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排序方式: 共有323条查询结果,搜索用时 15 毫秒
91.
Muk Chen Ong Torbjørn Utnes Lars Erik Holmedal Dag Myrhaug Bjørnar Pettersen 《Coastal Engineering》2010
High Reynolds number flows around a circular cylinder close to a flat seabed have been computed using a two-dimensional standard high Reynolds number k–ε turbulence model. The effects of gap to diameter ratio, Reynolds number and flat seabed roughness for a given boundary layer thickness of the inlet flow upstream of the cylinder have been investigated. Hydrodynamic quantities and the resulting bedload transport have been predicted, and the vortex shedding mechanisms have been investigated. Predictions of hydrodynamic quantities around a cylinder located far away from the bed (so that the effect of the bed is negligible) are in satisfactory agreement with published experimental data and numerical results obtained for the flow around an isolated cylinder. Results for lower Reynolds number flows have also been computed for comparison with the high Reynolds number flow results. Overall it appears that the present approach is suitable for design purposes at high Reynolds numbers which are present near the seabed in the real ocean. 相似文献
92.
A depth-integrated model for weakly dispersive, turbulent, and rotational fluid flows 总被引:2,自引:0,他引:2
A set of weakly dispersive Boussinesq-type equations, derived to include viscosity and vorticity terms in a physically consistent manner, is presented in conservative form. The model includes the approximate effects of bottom-induced turbulence, in a depth-integrated sense, as a second-order correction. Associated with this turbulence, vertical and horizontal rotational effects are captured. While the turbulence and horizontal vorticity models are simplified, a model with known physical limitations has been derived that includes the quadratic bottom friction term commonly added in an ad hoc manner to the inviscid equations. An interesting result of this derivation is that one should take care when adding such ad hoc models; it is clear from this exercise that (1) it is not necessary to do so – the terms can be included through a consistent derivation from the viscous primitive equations – and (2) one cannot properly add the quadratic bottom friction term without also adding a number of additional terms in the integrated governing equations. To solve these equations numerically, a highly accurate and stable model is developed. The numerical method uses a fourth-order MUSCL-TVD scheme to solve the leading order (shallow water) terms. For the dispersive terms, a cell averaged finite volume method is implemented. To verify the derived equations and the numerical model, four cases of verifications are given. First, solitary wave propagation is examined as a basic, yet fundamental, test of the models ability to predict dispersive and nonlinear wave propagation with minimal numerical error. Vertical velocity distributions of spatially uniform flows are compared with existing theory to investigate the effects of the newly included horizontal vorticity terms. Other test cases include comparisons with experiments that generate strong vorticity by the change of bottom bathymetry as well as by tidal jets through inlet structures. Very reasonable agreements are observed for the four cases, and the results provide some information as to the importance of dispersion and horizontal vorticity. 相似文献
93.
Robert B. Scott Brian K. Arbic Christina L. Holland Ayon Sen Bo Qiu 《Ocean Modelling》2008,23(3-4):102-112
Global, high-quality, satellite-based observation of oceanic currents over the past 13 years has revealed ubiquitous quasi-horizontal eddies in the mesoscale (tens to hundreds of kilometers), confirming the view of a highly turbulent ocean suggested by observational programs in the 1970s. Idealized quasigeostrophic turbulence models suggest mesoscale turbulent flow can vary between isotropic, and highly anisotropic zonal jets. Here we compare the zonal and meridional velocity variance from satellite altimetry. We find that, for an unexplained reason and despite the chaotic nature of turbulence, the surface flow is organized into mesoscale patches where either zonal or meridional velocity variance dominates. The patches persist over 13 years, much longer than the turbulent timescale of a few months. Implications include potentially highly anisotropic redistribution of tracers by the mesoscale flow. Zonally averaged velocity variances reveal a slight preference for meridional over zonal velocity variance. Realistic primitive equation models succeed in reproducing both the patchy structure in local preference for either zonal or meridional velocity variance, and the zonally averaged preference for meridional variance. Idealized models of fully developed, quasigeostrophic turbulence fail in both regards. 相似文献
94.
Michael Frech 《Boundary-Layer Meteorology》2007,123(3):385-393
This note reports on the influence of aircraft wake vortices on the estimation of the turbulent energy dissipation rate using
sonic anemometer measurements near the runway threshold. The wake vortex traces, which are generated at a height of about
65 m and subsequently evolve in ground effect, are clearly visible in the velocity components and temperature. The observed
temperature increase of 1 K appears related to the stably stratified atmospheric surface layer. The dissipation rate is estimated
from the longitudinal velocity power spectrum for a sample in a nocturnal boundary layer with and without a wake vortex signal.
