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921.
The stability, mixing and effect of downstream control on axisymmetric turbulent buoyant jets discharging vertically into
shallow stagnant water is studied using 3D Reynolds-averaged Navier–Stokes equations (RANS) combined with a buoyancy-extended
k –ε model. The steady axisymmetric turbulent flow, temperature (or tracer concentration) and turbulence fields are computed using
the finite volume method on a high resolution grid. The numerical predictions demonstrate two generic flow patterns for different
turbulent heated jet discharges and environmental parameters (i) a stable buoyant discharge with the mixed fluid leaving the
vertical jet region in a surface warm water layer; and (ii) an unstable buoyant discharge with flow recirculation and re-entrainment
of heated water. A stratified counterflow region always appears in the far-field for both stable and unstable buoyant discharges.
Provided that the domain radius L exceeds about 6H, the near field interaction and hence discharge stability is governed chiefly by the jet momentum length scale to depth ratio
lM/H, regardless of downstream control. The near field jet stability criterion is determined to be lM/H = 3.5. A radial internal hydraulic jump always exists beyond the surface impingement region, with a 3- to 6-fold increase
in dilution across the jump compared with vertical buoyant jet mixing. The predicted stability category, velocity and temperature/concentration
fields are well-supported by experiments of all previous investigators. 相似文献
922.
Multiple mating by social insect queens is a common phenomenon despite likely imposing substantial costs on queens. Mating with several males could be adaptive if a more genetically diverse worker force is better able to always handle any task sufficiently well, leading to a higher colony homeostasis. If multiple-paternity colonies are more homeostatic, then I propose that they may constitute less stressful rearing environments for developing sexuals. The effective stress levels experienced by developing males and queens may, however, also depend on colony productivity and sex-ratio preferences. I tested these hypotheses in the ant Lasius niger by examining whether the fluctuating asymmetry, means and coefficients of variation of a set of phenotypic traits in males and new queens co-varied with the effective number of patrilines per colony, colony productivity or sex ratio.Little support was found that the level of intra-colonial genetic diversity affects the variation of phenotype in sexuals. In 1 out of 2 years, however, females from colonies with high effective patriline numbers were heavier relative to their head width than were females from colonies with few patrilines. Support was found for the hypothesis that colonies with more resources may invest more in individual sexuals, and tendencies suggested that sexuals may receive better treatment when they belong to the majority sex of their colony.Communicated by J. Heinze 相似文献
923.
草炭溶解态有机物质与Cu2+、Cd2+络合稳定性的研究 总被引:2,自引:0,他引:2
研究了由草炭提取的胡敏酸(HA)、富里酸(FA)、水溶解态有机物质(WSOM)的基团特征,分子量分布及其与Cu2+、Cd2+两种金属离子之间的络合稳定性。对HA、FA及WSOM的红外光谱研究显示三者的官能团组成相似,只是含量略有不同。固态13C-NMR对HA、FA及WSOM结构特征的描述以及水相凝胶色谱法对三种有机质样品的分子量及分子量分布的测定结果显示HA分子最大且含有大量的长链烷烃,而FA分子小得多,结构以碳水化合物及多糖为主,WSOM的分子量略低于FA,结构与FA相近。离子交换平衡法对三种有机质样品与Cu2+、Cd2+的络合配位数和稳定常数的测定结果显示两种介质条件下络合配位数(x)、络合稳定常数(logk)的次序均为:Cu2+>Cd2+;同一介质条件下,对不同有机质而言均满足:M-HA>M-FA>M-WSOM。 相似文献
924.
