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 To be able to locate the point of inception of air entrainment is of considerable significance in the design of skijump spillways for high dams, in that this is not only prerequisite to the theoretical analysis of jet diffusion in the air and the subsequent alleviation of erosion as the aerated jet dives to the bed, but also essential if the possible benefit of aeration in the reduction of concrete pitting is to be evaluated. Although past contributions to this problem are numerous, no method has yet been available... To be able to locate the point of inception of air entrainment is of considerable significance in the design of skijump spillways for high dams, in that this is not only prerequisite to the theoretical analysis of jet diffusion in the air and the subsequent alleviation of erosion as the aerated jet dives to the bed, but also essential if the possible benefit of aeration in the reduction of concrete pitting is to be evaluated. Although past contributions to this problem are numerous, no method has yet been available to yield the correct prediction of the onset of aeration in or downstream of the curvilinear portion of the spillway which is known to take place much earlier than usual. Even for the straight portion of the spillway, calculated positions of aeration inception do not always match with the observed values (see table 1, and compare columns 4 to 7). In this paper is presented a rational and yet rather simple procedure with which one may treat the general problem of locating the position of aeration inception no matter if the spillway contains a curve or not. In the first place, the irrotational or "ambient" flow outside of the boundary layer is studied. In view of the fact that the flow over the spillway of a high dam is much smaller in extent laterally than longitudinally, an approximation similar to that used in the derivation of boundary layer equations from the NavierStokes equations is applied to the Euler equations. The resulting expressions indicate that the usual assumption of concentric streamlines is justifiable. The depth of flow is taken as that so calculated plus the displacement thickness of the boundary layer. Since on the plane of the complex potential, the flow over a spillway may be formulated as a simplymixed boundaryvalue problem, Wood's exact method is applied to a numerical example with gravitational effect taken into consideration. The result of calculation indicates that both methods yield practically the same depth of flow. The inception of aeration is, as usual, assumed to occur as the boundary layer meets the free surface. Under the combined influence of gravity and boundary geometry, the flow over a dam is continuously accelerated or decelerated. In such case, boundarylayer computation by usual method is both involved and of doubtful accuracy. It is found, however, that in the case of flow over a spillway, the flow outside of the turbulent boundary layer conforms to a condition of selfpreservation as proposed by Townsend. Since the Reynolds number for high dams may surely reach very high values, the turbulent boundary layer itself may be assumed to be approximately selfpreserving. In short, this means that Townsend's theory of approximate selfpreservation for boundary layers under the influence of longitudinal acceleration may be applied. This also means that the computation of boundary layer development may be much simplified. Based on an analysis of prototype data, it is found that in the present case involving airentrainment inception, thickness of the boundary layer should be defined as that at which the mean velocity is within 0.1% of the velocity of flow outside the boundary layer. In the carrying out of the computations, boundary layer thickness at various sections are first estimated, then the irrotational flow outside the boundary layer is analysed by the simplified method to obtain the surface profile and the parameter "a" denoting the variation of velocity along the surface, and finally Townsend's theory of approximate selfpreservation is applied to recompute the thickness of the boundary layer along the spillway. As any error made in the estimation of boundarylayer thickness has little effect on the computation of surface profile and hence on any subsequent computations, reasonably experienced computers should find it unnecessary to repeat the computations. Results of computations are found to be within 10% of the observed data obtained at two dams.  鉴于现有掺气发生点的计算方法误差可达观测值的70％以上,本文根据原型掺气发生点观测资料探讨较准确的计算方法,在无特殊干扰的条件下,掺气的发生取决于紊流边界层的发展.坝面边界层流动的特点是:雷诺数高(u_1x/,v在10~8至10~(10)之间),坝面粗糙,和水流先加速后减速因而纵向流速梯度对边界层发展的影响必须考虑.针对这些特点和坝面及陡槽高速水流具有自模性质,本文建议应用自模理论进行计算.计算的掺气点位置与实测位置比较误差减至10％左右,从而为掺气发生点或坝面紊流边界层的计算提供了较可靠的方法.  The paper describes the compaction characteristics of clayey soil. In view of the basic conception that the optimum degree of saturation Srop under various compaction energy is constant, and the optimum moisture content of the standard compaction energy is approximately the plastic limit, a method by using plastic limit wP and optimum saturation for estimation of the maximum dry density is presented as follows:γdmax=Srop·G3/G3wp+Sropwhere G3 is the specific gravity of the soil particles,wP is the plastic limit.Methods... The paper describes the compaction characteristics of clayey soil. In view of the basic conception that the optimum degree of saturation Srop under various compaction energy is constant, and the optimum moisture content of the standard compaction energy is approximately the plastic limit, a method by using plastic limit wP and optimum saturation for estimation of the maximum dry density is presented as follows:γdmax=Srop·G3/G3wp+Sropwhere G3 is the specific gravity of the soil particles,wP is the plastic limit.Methods of determining the compaction criterion of clayey soil are discussed and the coefficient of construction m is introduced and demonstrated with suggestion value 0.95~0.97 for medium and low dams, and 0.97~0.99 for high dams.  本文总结了粘性土的压实特性,从不同击实功能下土料的压实最优饱和度S_(rop)为常数、标准击实功能的最优含水量ω_(op)约等于塑限ω_p这些基本概念出发,提出了用塑限、最优饱和度来估算粘性土的最大干容重γdmax。即S_(rop)=C,W_(op)=ω_p且当W_p≤17％,S_(rop)=(3ω_p+35)％ω_p>17％,S_(rop)=(0.3ω_p+80)％γdmax=(Gs·S_(rop))/(Gs·ω_p+S_(rop))在文中讨论了确定粘性土压实标准的方法,并对施工条件系数m进行了论证,提出了建议数值。即对于中低坝:m=0.95～0.97对于高坝:m=0.97～0.99  A curve of equal cavitation number ( abbreviated as ECN curve ) is proposed for buckets of highdam spillways. Based on simplified analysis, equations for the calculation of such a curve have been developed. These equations may be conveniently solved by a RungeKutta scheme. Applications to highdam spillways show that the effective head of a spillway as far as cavitation potential is concerned may be materially reduced by adopting the ECN curve for the bucket. In one case, the effective head of... A curve of equal cavitation number ( abbreviated as ECN curve ) is proposed for buckets of highdam spillways. Based on simplified analysis, equations for the calculation of such a curve have been developed. These equations may be conveniently solved by a RungeKutta scheme. Applications to highdam spillways show that the effective head of a spillway as far as cavitation potential is concerned may be materially reduced by adopting the ECN curve for the bucket. In one case, the effective head of a spillway with a drop of 111.5m is thus reduced to 62m.  本文给出了等空化数线的计算方程和应用龙格——库塔法的计算框图。采用等空化数线反弧可使最小泄洪空化数显著提高。   << 更多相关文摘 
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