In fluid dynamics, laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of fluids. In laminar flow, the motion of the particles of the fluid is very orderly with particles close to a solid surface moving in straight lines parallel to that surface. Laminar flow is a flow regime characterized by high momentum diffusion and low momentum convection. Wikipedia

在流体动力学中，层流（laminar flow）的特征是流体粒子在层中遵循平滑的路径，每层平滑地移动经过相邻层，很少或没有混合。在低速时，流体倾向于流动而没有横向混合，相邻的层像扑克牌一样滑过彼此。没有垂直于流动方向的横流，也没有流体的涡流或漩涡。在层流中，流体颗粒的运动非常有序，靠近固体表面的颗粒沿平行于该表面的直线运动。层流是一种以高动量扩散和低动量对流为特征的流动状态。

从分子的角度，同层内的分子同向运动，V的大小和方向都相等。不同层间的分子，V的大小不同，方向相同。质点在层流时都走直线。