When two layers of fluid move relative to each other, a friction force develops between them and the slowest layer tries to slow down the fastest. This internal resistance to flow is quantified by the viscosity property of the fluid, which is a measure of its internal adherence. Viscositynity is caused by the cohesion forces between molecules, in liquids, and by molecular collisions in gases. There is no fluid with zero viscosity and consequently, viscous effects are involved to some degree in all fluid flows. Flows where the effects of friction are significant are called viscous flows. But, in many flows of practical interest, you have regions (usually regions that are not close to solid surfaces) where the viscous forces are negligibly small at comparison with inertia or pressure forces. Neglecting the viscous terms in those non-viscous regions of flow greatly simplifies the analysis, without considerable loss in accuracy.
Internal flow compared to external
A flow of a fluid is classified as internal or external, depending on whether that fluid is forced to flow in a confined channel or on a surface. The flow of an unrestricted fluid on a surface, such as a plate, wire, or tube, is external flow. Flow in a tube or duct is internal flow if the fluid is completely limited by solid surfaces. For example, the flow of water in a tube is internal flow and the flow of air over a ball or over an exposed tube during a constant wind day is external flow. Water flows in rivers and irrigation ditches are examples of these flows.
Compressible flow compared to incompressible
A flow is classified as compressible or incompressible, depending on the level of variation in fluid density during that flow. Incompresibility is an approximation and the flow is said to be incompressible if the density remains approximately constant throughout the flow. Therefore, the volume of all portions of the fluid remains unchanged over the course of its movement when the flow (or the fluid) is incompressible. In essence, the densities of liquids are constant, and thus their flow is typically incompressible. Therefore, it is often said that liquids are incompressible substances. For example, a pressure of 210 atm causes the density of liquid water at 1 atm to change by only 1 percent. Furthermore, the gases are intensely compressible. For example, a pressure change of only 0.
Laminar flow compared to turbulent
Some flows are smooth and orderly while others are considered chaotic. The intensely ordered movement of a fluid, characterized by undisturbed layers of it, is referred to as laminar. The word laminar comes from the movement of adjacent fluid particles together, in “sheets”. The flow of intensely viscous fluids, such as oils at low speeds, is generally laminar. The intensely disordered movement of a fluid, which is common occurs at high speeds and is characterized by fluctuations in velocity, is called turbulent. The flow of low-viscosity fluids, such as air at high speeds, is usually turbulent. The flow regime significantly influences the power required for pumping. A flow that alternates between laminar and turbulent is known as transition.
Natural (or non-forced) flow compared to forced.
The flow of a fluid is said to be natural or forced, depending on how the movement of that fluid begins. In forced flow, a fluid is forced to flow on a surface or in a tube by external means, such as a pump or fan. In natural flows, any movement of the fluid is due to natural means, such as the buoyancy effect, which manifests itself as the elevation of the hottest (and therefore lightest) fluid and the fall of the coldest fluid (and therefore Therefore, the densest) For example, in solar cell systems for hot water, it is common for the thermosiphon effect to be applied to replace the pumps when the water tank is placed high enough above the solar collectors.