In both cases an inertial subrange is found. For the analyzed sample the estimated dissipation rate is a factor of ten larger
compared to the undisturbed sample. Implications for operational wake avoidance systems are discussed. 相似文献
95.
A. R. Brown 《Boundary-Layer Meteorology》2005,114(1):233-239
Numerical simulations are presented of flow over small-scale three-dimensional hills embedded within the stable boundary layer. Large surface forces are associated with internal gravity waves excited by Fourier modes aligned with the ridge axes closely parallel to the wind. Even moderate anisotropy of the topography may then lead to the surface forces (in a frame aligned with the wind) being strongly sensitive to wind direction. However, the impact of the waves on forces in the direction of the geostrophic wind is relatively minor.The British Crowns right to retain a non-exclusive royalty-free license in and to any copyright is acknowledged. 相似文献
96.
Because of the combination of smallconcentrations and/or small fluxes, the determinationof air–sea gas fluxes presents unusual measurementdifficulties. Direct measurements (i.e., eddycorrelation) of the fluxes are rarely attempted. Inthe last decade, there has been an intense scientificeffort to improve measurement techniques and to placebulk parameterizations of gas transfer on firmertheoretical grounds. Oceanic tracer experiments,near-surface mean concentration profiles, eddyaccumulation, and direct eddy covariance methods haveall been used. Theoretical efforts have focusedprimarily in the realm of characterizing the transferproperties of the oceanic molecular sublayer. Recentmajor field efforts organized by the U.S.A. (GASEX-98) andthe European Union (ASGAMAGE) have yielded atmospheric-derivedresults much closer to those from oceanographicmethods. In this paper, we review the physical basisof a bulk-to-bulk gas transfer parameterization thatis generalized for solubility and Schmidt number. Wealso discuss various aspects of recent sensor andtechnique developments used for direct measurementsand demonstrate experimental progress with resultsfrom ASGAMAGE and GASEX-98. It is clear that sensornoise, sensitivity, and cross talk with other speciesand even ship motion corrections still need improvement foraccurate measurements of trace gas exchange over theocean. Significant work remains to resolve issuesassociated with the effects of waves, bubbles, andsurface films. 相似文献
97.
98.
Turbulent wall pressure data acquired during tests of a full-scale experimental towed array over a range of tow speeds in straight tows and turns is presented. The experimental towed array contained a linear array of sensors mounted at the fluid–solid interface to measure the spectra of the wall pressure fluctuations due to the cylindrical turbulent boundary layer. The physics are dominated by the growth of a thick, high Reynolds number turbulent boundary layer at arc length Reynolds numbers as high as 9×108. The measured wavenumber-frequency spectra, autospectra, cross-spectral decay and convection velocities are presented. A well-defined convective ridge exists in the wavenumber-frequency spectra obtained during straight tows and turns. Turns give rise to a complicated fluid–structure interaction problem, but do not lead to the separation of the turbulent boundary layer. As the array moves through a turn, flow-induced vibrations of the array are shown to dominate the spectra at low frequencies, with more rapid decay in the measured coherence occurring at higher frequencies. The use of tow speed as a velocity scale is shown to collapse autospectra and convection velocities. 相似文献
99.
Characteristics of turbulent boundary layers over a rough bed under saw-tooth waves and its application to sediment transport 总被引:1,自引:0,他引:1
A large number of studies have been done dealing with sinusoidal wave boundary layers in the past. However, ocean waves often have a strong asymmetric shape especially in shallow water, and net of sediment movement occurs. It is envisaged that bottom shear stress and sediment transport behaviors influenced by the effect of asymmetry are different from those in sinusoidal waves. Characteristics of the turbulent boundary layer under breaking waves (saw-tooth) are investigated and described through both laboratory and numerical experiments. A new calculation method for bottom shear stress based on velocity and acceleration terms, theoretical phase difference, φ and the acceleration coefficient, ac expressing the wave skew-ness effect for saw-tooth waves is proposed. The acceleration coefficient was determined empirically from both experimental and baseline k–ω model results. The new calculation has shown better agreement with the experimental data along a wave cycle for all saw-tooth wave cases compared by other existing methods. It was further applied into sediment transport rate calculation induced by skew waves. Sediment transport rate was formulated by using the existing sheet flow sediment transport rate data under skew waves by Watanabe and Sato [Watanabe, A. and Sato, S., 2004. A sheet-flow transport rate formula for asymmetric, forward-leaning waves and currents. Proc. of 29th ICCE, ASCE, pp. 1703–1714.]. Moreover, the characteristics of the net sediment transport were also examined and a good agreement between the proposed method and experimental data has been found. 相似文献
100.