论流动的恒定性与非恒定性

恒定流与非恒定流是流动现象的最基本分类形式之一.非恒定流是实际存在的,是绝对的.恒定流是理想化的,是相对的.通常重视的是由于主动原因导致的流动整体非恒定性,运动要素整体上随时间变化在允许幅度内可以近似作为恒定流处理.往往被忽略的是由于被动原因导致的流动局部非恒定性,运动要素随时间明显变化的空间在允许范围内可以近似作为恒定流处理;否则则呈现为周期的或不规则的非恒定流.流动局部非恒定性是由流动实质所决定的,可由它所遵循的控制方程在相应局部边界条件下进行分析.现有一些流动的计算示例和工程实例就是该种类型非恒定流存在的佐证.

Steady and Unsteady Characteristics of Flow

The steady and unsteady flow is one of the most basic form in the classification of flow phenomena inhydrodynamics. Unsteady flow is real and absolute,while steady flow is idealized and relative. What is usually valuedis that the unsteady of integrated flow is caused by active reasons. The motion elements change with time in anallowable range,so that the flow can be approximated as a steady flow. The unsteady of local flow caused by passivereasons is generally neglected,and if the motion elements of local unsteady flow change with time in an allowablerange,the flow can be approximated as a steady flow,otherwise it appears as periodic or irregular unsteady flow. Theunsteady of local flow is determined by the essence of the flow,which can be analyzed under the corresponding localboundary conditions by the control equation the flow follows. The existing flow calculation examples and engineeringexamples are the evidences for the existence of the unsteady flow.