共查询到20条相似文献，搜索用时 31 毫秒
1.
Estimation of orographically induced wave drag in the stable boundary layer during the CASES-99 experimental campaign 总被引：1，自引：0，他引：1
This paper addresses the quantification of gravity wave drag due to small hills in the stable boundary layer. A single column
atmospheric model is used to forecast wind and temperature profiles in the boundary layer. Next, these profiles are used to
calculate vertical profiles of gravity wave drag. Climatology of wave drag magnitude and “wave drag events” is presented for
the CASES-99 experimental campaign. It is found that gravity wave drag events occur for several relatively calm nights, and
that the wave drag is then of equivalent magnitude as the turbulent drag. We also illustrate that wave drag events modify
the wind speed sufficiently to substantially change the surface sensible heat flux. 相似文献
2.
Vegetation is a key aspect of water resources and ecology in natural rivers, floodplains and irrigation channels. The hydraulic resistance of the water flow is greatly changed when submerged vegetation is present. Three kinds of drag coefficients, i.e., the drag coefficient for an isolated cylinder, the bulk drag coefficient of an array of cylinders and the vertically distributed or local drag coefficient, have been commonly used as parameters to represent the vegetation drag force. In this paper, a comprehensive experimental study of submerged stems in an open channel flow is presented. Empirical formulae for the three drag coefficients were obtained based on our experimental results and on data from previous studies. A two-layer model was developed to solve the mean momentum equation, which was used to evaluate the vertical mean velocity profile with each of the drag coefficients. By comparing the velocity distribution model predictions and the measurement results, we found that the model with the drag coefficient for an isolated cylinder and the local drag coefficient was good fit. In addition, the model with the bulk drag coefficient gave much larger velocity values than measurements, but it could be improved by adding the bed friction effect and making choice of the depth-averaged velocity within the canopy layer. 相似文献
3.
ZHANG Yi & TAN ZheMin? Key Laboratory of Mesoscale Severe Weather of Ministry of Education and Department of Atmospheric Sciences Nanjing Univer- sity Nanjing China 《中国科学D辑(英文版)》2007,50(2):306-319
Using a three-dimensional nonhydrostatic mesoscale numerical model (MM5), the evolution and structures of baroclinic waves with and without surface drag in case of dry and moist atmosphere are simulated, with special emphases on the effects of surface drag on the low-level frontal structure and frontogenesis. There are two different effects of surface drag on the low-level frontogenesis in the dry case. On one hand, the surface drag weakens the low-level frontogenesis and less inclined to develop the baroclinic wave due to the dissipation. But on the other hand, the surface drag induces a strong ageostrophic flow, which prolongs the low-level frontogenesis and finally leads to the enhancement of cold front. Compared with the no surface drag case, the surface drag increases the frontal slope espe- cially in the boundary layer, where the front is almost vertical to the surface, and then enhances the prefrontal vertical motion. All these conclusions expanded the analytical theory of Tan and Wu (1990). In the moist atmosphere, the influence of surface drag on frontal rainbands is also obvious. The surface drag weakens the convection, and reduces the energy dissipation near the surface when the initial relative humidity is relatively weak. At this time, the confluence induced post-frontal updrafts moves across the cold front and reinforces the prefrontal convection, which is beneficial to the maintenance of the rainband in cold sector. Given the enhancement of relative humidity, the moist convection domi- nates the low-level frontogenesis while the retardation of surface drag on energy dissipation is not obvious, therefore the effects of surface drag on the low-level frontogenesis and precipitation are re- duced. 相似文献
4.
5.
The response of gobi surfaces to the near-surface air flow can be characterized quantitatively by drag coefficients. By using wind tunnel tests, an attempt is made to define the relationship between the drag coefficients of gobi surfaces and gravel size and coverage. It is concluded that the drag coefficients of gobi surfaces tend to be constants when gravel coverage is over 40%-50%. Consequently, we think that the gobi deflation planes expanding vastly in the arid Northwestern China are aerodynamically stable, at least not the supplying sources of current dust storms, and therefore the emphasis on dust storm control should be paid on the so-called "earth gobi" that has low gravel coverage. The prediction model for drag coefficients of gobi surfaces has been developed by regressing drag coefficients on gravel size and coverage, the predicted results are in reasonably good agreement with wind tunnel results (R 2 = 0.94). The change of drag coefficients with gravel friction Reynolds number implies that the development extent of drag effect increases with gravel size and coverage. 相似文献
6.