Watershed ordinances in Austin, Texas, USA, are intended to protect streams from stormwater degradation. Their adequacy is
being questioned, however, where development is advancing into the Hill Country northwest and southwest of the city. Detailed
investigation into hillslope runoff reveals that several important facts were overlooked in the ordinances, including locally
high infiltration rates and drainage basins, which function as partial area systems. As a result, development planning is
not taking advantage of the natural mitigation potential of the land. Roads cut across infiltration and moisture retention
areas on side slopes, enlarging the partial area system feeding streams with stormflows. In addition, most residential planning
is not responsive to the stepped microtopography of Hill Country drainage basins and the critical scale at which local runoff
processes operate. Recommendations include adjusting the scale and configuration of development to conform with local runoff
processes and features and taking advantage of the water-absorbing capacities of basin side slopes. The lesson for ordinance
writers is that standard models of community stormwater ordinances are not appropriate for all terrains, especially complex
ones like the Texas Hill Country. 相似文献
925.
Assemblage stability in stream fishes: A review 总被引:6,自引:0,他引:6
We quantified the stability of nine stream fish assemblages by calculating coefficients of variation of population size for
assemblage members. Coefficients of variation were high and averaged over 96%; indicating that most assemblages were quite
variable. Coefficient of variation (CV) estimates were not significantly affected by: (1) years of study, (2) mean abundance,
(3) familial classification, or (4) mean interval between collections. We also detected minor regional differences in CVs.
The high variability exhibited by many stream fish assemblages suggests that it may be difficult to detect the effects of
anthropogenic disturbances using population data alone. Consequently, we urge managers to exercise caution in the evaluation
of the effects of these disturbances. More long-term studies of the ecological characteristics of undisturbed stream fish
assemblages are needed to provide a benchmark against which disturbed systems can be compared.
We suggest that CVs are a better estimator of population/assemblage stability, than either Kendall's W or the standard deviation
of the logarithms of numerical censuses. This conclusion is based on the following reasons. First, CVs scale population variation
by the mean and, hence, more accurately measure population variability. Second, this scaling permits the comparison of populations
with different mean abundances. Finally, the interpretation of CV values is less ambiguous than either of the aforementioned
metrics. 相似文献
926.
Thomas J. Myers Sherman Swanson 《Journal of the American Water Resources Association》1991,27(4):667-677
ABSTRACT: The quality of stream habitat varies for a variety of natural and anthropogenic reasons not identified by a condition index. However, many people use condition indices to indicate management needs or even direction. To better sort natural from livestock influences, stream types and levels of ungulate bank damage were regulated to estimates of aquatic habitat condition index and stream width parameters in a large existing stream inventory data base. Pool/riffle ratio, pool structure, stream bottom materials, soil stability, and vegetation type varied significantly with stream type. Pool/riffle ratio, soil and vegetation stability varied significantly with ungulate bank damage level. Soil and vegetation stability were highly cross-correlated. Riparian area width did not vary significantly with either stream type or ungulate bank damage. Variation among stream types indicates that riparian management and monitoring should be stream type and reach specific. 相似文献
927.
928.
929.
采用量纲分析的方法给出直边坡几何参数和物理力学参数决定边坡稳定系数的通式。首先,边坡高度H和边坡角α被确定为直边坡的几何参数,抗拉强度σt、黏聚力C、内摩擦角和岩土比重γ被确定为直边坡的物理力学参数;其次,采用量纲分析的方法,形式上给出边坡稳定系数F的通式,F=qαpα(C/Hγ)pcφpφ(σt/Hγ)pσ;再次,为了得到待定系数q和待定指数pα、pc、pφ、pσ,通过最小二乘法对数万个计算结果进行回归分析(每个结果都是通过FLAC计算将确定的直边坡参数与它的稳定系数形成对应)。结果表明,在一定的取值范围内,边坡稳定系数的表达式为F=4.63/α(C/Hγ)0.17φ0.83。可见,受MohrCoulomb和拉应力两个强度准则控制的岩土材料形成的直边坡的稳定性受无量纲的黏聚力、边坡角(角度)、内摩擦角(角度)的控制,与抗拉强度关系不大。无量纲黏聚力是黏聚力除以边坡高度和岩土比重。 相似文献
930.