第三单元_流体流动现象.ppt

* * Lesson3 Fluid-flow phenomena 流体流动现象 The behavior of a flowing fluid depends strongly on weather or not the fluid is under the influence of solid boundaries． In the region where the influence of the wall is small，the shear stress maybe negligible and the fluid behavior may approach that of an ideal fluid，one which is incompressible and has zero viscosity． The flow of such an ideal fluid is called potential flow and is completely described by the principles of Newtonian mechanics and conservation of mass. 流体流动主要取决于流体是否受固体边界层的影响。 在边界层影响最小的区域，剪切应力很小，流体的行为接近于理想流体 的流动状态，理想流体是不可压缩，黏度为0的流体。 理想流体的流动称为势流，势流的流动现象遵循牛顿力学定律和质量守恒定律。 Potential flow has two important characteristics：(i) neither circulations nor eddies can form within the stream, so that potential flow is also called irrotational flow，and (ii) friction cannot develop，so that there is no dissipation of mechanical energy into heat． 势流有两个重要的特征：1）在势流中既无环流也无涡流的产生，所以 势流又称为无旋流。2）在势流中摩擦不存在，也因此没有机械能转化 为热能的散逸。 Potential flow can exist at distances not far from a solid boundary． A fundamental principle of fluid mechanics，originally stated by Prandtl in 1904，is that，except for fluids moving at low velocities or possessing high viscosities，the effect of the solid boundary on the flow is confined to a 1ayer of the fluid immediately adjacent to the solid wall． This layer is called the boundary layer，and shear forces are confined to this part of the fluid．Outside the boundary layer，potential flow survives． 势流一般存在于固体边界层附近。 1904年，Prandtl首次提出流体力学的一个基本准则，即 除了流体流动速度很慢或者拥有高黏度以外，固体边界层对流体流动的影响 仅局限于临近于固体边界的流体层。 这一流体层称为边界层，剪切应力也仅作用于固体边界层流体. 在边界层以外，即存在势流。 Most technical flow processes are best studied by considering the fluid stream as two parts，the boundary layer and the remaining fluid． In some situations such as flow in a converging nozzle，the boundary layer maybe neglected，and in others such as flow through pipes the boundary layer fills the entire channel，and there is no potential flow． 大部分关于流体流动现象的研究都是将流体