共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
Umberto Rizza Cristina Mangia Jonas C. Carvalho Domenico Anfossi 《Boundary-Layer Meteorology》2006,120(1):25-37
The inertial subrange Kolmogorov constant C
0, which determines the effective turbulent diffusion in velocity space, plays an important role in the Lagrangian modelling of pollutants. A wide range of values of the constant are found in the literature, most of them determined at low Reynolds number and/or under different assumptions. Here we estimate the constant C
0 by tracking an ensemble of Lagrangian particles in a planetary boundary layer simulated with a large-eddy simulation model and analysing the Lagrangian velocity structure function in the inertial subrange. The advantage of this technique is that it easily allows Reynolds numbers to be achieved typical of convective turbulent flows. Our estimates of C
0 is C
0=4.3±0.3 consistent with values found in the literature 相似文献
3.
Donald O. Hodgins 《大气与海洋》2013,51(3):285-298
Abstract Compliant fairing made of plastic ribbons or flexible rubber filaments can be attached to the cables of taut‐line moorings to reduce the drag force of the currents. The efficiency of such fairing depends upon its orientation in the flow and the Reynolds number of the cable. Results of using filament fairing manufactured by ENDECO Inc. on a 337‐m long mooring in the mouth of Hudson Strait are discussed. The fairing was found to be detrimental in this application because of twisting of the wire rope, producing a “bottle‐brush” configuration, and because the normal drag coefficient Cd ? 2.5 ± 0.45 was found to exceed that of a bare cable by 65 per cent on average. The Reynolds number range for this drag coefficient was 2.6 × 103 to 4.2 × 103 and was determined by force measurements in a flume. It is concluded that the problems of correctly orienting the fairing in the flow, and the true drag coefficient for Reynolds numbers less than about 5 × 104 must be carefully addressed in the design of conventional taut‐line moorings. Drag coefficients found in this study imply that compliant fairings would not be warranted for Reynolds numbers below about 5 × 104 unless strumming was expected for bare cables. 相似文献
4.
We use large-eddy simulation (LES) to study the turbulent pressure field in atmospheric boundary layers with free convection, forced convection, and stable stratification. We use the Poisson equation for pressure to represent the pressure field as the sum of mean-shear, turbulence–turbulence, subfilter-scale, Coriolis, and buoyancy contributions. We isolate these contributions and study them separately. We find that in the energy-containing range in the free-convection case the turbulence–turbulence pressure dominates over the entire boundary layer. That part dominates also up to midlayer in the forced-convection case; above that the mean-shear pressure dominates. In the stable case the mean-shear pressure dominates over the entire boundary layer.We find evidence of an inertial subrange in the pressure spectrum in the free and forced-convection cases; it is dominated by the turbulence–turbulence pressure and has a three-dimensional spectral constant of about 4.0. This agrees well with quasi-Gaussian predictions but is a factor of 2 less than recent results from direct numerical simulations at moderate Reynolds numbers. Measurements of the inertial subrange pressure spectral constant at high Reynolds numbers, which might now be possible, would be most useful. 相似文献
5.
Flow over surface obstructions can produce significantly large wind shears such that adverse flying conditions can occur for aeronautical systems (helicopters, V/STOL vehicles, etc.). The purpose of this analysis is to determine the kinds of flow fields that can result from surface obstructions in an otherwise horizontally homogeneous statistically stationary flow. The technique is based on the boundary-layer/Boussinesq-approximated equations of motion. The pressure gradient resulting from the surface obstruction is that consistent with a potential flow over a two-dimensional cylinder with elliptical cross-section, an approach commonly used for boundary-layer analyses in the engineering community. The dissipative effects of atmospheric turbulence on the mean flow are represented with eddy-viscosity models of the Reynolds stresses. The upstream flow is a neutral one and is characterized by a logarithmic profile for the mean wind. The following conclusions result from the analysis: (1) localized maxima in wind speed occur at the top of a surface obstruction, which are expected in physically real flow situations, (2) an increase in the elliptical aspect ratio decreases the wind speed within the boundary layer at the top of the ellipse and returns it to the logarithmic distribution characteristic of undisturbed flow, (3) increases in surface roughness affect the flow by decreasing the velocity in the boundary layer, with the most pronounced effect occurring near the surface of the smaller aspect-ratio ellipse, (4) Reynolds number has a negligible effect on the overall flow for the range of Reynolds numbers considered in this study, (5) a decrease in the elliptical aspect ratio and an increase in the surface roughness cause larger separation regions. 相似文献
6.