The dimensionless Darcy–Weisbach coefficient of friction is used to evaluate the drag in channel flows. A developed turbulent flow with a quadratic drag law is considered. The dependence of the coefficient of friction on the cross-section shape of the channel flow is examined. A coefficient of the channel shape is introduced, which depends on the wetted perimeter and the flow width and allows the complicated geometry of the river cross-section to be taken into account in calculating the drag. The drag estimates calculated using the suggested technique are compared with other authors' estimates for flumes and rivers. 相似文献
7.
An analytical expression for the 10 m drag law in terms of the 10 m wind speed at the maximum in the 10 m drag coefficient, and the Charnock constant is presented, which is based on the results obtained from a model of the air-sea interface derived in Bye et al. (2010). This drag law is almost independent of wave age and over the mid-range of wind speeds (5?17 ms?1) is very similar to the drag law based on observed data presented in Foreman and Emeis (2010). The linear fit of the observed data which incorporates a constant into the traditional definition of the drag coefficient is shown to arise to first-order as a consequence of the momentum exchange across the air-sea boundary layer brought about by wave generation and spray production which are explicitly represented in the theoretical model. 相似文献
8.
Gregory M. Zhikharev 《地球物理与天体物理流体动力学》2013,107(1-4):101-108
Abstract Isallobaric effect of a slowly varying quasi-geostrophic flow represented by propagating waves may give rise to a mean steady topographic drag component which turns out to be the principal one when viscous effects are negligibly small. This drag component decreases, in contrast to the quasi-geostrophic component, when statistical properties of the topography become isotropic. When the phase velocity of the incident wave is much larger (smaller) than the phase speed of Rossby waves, the isallobaric drag becomes independent (dependent) on the sign of that velocity. 相似文献
9.
Ismail Albayrak Vladimir Nikora Oliver Miler Matthew O’Hare 《Aquatic Sciences - Research Across Boundaries》2012,74(2):267-286
The effects of leaf shape, serration, roughness and flexural rigidity on drag force imposed by flowing water and its time
variability were experimentally studied in an open-channel flume at seven leaf Reynolds numbers ranging from 5 to 35 × 103. The study involved artificial leaves of the same surface area but with three shapes (‘elliptic’, ‘rectangular’ and ‘pinnate’),
three flexural rigidities, smooth-edge and sawtooth-like serration, and three combinations of surface roughness (two-side
rough, one-side rough/one-side smooth, and two-side smooth). Shape was the most important factor determining flow-leaf interactions,
with flexural rigidity, serration and surface roughness affecting the magnitude but not the direction of the effect on drag
control. The smooth-edge elliptic leaf had a better hydrodynamic shape as it experienced less drag force, with the rectangular
leaf showing slightly less efficiency. The pinnate leaf experienced higher drag force than the other leaves due to its complex
geometry. It is likely that flow separation from 12 leaflets of the pinnate leaf prevented leaf reconfiguration such as leaflets
folding and/or streamlining. Flexural rigidity strongly influenced the leaf reconfiguration and augmented the serration effect
since very rigid leaves showed a strong effect of serration. Furthermore, serration changed the turbulence pattern around
the leaves by increasing the turbulence intensity. Surface roughness was observed to enhance the drag force acting on the
leaf at high Reynolds numbers. The results also suggest that there are two distinctly different flow-leaf interaction regimes:
(I) regime of passive interaction at low turbulence levels when the drag statistics are completely controlled by the turbulence
statistics, and (II) regime of active interaction at high turbulence levels when the effect of leaf properties on the drag
statistics becomes comparable to the turbulence contribution. 相似文献
10.
