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1.
A model for the roughness length and its correlation with the roughness shear stress on organized rough walls of varying geometry are presented and verified. The roughness length is nondimensionalized by the characteristic roughness length and is expressed as a function of roughness density with a wake-interference parameter. The dimensionless roughness length is independent of Reynolds number. When the model is applied to the whole range of roughness densities, the rough walls can be smooth, transitionally rough, and fully rough. A large number of data from classical experiments and recent simulations are analyzed to evaluate the proposed correlations, which are found to be consistent with the analyzed datasets. The proposed expression for the dimensionless roughness length and the expression for the dimensionless roughness shear stress, proposed previously by the author (Boundary-Layer Meteorology, 2020, Vol. 174, 393–410), are found to be identical in form. Numerous extant measurements of the two roughness parameters can be reproduced when the wake-interference parameters in the two models are treated as identical. The parameters of the roughness-length model are closely related to the geometry of the roughness elements. Different types of roughness elements can be distinguished by the values of the parameters. These results provide the foundation for constructing the unified roughness model for organized rough walls of varying geometry.
相似文献2.
L. Mahrt Dean Vickers Jim Edson James M. Wilczak Jeff Hare Jørgen Højstrup 《Boundary-Layer Meteorology》2001,100(1):47-61
The adjustment of the boundary layer immediately downstream froma coastline is examined based on two levels of eddy correlation data collected on a mast at the shore and six levels of eddy correlation data and profiles of mean variables collected from a mast 2 km offshore during the Risø Air-Sea Experiment. The characteristics of offshore flow are studied in terms of case studies and inter-variable relationships for the entire one-month data set. A turbulent kinetic energy budget is constructed for each case study.The buoyancy generation of turbulence is small compared to shear generation and dissipation. However, weakly stable and weakly unstable cases exhibit completely different vertical structure. With flow of warm air from land over cooler water, modest buoyancy destruction of turbulence and reduced shear generation of turbulence over the less rough sea surface cause the turbulence to rapidly weaken downstream from the coast. The reduction of downward mixing of momentum by the stratification leads to smaller roughness lengths compared to the unstable case. Shear generation at higher levels and advection of stronger turbulence from land often lead to an increase of stress and turbulence energy with height and downward transport of turbulence energy toward the surface.With flow of cool air over a warmer sea surface, a convective internal boundary layer develops downstream from the coast. An overlying relatively thick layer of downward buoyancy flux (virtual temperature flux) is sometimes maintained by shear generation in the accelerating offshore flow. 相似文献
3.
A Wind-Tunnel Investigation of Wind-Turbine Wakes: Boundary-Layer Turbulence Effects 总被引：1，自引：1，他引：0
Wind-tunnel experiments were performed to study turbulence in the wake of a model wind turbine placed in a boundary layer
developed over rough and smooth surfaces. Hot-wire anemometry was used to characterize the cross-sectional distribution of
mean velocity, turbulence intensity and kinematic shear stress at different locations downwind of the turbine for both surface
roughness cases. Special emphasis was placed on the spatial distribution of the velocity deficit and the turbulence intensity,
which are important factors affecting turbine power generation and fatigue loads in wind energy parks. Non-axisymmetric behaviour
of the wake is observed over both roughness types in response to the non-uniform incoming boundary-layer flow and the effect
of the surface. Nonetheless, the velocity deficit with respect to the incoming velocity profile is nearly axisymmetric, except
near the ground in the far wake where the wake interacts with the surface. It is found that the wind turbine induces a large
enhancement of turbulence levels (positive added turbulence intensity) in the upper part of the wake. This is due to the effect
of relatively large velocity fluctuations associated with helicoidal tip vortices near the wake edge, where the mean shear
is strong. In the lower part of the wake, the mean shear and turbulence intensity are reduced with respect to the incoming
flow. The non-axisymmetry of the turbulence intensity distribution of the wake is found to be stronger over the rough surface,
where the incoming flow is less uniform at the turbine level. In the far wake the added turbulent intensity, its positive
and negative contributions and its local maximum decay as a power law of downwind distance (with an exponent ranging from
−0.3 to −0.5 for the rough surface, and with a wider variation for the smooth surface). Nevertheless, the effect of the turbine
on the velocity defect and added turbulence intensity is not negligible even in the very far wake, at a distance of fifteen
times the rotor diameter. 相似文献
4.
P. A. Taylor 《Boundary-Layer Meteorology》1972,2(3):381-389
Results are presented from a numerical experiment of wind and shear stress profile development away from a shore line; the water surface is assumed to obey the Charnock-Ellison relation between surface roughness and friction velocity. In typical cases the upwind, land surface is rough relative to the sea and the velocity and shear stress results are qualitatively similar to those for flows from relatively rough to relatively smooth solid surfaces. In the present case, however, the downwind surface roughness and friction velocity vary with position and we find that wind profile development may play a significant role in the relationship between sea surface roughness and fetch. 相似文献
5.