Johannes Peter Bhm 《Atmospheric Research》1992,27(4)
A general parameterization for solid and liquid hydrometeors is presented. hydrometeors basically are viewed as porous spheroids with the following variable parameters: diameter, axial ratio, mass, and porosity. Based on this parameterization a functional dependence on the Reynolds number of the drag of hydrometeors is derived, which is based on boundary layer theory. The basic form of this functional dependence is consistent with viscous theory and the inertial drag at low Reynolds numbers is predicted with good accuracy by matching the results from the boundary layer theory with Oseen's theory of creeping motion. Based on this solution a general semi-empirical expression for the Reynolds number and fall speed of particles is found. The results from the present theory are in remarkable agreement with experiments: The errors generally are < 5–10% for a wide variety of hydrometeors in the range of Reynolds numbers 0<NRe<5×105, including columnar and variously branched planar ice crystals, rimed and unrimed aggregates, lump, conical, and hexagonal graupel, hail, and rain drops. The present parameterisation aims far beyond the limits of the conventional methods since it is suitable for mixed-phase models of the microphysics of precipitation with continuously varying particle mass and shape characteristics and including processes such as depositional growth of ice crystals under varying environmental conditions, collisional growth of particles, and melting. 相似文献
7.
Kiyosi Kawanisi Mahmoud Faghfour Maghrebi Shōitirō Yokosi 《Boundary-Layer Meteorology》1993,64(1-2):1-14
Instantaneous three-dimensional velocity fields in the wake of a hemisphere located on an open channel bed are obtained by using multi-section flow visualization and a Mass-Consistent model. Instantaneous pictures of the organized structures are taken at a short time interval to investigate their behaviors at high Reynolds number. At high Reynolds number, the large vortex in the hemisphere wake is similar to the hairpin vortex observed at low Reynolds number. However, the long legs of the hairpin in the streamwise direction are not observed. A secondary hairpin vortex has been generated on the upstream side of the large hairpin vortex. Instantaneous spatial distributions of fluctuating velocity, vorticity production, etc. are presented and discussed. 相似文献
8.
9.
Specification of the eddy exchange coefficients is perhaps one of the most difficult problems in the numerical modeling of the planetary boundary layer. These coefficients have been computed from finite-difference analogs to analytical expressions associated with surface boundary-layer similarity theory, which is based on observations in an equilibrium surface layer. This procedure leads to erroneous results in the region above the surface layer and in a non-equilibrium surface layer. In addition, differencing problems arise in regions of small vertical wind shear. A new turbulence transport model has been obtained through the closure procedures for the transport equations of the Reynolds stress and the turbulent length scale. The new approach could be used to calculate Reynolds stresses and eddy exchange coefficients throughout a non-neutral planetary boundary layer under non-equilibrium conditions. 相似文献
10.
Observations made with the help of a movable-bed tank designed and operated to freeze the motion of gravity current fronts indicate that both the scale of the apparatus and the magnitude of the current Reynolds number have a strong influence on mixing rates. Low mixing rates are associated with fronts in relatively shallow fresh water depths and low Reynolds numbers. Mixing rates increase more rapidly as the ratio of current thickness to fresh water depth increases. For the deepest fresh water flow conditions, when scale effects can be expected to be negligible, the dimensionless mixing rate is approximately equal to three times the relative current thickness and has an upper limit of about 0.3. Lower mixing rates are observed as the current Reynolds number decreases, suggesting that viscous effects can still exist even for deep fresh water conditions. Application of the experimental results to estimate some parameters of a hypothetical gravity current on the continental shelf yield reasonable values. However, it is clear that, owing to the scale and Reynolds number effects that might be present in laboratory experiments, particular care should be exercised when trying to extrapolate results to gravity currents in nature. 相似文献
11.
