Before going to viscous flow first understand the concept of Viscosity. Viscosity is an internal property of fluid which resist the fluid to flow. This resistance is offered layer by layer to the adjacent layers. It is caused by cohesive forces between the molecules in liquids and by molecular collision in gases. To understand the concept of viscosity, assume a fluid is flow inbetween two plates. If the top plate is pulled with a force F, then the fluid layer adjacent to the top plate starts moving along with the plate.  Say for example if the fluid layer adjacent to the top plate is moving with a velocity of 5 cm/s, then the adjacent layer will definitely move with velocity less than 5 cm/s. Similary it goes down till the bottom of the plate and at the bottom of the plate velocity is 'zero'. Thus a friction force(resistance) develops between the layers and the slower layer tries to slow down the faster layer. This is because of the property of viscosity. Viscous flow: Flows in which this frictional effects(resistance) are significant and are called "viscous flows" Inviscid flow: However in practical purposes, there are regions where viscous forces are negligible and these flows are "inviscid flows"......But in real cases there are no fluids with 'zero' viscosity..... Note: Fluids are generally 'liquids and gases'

Posted By: Uthanda