Drag Partition for Regularly-Arrayed Rough Surfaces 总被引：1，自引：0，他引：1
Vegetation and other roughness elements distributed across a surface can providesignificant protection against wind erosion by extracting momentum from the flowand thereby reducing the shear stress acting at the surface. A theoretical model haspreviously been presented to specify the partition of drag forces for rough surfacesand to predict required vegetation density to suppress wind erosion. However, themodel parameters have not yet been constrained and the predictive capacity of themodel has remained uncertain. A wind-tunnel study was conducted to measure thedrag partition for a range of roughness densities and to parameterise the model inorder to improve its range of potential applicability. The drag forces acting on bothan array of roughness elements and the intervening surface were measured independentlyand simultaneously using new drag balance instrumentation. A detailed measure of thespatial heterogeneity of surface shear stresses was also made using Irwin sensors. Thedata agreed well with previous results and confirmed the general form of the model.Analysis of the drag partition confirmed the parameter definition = CR/CS(where CR and CS are roughness element and surface drag coefficients,respectively) and a constant proportional difference between the mean and maximumsurface shear stress was found. The results of this experiment suggest that the definitionfor m, the surface shear stress inhomogeneity parameter, should be revised, although thetheoretical and physical reasons for including this parameter in the model appear to bevalid. Best-fit values for m ranged from 0.53 to 0.58. 相似文献
6.
Turbulent flow over a very rough,random surface 总被引：1，自引：1，他引：1
A knowledge of the nature of turbulent flow over very rough surfaces is important for an understanding of the environment of crops, forests, and cities. For this reason, a wind-tunnel investigation was carried out on the variations in mean velocity, Reynolds shear-stress, and other turbulence quantities in a deep turbulent flow over a rough surface having a fair degree of randomness in the shapes, sizes, and positions of its elements.There was a layer close to the surface with considerable variations in both mean velocity and shear-stress, and the horizontal scale over which the mean velocity varied was much larger than the average distance between roughness elements. Above this layer, whose depth was of the order of the spacing between roughness elements, shear stress was constant with height, and the velocity profile had a logarithmic form. The usefulness of both mean profile and eddy-correlation methods for estimating fluxes above very rough terrain is discussed in the light of these findings. 相似文献
7.
The response of a neutrally stratified atmospheric surface layer to sudden changes in surface roughness and temperature is investigated by solving the steady-state boundary-layer equations. Near the discontinuities, the distributions of velocity and shear stress are found to be critically dependent upon whether or not the pressure and buoyancy terms are retained in the governing equations. Computed distributions of velocity and shear stress are found to be in good agreement with windtunnel and field data. The growth of the internal boundary layer under neutral conditions for smooth to rough transitions follows the 0.8 power law; however, rough to smooth transitions are associated with a 0.7 power law. 相似文献
8.
Drag partition measurements were made in the atmospheric inertial sublayer for six roughness configurations made up of solid
elements in staggered arrays of different roughness densities. The roughness was in the form of a patch within a large open
area and in the shape of an equilateral triangle with 60 m long sides. Measurements were obtained of the total shear stress
(τ) acting on the surfaces, the surface shear stress on the ground between the elements (τS) and the drag force on the elements for each roughness array. The measurements indicated that τS quickly reduced near the leading edge of the roughness compared with τ, and a τS minimum occurs at a normalized distance (x/h, where h is element height) of
(downwind of the roughness leading edge is negative), then recovers to a relatively stable value. The location of the minimum
appears to scale with element height and not roughness density. The force on the elements decreases exponentially with normalized
downwind distance and this rate of change scales with the roughness density, with the rate of change increasing as roughness
density increases. Average τS : τ values for the six roughness surfaces scale predictably as a function of roughness density and in accordance with a shear
stress partitioning model. The shear stress partitioning model performed very well in predicting the amount of surface shear
stress, given knowledge of the stated input parameters for these patches of roughness. As the shear stress partitioning relationship
within the roughness appears to come into equilibrium faster for smaller roughness element sizes it would also appear the
shear stress partitioning model can be applied with confidence for smaller patches of smaller roughness elements than those
used in this experiment. 相似文献
9.
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
10.