This work questions, starting from dimensional considerations, the generality of the belief that the marine drag coefficient levels off with increasing wind speed. Dimensional analysis shows that the drag coefficient scales with the wave steepness as opposed to a wave-age scaling. A correlation equation is employed here that uses wave steepness scaling at low aspect ratios (inverse wave steepnesses) and a constant drag coefficient at high aspect ratios. Invoked in support of the correlation are measurements sourced from the literature and at the FINO1 platform in the North Sea. The correlation equation is then applied to measurements recorded from buoys during the passage of hurricanes Rita, Katrina (2005) and Ike (2008). Results show that the correlation equation anticipates the expected levelling off in deeper water, but a drag coefficient more consistent with a Charnock type relation is also possible in more shallower water. Some suggestions are made for proceeding with a higher-order analysis than that conducted here. 相似文献
11.
《Limnologica》2020
Modern ethohydraulics is the study of the behavioral responses of swimming fish to flow fields. However, the exact drag forces experienced by fish remain poorly studied; this information is required to obtain a better understanding of the behavioral responses of fish and their current resistance strategies. We measured near-ground frontal drag forces on preserved individuals of three benthic fish species, round goby (Neogobius melanstomus), gudgeon (Gobio gobio) and bullhead (Cottus gobio), in a flow channel. The forces were compared to acoustic Doppler velocity (ADV) measurements and fish tracking data based on video observations of live fish in the flow channel. Overall, we observed drag coefficients (CD) of ∼10−3 at Reynolds numbers ∼105. The frontal drag forces acting on preserved fish with non-spread fins ranged from -1.96 mN*g-1 (force per fish wet weight, velocity 0.55 m*s-1) to 11.01 mN*g-1 (velocity 0.85 m*s-1). Spreading the fins strongly increased the drag forces for bullhead and round goby. In contrast, the drag forces were similar for gudgeon with spread fins and all fish with non-spread fins. Video tracking revealed no clear relationship between the position of the fish in the flow field and the forces experienced by the preserved fish at these positions. Collectively, these results suggest that i) the differences in frontal drag forces between species are small in homogenous flow, ii) individuals chose their position in the flow field based on factors other than the drag forces experienced, and iii) whether fins are spread or non-spread is an essential quality that modulates species-specific differences. The methodology and results of this study will enable integration of flow measurements, fish behavior and force measurements and inform ethohydraulics research. More advanced force measurements will lead to a detailed understanding of the current resistance strategies of benthic fish and improve the design of fish passes. 相似文献
12.
Month-long observations of waves and tidal currents at Ponce de Leon Inlet, North Florida are used to investigate the importance of wave-induced bottom drag as a mechanism for overtide generation in estuaries. While bottom drag can in theory lead to overtide generation, in practice, resolving unambiguously this effect is difficult as it tends to be overshadowed by the stronger effect of diurnal–semidiurnal tidal variance. Bottom boundary layer numerical simulations based on observational data suggest that waves can cause the bottom drag experienced by currents to increase by a factor of 1.7, compared with relatively calm conditions. Despite the relatively short duration and limited scope of the experiment, the analysis suggests that overtide modulations (East–West velocity components of the 5th and 6th diurnal constituents) are correlated with wave-enhanced drag trends. Therefore, wave-enhanced bottom drags may be enhancing generation of overtides. Further work is necessary to understand the scope and the strength of this mechanism, in relation to the characteristics (e.g., flow direction) of individual overtides. 相似文献
13.
We present an immersed structure approach for modeling the interaction between surface flows and vegetation. Fluid flow and rigid and flexible vegetative obstacles are coupled through a local drag relation that conserves momentum. In the presented method, separate meshes are used for the fluid domain and vegetative obstacles. Taking techniques from immersed boundary finite element methods, the effects of the fluid on the vegetative structures and vice versa are calculated using integral transforms. Using a simple elastic structure model we incorporate bending and moving vegetative obstacles. We model flexible vegetation as thin, elastic, inextensible cantilever beams. Using the immersed structure approach, a fully coupled fluid-vegetation interaction model is developed assuming dynamic fluid flow and quasi-static bending. This relatively computationally inexpensive model allows for thousands of vegetative obstacles to be included in a simulation without requiring an extremely refined fluid mesh. The method is validated with comparisons to mean velocity profiles and bent vegetation heights from experiments that are reproduced computationally. We test the method on several channel flow setups. We calculate the bulk drag coefficient in these flow scenarios and analyze their trends with changing model parameters including stem population density and flow Reynolds number. Bulk drag models are the primary method of incorporating small-scale drag from individual plants into a value that can be used in larger-scale models. Upscaled bulk drag quantities from this method may be utilized in larger-scale simulations of flow through vegetation regions. 相似文献
14.
