other side of the flight track. 相似文献
20.
Momentum Absorption in Rough-Wall Boundary Layers with Sparse Roughness Elements in Random and Clustered Distributions 总被引：1，自引：1，他引：1
A wind-tunnel experiment has been used to investigate momentum absorption by rough surfaces with sparse random and clustered distributions of roughness elements. An unusual (though longstanding) method was used to measure the boundary-layer depth δ and friction velocity u
* and thence to infer the functional relationship z
0/h = f(λ) between the normalised roughness length z
0/ h and the roughness density λ (where z
0 is the roughness length and h the mean height of the roughness elements). The method for finding u
* is based on fitting the velocity defect in the outer layer to a functional form for the dimensionless velocity-defect profile in a canonical zero-pressure-gradient boundary layer. For the conditions investigated here, involving boundary layers over sparse roughness with strong local heterogeneity, this velocity-defect-law method is found to be more robust than several alternative methods for finding u
* (uw covariance, momentum integral and slope of the logarithmic velocity profile).The experimental results show that, (1) there is general agreement in the relationship z
0/h = f(λ) between the present experiment with random arrays and other wind-tunnel experiments with regular arrays; (2) the main effect of clustering is to increase the scatter in the z
0/h = f(λ) relationship, through increased local horizontal heterogeneity; (3) this scatter obscures any trend in the z
0/h = f(λ) relationship in response to clustering; and (4) the agreement between the body of wind-tunnel data (taken as a whole) and field data is good, though with scatter for which it is likely that a major contribution stems from local horizontal heterogeneity in the field. 相似文献