Water-flume experiments are conducted to study the structure of turbulent flow within and above a sparse model canopy consisting
of two rigid canopies of different heights. This difference in height specifies a two-dimensional step change from a rough
to a rougher surface, as opposed to a smooth-to-rough transition. Despite the fact that the flow is in transition from a rough
to a rougher surface, the thickness of the internal boundary layer scales as x
4/5, consistent with smooth-to-rough boundary layer adjustment studies, where x is the downstream distance from the step change. However, the analogy with smooth-to-rough transitions no longer holds when
the flow inside the canopy and near the canopy top is considered. Results show that the step change in surface roughness significantly
increases turbulence intensities and shear stress. In particular, there is an adjustment of the mean horizontal velocity and
shear stress as the flow passes over the rougher canopy, so that their vertical profiles adjust to give maximum values at
the top of this canopy. We also observe that the magnitude and shape of the inflection in the mean horizontal velocity profile
is significantly affected by the transition. The horizontal and vertical turbulence spectra compare well with Kolmogorov’s
theory, although a small deviation at high frequencies is observed in the horizontal spectrum within the canopy. Here, for
relatively low leaf area index, shear is found to be a more effective mechanism for momentum transfer through the canopy structure
than vortex shedding. 相似文献
11.
Near Wall Flow over Urban-like Roughness 总被引：3，自引：35，他引：3
In this study, comprehensive measurements over a number of urban-type surfaces with the same area density of 25% have been performed in a wind tunnel. The experiments were conducted at a free stream velocity of 10 m s-1 and the main instrumentation was 120 ° x-wire anemometry, but measurement accuracy was checked using laser Doppler anemometry.The results haveconfirmed the strong three-dimensionalityof the turbulent flow inthe roughness sublayer and the depths of the inertial sublayer (log-law region) and roughness sublayer for each surface have been determined. Spatial averaging has been used to remove the variability of the flow in the roughness sublayer due to individual obstacles and it is shown that the spatially averaged mean velocity in the inertial sublayer and roughness sublayer can,together, be described by a single log-law with a mean zero-plane displacement and roughness length for the surface, provided that the proper surface stress is known. The spatially averaged shear stresses in the inertial sublayer and roughness sublayer are compared with the surface stress deduced from form drag measurements on the roughness elements themselves.The dispersive stress arising from the spatial inhomogeneity in the mean flow profiles was deduced from the data and is shown to be negligible compared with the usual Reynolds stresses in the roughness sublayer. Comparisons have been made between a homogeneous (regular element array) surface and one consisting of random height elements of the same total volume. Although the upper limits of the inertial sublayer for both surfaces were almost identical at equivalent fetch, the roughness sublayer was much thicker for the random surface than for the uniform surface, the friction velocity and the roughness length were significantly larger and the `roughness efficiency' was greater. It is argued that the inertial sublayer may not exist at all in some of the more extreme rough urban areas. These results will provide fundamental information for modelling urban air quality and forecasting urban wind climates. 相似文献
12.
A wind-tunnel experiment was designed and carried out to study the effect of a surface roughness transition on subfilter-scale
(SFS) physics in a turbulent boundary layer. Specifically, subfilter-scale stresses are evaluated that require parameterizations
and are key to improving the accuracy of large-eddy simulations of the atmospheric boundary layer. The surface transition
considered in this study consists of a sharp change from a rough, wire-mesh covered surface to a smooth surface. The resulting
magnitude jump in aerodynamic roughnesses, M = ln(z
01/z
02), where z
01 and z
02 are the upwind and downwind aerodynamic surface roughnesses respectively, is similar to that of past experimental studies
in the atmospheric boundary layer. The two-dimensional velocity fields used in this study are measured using particle image
velocimetry and are acquired at several positions downwind of the roughness transition as well as over a homogeneous smooth
surface. Results show that the SFS stress, resolved strain rate and SFS transfer rate of resolved kinetic energy are dependent
on the position within the boundary layer relative to the surface roughness transition. A mismatch is found in the downwind
trend of the SFS stress and resolved strain rate with distance from the transition. This difference of behaviour may not be
captured by some eddy-viscosity type models that parameterize the SFS stress tensor as proportional to the resolved strain
rate tensor. These results can be used as a benchmark to test the ability of existing and new SFS models to capture the spatial
variability SFS physics associated with surface roughness heterogeneities. 相似文献
13.
The budget equations of turbulent kinetic energy and shear stress contain interaction terms of velocity-pressure and velocity-pressure gradient. These terms were estimated in the surface layer using the air pressure observed at the surface and wind velocity components over plant canopies. The magnitude of the pressure interaction terms was significantly large; it was not negligible compared with the production terms in each budget equation. The present results obtained over a rough surface also confirmed previous results that pressure terms play an important role in the turbulent kinetic energy budgets and the shear stress budget. The height dependency of nondimensional pressure terms versus (z - d)/z
0 was not clear. 相似文献
14.