The turbulent flow in and above plant canopies is of fundamental importance to the understanding oftransport processes of momentum,heat and mass between plant canopies and atmosphere,and to microme-teorology.The Reynolds stress equation model(RSM)has been applied to calculate the turbulence in cano-pies in this paper.The calculated mean wind velocity profiles,Reynolds stress,turbulent kinetic energy andviscous dissipation rate in a corn canopy and a spruce forest are compared with field observed data and withWilson's and Shaw's model.The velocity profiles and Rynolds stress calculated by both models are in goodagreement,and the length scale of turbulence appears to be similar. 相似文献
12.
Adrie F. G. Jacobs 《Boundary-Layer Meteorology》1985,32(4):329-335
In a full-scale field study, the drag coefficient of a thin closed fence was estimated. A measurement plate was constructed more or less in the center of the barrier in order to estimate the normal-force on the fence. The normal-force coefficient was studied under near-neutral stratification for various Reynolds numbers and different angles of attack. 相似文献
13.
Entrainment fluxes in a shear-free convective boundary layer have been measured with a saline water tank set-up. The experiments were targeted towards measuring the entrainment behaviour for medium to high Richardson numbers and use a two-layer design, i.e. two stacked non-stratified (neutral) layers with different densities. With laser induced fluorescence (LIF), the entrainment flux of a fluorescent dye is measured for bulk Richardson numbers in the range 30–260. It is proposed that a carefully chosen combination of top-down and bottom-up processes improves the accuracy of LIF-based entrainment observations. The observed entrainment fluxes are about an order of magnitude lower than reported for thermal water tanks: the derived buoyancy entrainment ratio, $A$ , is found to be $A \approx 0.02$ , which is to be compared with $A\approx 0.25$ for a thermal convection tank (Deardorff et al., J Fluid Mech 100:41–64, 1980). An extensive discussion is devoted to the influence of the Reynolds and Prandtl numbers in laboratory experiments on entrainment. 相似文献
14.
A Note On The Contribution Of Dispersive Fluxes To Momentum Transfer Within Canopies 总被引：5，自引：12，他引：5
Dispersive flux terms are formed when the time-averaged meanmomentum equation is spatially averaged within the canopy volume.These fluxes represent a contribution to momentum transfer arisingfrom spatial correlations of the time-averaged velocity componentswithin a horizontal plane embedded in the canopy sublayer (CSL).Their relative importance to CSL momentum transfer is commonlyneglected in model calculations and in nearly all fieldmeasurement interpretations. Recent wind-tunnel studies suggestthat these fluxes may be important in the lower layers of thecanopy; however, no one study considered their importance acrossall regions of the canopy and for a wide range of canopy roughnessdensities. Using detailed laser Doppler anemometry measurementsconducted in a model canopy composed of cylinders within a largeflume, we demonstrate that the dispersive fluxes are onlysignificant (i.e., >10%) for sparse canopies. These fluxes arein the same direction as the turbulent flux in the lower layers ofthe canopy but in the opposite direction near the canopy top. Fordense canopies, we show that the dispersive fluxes are <5% atall heights. These results appear to be insensitive to theReynolds number (at high Reynolds numbers). 相似文献
15.
《Dynamics of Atmospheres and Oceans》1987,11(1):59-86
The flow of a homogenous, incompressible, rotating (vertically upward) fluid past cylinders of triangular and semi-circular cross-section mounted on either the left or right wall (facing downstream) of a channel is investigated experimentally. The pertinent system parameters are the Rossby and Ekman (or Reynolds) numbers and the obstacle width to fluid depth ratio. The experiments indicate that the shedding of tip eddies from the triangular obstacle leads to a rather complex wake motion which is critically dependent on the system parameters. For certain parameter combinations the tip eddies advect downstream as single entities while in other regions of parameter space two or more eddies merge and advect downstream as large-scale eddy structures.The Strouhal numbers for both the shedding of the tip eddies as well as of the large-scale structures are measured as functions of the system parameters. Measurements of the dimensionless size of the large-scale starting eddies are made as functions of a dimensionless time and other system parameters. It is shown that eddies formed in the lee of obstacles mounted on the right (anticyclonic) tend to shed more quickly, other parameters being fixed, than those on the left (cyclonic).Measurements of the dimensionless vorticity of the cores of the large-scale structures at a fixed dimensionless time indicate that, within the accuracy of the experiments and for the range of parameters considered, this quantity is independent of the Rossby and Reynolds numbers and the side to which the obstacle is mounted. Finally some of the experimental flow patterns are shown to be similar to a recent observation of a southeastward ocean current past the western tip of Grand Bahama Island. 相似文献
16.
