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82.
Numerical modelling of flow structures over idealized transverse aeolian dunes of varying geometry 总被引：2，自引：0，他引：2
A Computational Fluid Dynamics (CFD) model (PHOENICS™ 3.5) previously validated for wind tunnel measurements is used to simulate the streamwise and vertical velocity flow fields over idealized transverse dunes of varying height (h) and stoss slope basal length (L). The model accurately reproduced patterns of: flow deceleration at the dune toe; stoss flow acceleration; vertical lift in the crest region; lee-side flow separation, re-attachment and reversal; and flow recovery distance. Results indicate that the flow field over transverse dunes is particularly sensitive to changes in dune height, with an increase in height resulting in flow deceleration at the toe, streamwise acceleration and vertical lift at the crest, and an increase in the extent of, and strength of reversed flows within, the lee-side separation cell. In general, the length of the separation zone varied from 3 to 15 h from the crest and increased over taller, steeper dunes. Similarly, the flow recovery distance ranged from 45 to >75 h and was more sensitive to changes in dune height. For the range of dune shapes investigated in this study, the differing effects of height and stoss slope length raise questions regarding the applicability of dune aspect ratio as a parameter for explaining airflow over transverse dunes. Evidence is also provided to support existing research on: streamline curvature and the maintenance of sand transport in the toe region; vertical lift in the crest region and its effect on grainfall delivery; relations between the turbulent shear layer and downward forcing of flow re-attachment; and extended flow recovery distances beyond the separation cell. Field validation is required to test these findings in natural settings. Future applications of the model will characterize turbulence and shear stress fields, examine the effects of more complex isolated dune forms and investigate flow over multiple dunes. 相似文献
83.
84.
Air flow inside an array of cubes is simulated. Cubes (edge length 0.15 m) are arranged in a regular array, separated by 0.15 m
in the streamwise and spanwise directions. Numerical simulations are performed based on Reynolds-averaged Navier–Stokes equations
(RANS), solved in a computational fluid dynamics model (CFD), with standard k–ε turbulent closure (two prognostic equations are solved for the turbulent kinetic energy k and its dissipation ε, respectively). Simulations are validated against wind-tunnel data using a technique based on hit-rate
calculations, and calculated statistical parameters. The results show that the horizontal velocity is very well modelled,
and despite some discrepancies, the model that fulfils the hit-rate test criteria gives useful results that are used to investigate
three-dimensional (3-D) flow structures. The 3-D analysis of the flow shows interesting patterns: the centre of the canyon
vortex is at 3/4 of the canyon height, and stronger downward than upward motions are present within the canyon. Such behaviour
is explained by the presence of a compensation flow through the side of the canyon, which enters the canyon from the upper
part and exits from the lower part. This complex 3-D structure affects the tracer dispersion, and is responsible for pollutant
transport and diffusion. 相似文献
85.
针对黄土地层,提出利用空气代替钻井液的反循环气体喷射技术。但现有技术对反循环气体喷射钻头切削土体的能力没有具体优化讨论。利用fluent软件,优化反循环喷射钻头结构参数,对不同底喷孔直径、底喷孔数量、底喷孔扩大段直径、底喷孔扩大段数量、喷射孔数量5个结构参数进行模拟分析。分析结果表明,在进风流量一定的情况下,增加底喷孔的数量或增大底喷孔的直径会削弱钻头的空气射流切削能力和反循环能力。底喷孔直径为3 mm,底喷孔个数为2个,底喷孔扩大段直径为8 mm,底喷孔扩大段长度为5 mm,内喷孔数量为5个时,反循环喷射钻头喷射能力和反循环能力最优。 相似文献
86.
The turbulence closure in atmospheric boundary-layer modelling utilizing Reynolds Averaged Navier–Stokes (RANS) equations
at mesoscale as well as at local scale is lacking today a common approach. The standard kɛ model, although it has been successful for local scale problems especially in neutral conditions, is deficient for mesoscale
flows without modifications. The k–ɛ model is re-examined and a new general approach in developing two-equation turbulence models is proposed with the aim of
improving their reliability and consequently their range of applicability. This exercise has led to the replacement of the
ɛ-transport equation by the transport equation for the turbulence inverse length scale (wavenumber). The present version of
the model is restricted to neutrally stratified flows but applicable to both local scale and mesoscale flows. The model capabilities
are demonstrated by application to a series of one-dimensional planetary boundary-layer problems and a two-dimensional flow
over a square obstacle. For those applications, the present model gave considerably better results than the standard k–ɛ model. 相似文献
87.
88.
Green water loads on moored or sailing ships occur when an incoming wave significantly exceeds the freeboard and water runs onto the deck. In this paper, a Navier–Stokes solver with a free surface capturing scheme (i.e. the VOF model; [Hirt and Nichols, 1981]) is used to numerically model green water loads on a moored FPSO exposed to head sea waves. Two cases are investigated: first, green water on a fixed vessel has been analysed, where resulting waterheight on deck, and impact pressure on a deck mounted structure have been computed. These results have been compared to experimental data obtained by [Greco, 2001] and show very favourable agreement. Second, a full green water incident, including vessel motions has been modelled. In these computations, the vertical motion has been modelled by the use of transfer functions for heave and pitch, but the rotational contribution from the pitch motion has been neglected. The computed water height on deck has been compared to the experimental data obtained by [Buchner, 1995a] and it also shows very good agreement. The modelling in the second case was performed in both 2-D and 3-D with very similar results, which indicates that 3-D effects are not dominant. 相似文献
89.
Wells turbine with end plates for wave energy conversion 总被引：1，自引：0，他引：1
Manabu Takao Toshiaki Setoguchi Yoichi Kinoue Kenji Kaneko 《Ocean Engineering》2007,34(11-12):1790-1795
In order to improve the performance of the Wells turbine for wave energy conversion, the effect of end plate on the turbine characteristics has been investigated experimentally by model testing. As a result, it is found that the characteristics of the Wells turbine with end plates are superior to those of the original Wells turbine, i.e., the turbine without end plate and the characteristics are dependent on the size and position of end plate. Furthermore, by using a computational fluid dynamics (CFD), reason of the performance improvement of the turbine has been clarified and the effectiveness of the end plate has been demonstrated. 相似文献
90.
The slightly compressible flow formulation is applied to the free-surface, three-dimensional turbulent flow around a Wigley hull. Two turbulence models (large eddy simulation and Baldwin–Lomax) are used and compared. The simulation conditions are the ones for which experimental and numerical results exist. The computational grid is built using an algebraic grid generator with the model fixed in space. The codes use the interface-capturing technique for computing the free-surface displacements and the Beam and Warming scheme for marching in time the numerical model. The results compare well with the experimental data available. 相似文献