Ismail Albayrak Vladimir Nikora Oliver Miler Matthew T. O’Hare 《Aquatic Sciences - Research Across Boundaries》2014,76(2):269-294
Flow–plant interactions are experimentally investigated at leaf, stem, and shoot scales in an open-channel flume at a range of Reynolds numbers. The experiments included measurements of instantaneous drag forces acting on leaves, stems, and shoots of the common freshwater plant species Glyceria fluitans, complemented with velocity measurements, high-resolution video recordings, and biomechanical tests of leaf and stem properties. The analyses of bulk statistics, power spectral densities, transfer functions, and cross-correlations of measured velocities and drag forces revealed that flow characteristics, drag force, and plant biomechanical and morphological properties are strongly interconnected and scale-dependent. The plant element–flow interactions can be subdivided into two classes: (I) passive interactions when the drag variability is due to the time variability of the wetted and frontal areas and squared approach velocity (due to the large-scale turbulence); and (II) active interactions representing a range of element-specific instabilities that depend on the element flexural rigidity and morphology. Implications of experimental findings for plant biophysics and ecology are briefly discussed. 相似文献
15.
The subduction of the Indian plate underneath Eurasian plate results not only in deformation and movement of the elastic upper crust, but also flow of the ductile lower crust in the high temperature and high pressure which drags the brittle upper crust to move at the same time. These two actions work together producing the present movement and deformation field in Tibetan plateau. The dynamics progress has been verified by GPS observation data. Therefore, in a two-dimension plain model, only the elastic deformation with the boundary action at the upper crust cannot explain the deformation well, the drag force acted on the base of upper crust by the drag of ductile flow of the lower crust also need to be considered. However, it's hard to figure out the magnitude and direction of the drag force. Thus, we established a two-dimension plain elastic finite element model, with the equivalent-body force approach to simulate the drag force. With the internal GPS observation data of Tibetan plateau as constraint condition, we calculated inversely the drag force of key nodes in the model with trial method, and the other nodes in the model with bilinear interpolation method. Finally, we got the drag forces(nodal forces, unit:N) caused by the difference flow of ductile lower crust dragging the brittle upper crust, which are distributed mainly in the region of 86°~100°E and 26°~32°N, the direction is east and south, and the maximum reaches to 1e8N; in some areas in the western part of the study region at 31°~36°N and 76°~80°E, the direction is west, and the maximum reaches to 1e7N. All these work provides a new thought for further research on long-term dynamic mechanism of surface deformation in Tibetan plateau and its surrounding area. 相似文献
16.
The flow of dense water along continental slopes is considered. There is a large literature on the topic based on observations and laboratory experiments. In addition, there are many analytical and numerical studies of dense water flows. In particular, there is a sequence of numerical investigations using the dynamics of overflow mixing and entrainment (DOME) setup. In these papers, the sensitivity of the solutions to numerical parameters such as grid size and numerical viscosity coefficients and to the choices of methods and models is investigated. In earlier DOME studies, three different bottom boundary conditions and a range of vertical grid sizes are applied. In other parts of the literature on numerical studies of oceanic gravity currents, there are statements that appear to contradict choices made on bottom boundary conditions in some of the DOME papers. In the present study, we therefore address the effects of the bottom boundary condition and vertical resolution in numerical investigations of dense water cascading on a slope. The main finding of the present paper is that it is feasible to capture the bottom Ekman layer dynamics adequately and cost efficiently by using a terrain-following model system using a quadratic drag law with a drag coefficient computed to give near-bottom velocity profiles in agreement with the logarithmic law of the wall. Many studies of dense water flows are performed with a quadratic bottom drag law and a constant drag coefficient. It is shown that when using this bottom boundary condition, Ekman drainage will not be adequately represented. In other studies of gravity flow, a no-slip bottom boundary condition is applied. With no-slip and a very fine resolution near the seabed, the solutions are essentially equal to the solutions obtained with a quadratic drag law and a drag coefficient computed to produce velocity profiles matching the logarithmic law of the wall. However, with coarser resolution near the seabed, there may be a substantial artificial blocking effect when using no-slip. 相似文献
17.
