Conjectures on certain boundary-layer equations and natural coordinates

J. Philip
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引用次数: 6

Abstract

The problem of exclusion of steady downward unsaturated seepage from underground cavities is reducible to a linear convection—diffusion equation with a no normal-flux condition at the cavity surface. Various exact solutions indicate that a roof boundary-layer analysis centred on the upstream stagnation point, and neglecting peripheral variation, gives to remarkable accuracy the quantity θmax, the crucial dimensionless potential determining whether or not water enters the cavity. The great accuracy of this analysis is attributed to the use of curvilinear coordinates natural to the cavity configuration. Global information (such as up to three separate characteristic lengthscales) is injected into the localized boundary-layer formulation via the metric coefficient of the natural coordinates. These are essential to the boundary-layer analysis. Cartesian coordinates, on the other hand, invariably suggest that no boundary layer exists! Definition of the natural coordinates is discussed and means of constructing them about arbitrary cavities are developed. Results for smooth cavities support the conjecture that roof geometry near the upstream stagnation point largely determines θmax, with downstream details unimportant. Comparison of solutions for flat-roofed rectangular and cylindrical cavities with those for strips and discs indicate, however, that the conjecture applies only in weak form to cavities of polygonal cross-section.
关于某些边界层方程和自然坐标的猜想
地下洞室稳定向下非饱和渗流的排除问题可简化为一个在洞室表面无正流条件下的线性对流扩散方程。各种精确解表明,以上游驻点为中心的顶板边界层分析,忽略外围变化,可以非常准确地得到θmax,即决定水是否进入空腔的关键无因次势。这种分析的高度准确性归功于对空腔结构的自然曲线坐标的使用。全局信息(如多达三个独立的特征长度尺度)通过自然坐标的度量系数注入局部边界层公式。这些对于边界层分析是必不可少的。另一方面,笛卡尔坐标总是暗示不存在边界层!讨论了任意空腔的自然坐标的定义和构造方法。光滑空腔的结果支持这样的猜想:靠近上游滞止点的顶板几何形状在很大程度上决定了θmax,而下游的细节不重要。然而,将平顶矩形和圆柱形空腔解与条形和盘形空腔解的比较表明,该猜想仅以弱形式适用于多边形截面空腔。
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