深凹地形边界层风场与湍流场结构及扩散规律的数值研究 总被引：1，自引：0，他引：1
建立了一个模拟小尺度复杂地形边界层平均流场和湍流场结构分布特征的三维非静力细网格高阶矩湍流闭合模式,并将此模式与一个拉格朗日粒子随机游动大气扩散数值模式相联结,以边界层气象模式的输出作为扩散模式的输入,构成了一个大气扩和模拟系统,成功地模拟了某地实际深凹露天矿区和理想凹坑地形两种情况下的平均流场、雷诺应力和湍流通量场,分析了其分布特征,并模拟了矿坑内污染物的散布规律 相似文献
17.
The vortex structure formed in front of and behind a hemispheric surface hump has been studied in a water recirculating flume. In the recirculation zone behind the hemisphere, arched vortex tubes were formed. Up to a certain critical Reynolds number, each tube was shed individually from the recirculation zone. However, when the Reynolds number exceeded a critical value, several of the tubes coalesced in the recirculation zone before shedding. In front of the hemisphere, some parabolic vortex tubes formed. Their number increased with Reynolds number below its critical value and decreased above this value. The dimensionless height of the parabolic vortex tube decreased with Reynolds number. 相似文献
18.
P. A. Iyer 《Boundary-Layer Meteorology》1973,5(1-2):53-66
An analysis is made of the stability of a buoyancy boundary layer existing on an inclined wall which is either heated or cooled relative to ambient, stably stratified fluid. A Boussinesq fluid, with various Prandtl numbers, is considered. Detailed calculations of the linear stability boundaries are made for both streamwise periodic, travelling disturbances and spanwise periodic, stationary disturbances. The former type is found to become unstable first for all angles of tilt, but calculations at a particular angle indicate that the latter can have a higher growth rate once the Reynolds number is sufficiently above the critical value. Energy integrals are evaluated at the critical Reynolds number for various angles of tilt in order to clarify the mechanisms for energy transfer to the disturbance. 相似文献
19.
We study turbulent flow over two-dimensional hills. The Reynolds stresses are represented by a second-order closure model, where advection, diffusion, production and dissipation processes are all accounted for. We solve a full set of primitive non-hydrostatic dynamic equations for mean flow quantities using a finite-difference numerical method. The model predictions for the mean velocity and Reynolds stresses are compared with the measured data from a wind-tunnel experiment that simulates the atmospheric boundary layer. The agreement is good. The performance of the second-order closure model is also compared withthat of lower level turbulence models, including the eddy-viscositymodel and algebraic Reynolds stress models. It is concluded that thepresent closure is a considerable improvement over the other modelsin representing various physical effects in flow over hills. Thefeasibility of running a finite-difference numerical simulationincorporating a full second-order closure model on an IBM workstationis also demonstrated. 相似文献
20.
A lattice Boltzmann (LB) model with overall second-order accuracy is applied to the 1.5-layer shallow water equation for a wind-driven double-gyre ocean circulation. By introducing the second-order integral approximation for the collision operator, the model becomes fully explicit. In this case, any iterative technique is not needed. The Coriolis force and other external forces are included in the model with second-order accuracy, which is consistent with the discretized accuracy of the LB equation. The numerical results show correct physics of the ocean circulation driven by the double-gyre wind stress with different Reynolds numbers and different spatial resolutions. An intrinsic low-frequency variability of the shallow water model is also found. The wind-driven ocean circulation exhibits subannual and interannual oscillations, which are comparable to those of models in which the conventional numerical methods are used. 相似文献