本文以德国低轨道卫星CHAMP为例,联合考虑地球扁率和大气阻力摄动的影响,对相应摄动方程进行数值积分,计算轨道根数变化,并进而计算得到卫星空间位置,由此模拟考察大气阻力引起的轨道高度衰减.模拟中使用综合考虑了太阳辐射和磁暴等多种因素影响的最新国际大气标准JB2008模式来计算热层大气密度. 选取CHAMP卫星轨道高度自然衰减（无点火提升卫星高度操作）的2005全年进行模拟;为了考察不同年份阻力系数的可能变化,对2002年1—3月处在较大高度的轨道也进行了模拟.考虑到CHAMP卫星的特殊几何构形及飞行高度的热层温度条件,取阻力系数大于2.8,并在一定范围内变化,以求得模拟与实际轨道衰减符合较好.结果表明,对于2005年,阻力系数为2.91时模拟得到的轨道高度的衰减与实际轨道衰减符合得最好,模拟与实际轨道半长轴全年的标准偏差为81m;在卫星高度稍高的2002年,模拟的最佳阻力系数为3.0;模拟所得最佳阻力系数值比传统使用的值2.2大30%以上.由于在模拟中忽略了高阶保守力分量,所得近/远地点高度没有出现实际轨道所显示的周期性起伏. 相似文献
18.
A model for the air–sea interface, based on the coupled pair of similarity relations for “aerodynamically” rough flow in both
fluids, is presented, which is applied to fetch-limited and high wind speed conditions which occur, for example, in hurricanes.
It is shown that the specification of the maximum 10-m drag coefficient and the 10-m wind speed and the peak wave speed at
which it occurs are sufficient to uniquely determine the drag law, which asymptotes at low wind speeds to a Charnock constant
similar to that for the fully developed wind wave sea and is almost independent of the peak wave speed at the maximum in drag
coefficient. A feature of the drag law is that it is of Charnock form, almost independent of the wave age, consistent with
the transfer of momentum to the wave spectrum being due to the smaller rather than the dominant wavelengths. The analysis
is also applied to a variable sea state in which either the surface wind or the surface Stokes drift vary, but the peak wave
speed is kept constant. The corresponding variability in the Charnock constant is in general accord with observations. 相似文献
19.
Shaking table tests on subgrade reaction of pipe embedded in sandy liquefied subsoil 总被引：3，自引：0，他引：3
I. Towhata W. Vargas-Monge R. P. Orense M. Yao 《Soil Dynamics and Earthquake Engineering》1999,18(5):1009
An interest in the behavior of liquefied sand during seismic flow failure led the authors to conduct shaking table tests in which an embedded pipe was pulled laterally and the required drag force was monitored. Test results showed that the amplitude of shaking acceleration affected the behavior of sand in both dry and water-saturated conditions. In dry sand, the induced inertia force decreased the shear strength and consequently the magnitude of the drag force. When the sand was saturated, a special consideration was made of the similitude of dilatancy between 1-G model tests and the in-situ situation. This goal was attained by employing very loose sand in model tests. The rate-dependency in which the drag force increased with the rate of pipe movement was focused on, leading to an apparently viscous behavior of sand. This is consistent with what several former studies reported. 相似文献
20.
In this paper, we use the inertial coupling relation as a similarity model for the air–sea boundary layer, to predict the 10-m drag coefficient. Excellent agreement with the commonly used statistical relationship of Garratt (1992) is found for a fully developed growing wind wave sea with a constant inertial drag coefficient, KI = 1.5 × 10–3. This suggests that the inertial coupling model can be used to realistically predict the 10 m drag coefficient under more general wind wave conditions.Acknowledgements The paper was completed while JATB was a Fellow at the Hanse-Wissenschaftskolleg in Delmenhorst, Germany, in July and August 2004. The comments of two anonymous reviewers are gratefully acknowledged. 相似文献