Turbulence above and within canopies has characteristics distinct from that over rough surfaces. The vertical transport of
momentum and scalars is dominated by coherent structures whose origin is now thought to be the result of the unstable inflexion
in the profile of the mean wind speed established by the application of canopy drag. This distinctive property leads to the
failure of the standard Monin–Obukhov flux–profile relationships over homogeneous canopies, relationships that are assumed
in many surface exchange schemes within numerical weather prediction and general circulation models. A modification of the
flux–profile relationships is presented that incorporates the effects of the canopy turbulence. The subsequent impacts on
the evolution of the surface energy balance and boundary-layer state are investigated within a simple numerical model for
the evolution of the boundary layer and canopy state. By comparing cases with and without the modification it is shown that
canopy-generated turbulence can lead, not only to the alteration of the flux–profile relationships above the canopy, but also
to a different evolution of the surface energy balance and differences in near-surface conditions that would be significant
in numerical weather prediction. More fundamentally, the modifications to the flux–profile relationships imply that parameters
such as the roughness length and displacement height for canopies should not be considered as invariant properties, but rather
as properties that depend on the flow and hence vary systematically with the diabatic stability of the boundary layer. 相似文献
15.
Miles' inviscid theory of surface wave generation by wind is (a) modified by replacing the logarithmic shear velocity profile with one which applies right down to the wave surface and which exhibits an explicit dependence on the roughness of the surface, and (b) extended to include the effects of the interaction of wave with air flow turbulence by considering the wave-modified mean flow as the mean of the actual turbulent air flow over water waves and using this in a mixing-length model.The surface pressure is shown to depend significantly on the flow conditions being aerodynamically smooth or rough. Its component in phase with the surface elevation is practically unaffected by the wave-turbulence interaction. However, such interaction tends to increase the rate of energy input ß from wind to waves travelling in the same direction, e.g., the increase is 2gk
2 for aerodynamically rough flow, where gk is the Von Karman constant. It also provides damping of waves in an adverse wind which can be about 10% of the growth rate in a favourable wind. 相似文献
16.
17.
Wind profile and eddy-correlation data obtained at two sites on a melting glacier surface in Iceland during the summer of 1996 are presented. Throughout the experiment the surface roughness increased rapidly from smooth to very rough, with the largest roughness element height obtained being about 1.7 m. In a layer close to the rough surface we find that the wind speed profiles were disturbed showing horizontal inhomogeneities as in a roughness sublayer. Its height was approximately two times the height of the main roughness elements (h) at both sites throughout the experiment. From the wind profiles and eddy-correlation data we calculated corrections for the displaced zero plane as a function of time and compared these with results obtained from a drag partitioning model. In general, the agreement was reasonable considering the ranges of uncertainty but the results indicate that the increasing horizontal anisotropy of the surface probably limits the use of the model. The values obtained for the roughness lengths are in good agreement with those calculated from a simple linear model, i.e., z0/h = 0.5 with the frontal area index. Above the roughness sublayer the wind profiles, normalised standard deviations of wind speed, and the balance of the turbulence kinetic energy budget behaved as over an ideal homogeneous surface thereby confirming similarity of the flow. 相似文献
18.
A turbulent energy model is developed to simulate the response of a neutrally stratified atmospheric boundary layer to sudden changes in surface roughness. A mechanism of turbulent energy transfer is proposed, based upon the results of numerical experiments, that explains the distribution of shear stress and hence the distribution of velocity profiles in the atmospheric surface layer. Two length scales associated with the turbulent energy equation are obtained from experimental data and the law of the wall. Turbulent energy is also predicted.The predicted growth of the internal boundary layer is slower than that obtained from mixing-length models. Also, the predicted surface shear stress obtained from the turbulent energy model is in better agreement with field data than that obtained from mixing-length models. 相似文献
19.
M. W. Rotach 《Boundary-Layer Meteorology》1993,65(1-2):1-28
20.
The three-dimensional wind velocity and dynamic pressure for stationary tornado-like vortices that developed over ground of different roughness categories were investigated to clarify the effects of ground roughness. Measurements were performed for various roughness categories and two swirl ratios. Variations of the vertical and horizontal distributions of velocity and pressure with roughness are presented, with the results showing that the tangential, radial, and axial velocity components increase inside the vortex core near the ground under rough surface conditions. Meanwhile, clearly decreased tangential components are found outside the core radius at low elevations. The high axial velocity inside the vortex core over rough ground surface indicates that roughness produces an effect similar to a reduced swirl ratio. In addition, the pressure drop accompanying a tornado is more significant at elevations closer to the ground under rough compared with smooth surface conditions. We show that the variations of the flow characteristics with roughness are dependent on the vortex-generating mechanism, indicating the need for appropriate modelling of tornado-like vortices. 相似文献