Among the many minima of the time rate change of kinetic energy density with time,the minimum with reference to the space coordinates is postulated to coincide with the location of fluid detachment from the solid surface.Inviscid flow solution can be used to compute for the velocity field from which the kinetic energy density can be found.Mapping function for potential flow makes access to solutions for complicated geometric configurations such that potential sites of flow separation can be readily determined and used for the design of streamline bodies to delay separation and/or reduce drag.For uniform flow over a stationary circular cylinder,the criterion predicts separation points at angles ±54.74° from the rear stagnation point for potential flow. Known experimental results show that separation occurs between ±50° to ±58°.For elliptical cross-section airfoil with an aspect ratio of 6,a severe stalling angle of attack between 7°and 8° was predicted. This is close to the known result for the Royal Airforce 34 airfoil with the same aspect ratio where stalling occurred in the range of 12° to 14° angle of attack. Results for other airfoil shapes are given and discussed in connection with how airfoil profile could be chosen to delay stalling.%在空间场中动能密度的时间变化率有许多极小值,定义流动分离发生在其最小值处.可利用流体的无粘流动解计算动能密度值,而采用保角变换的方法,则可获得复杂几何形状下的有势流场解,从而很容易确定出可能的流动分离位置.将此判据应用于流线外型的设计,对延迟流动分离及减少阻力是有帮助的.对于均匀来流的圆柱绕流问题,该判据预示流动分离发生在离后驻点角度为±54.74°处,而已知的实验数据指出:流动分离发生在±50°与±58°之间.对于长轴:短轴＝1:6的椭圆翼剖面,当来流攻角在7°与8°之间时,将开始发生严重的失速现象.英国皇家空军34翼剖面(具有相同的长短轴比)的实验数据表明,失速开始发生在12°与14°攻角之间,可见理论值与实验值接近.文中讨论了其它形状的翼剖面,说明如何通过选择机翼的形状来延迟失速现象的发生.
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