With any movement, energy losses occur - at least it will be a car, at least an airplane, at least a liquid in a pipeline. Part of the energy is always spent on overcoming resistance to movement. A decrease in the pressure of the liquid is usually defined as hydraulic resistance. In fact, there are two types of such resistance - local and linear. Local is associated with energy losses on valves, gate valves, bends, expansions and contractions of the pipe.
It should be noted that the source of losses is always the viscosity of the liquid. Local losses or hydraulic resistance, the calculation formula of which is associated with the parameters of valves, pipes and valves, is determined by a special method. But linear losses largely depend on the nature of the flow of fluid in the pipe.
Studies of fluid flow patterns were carried out by Reynolds in 1883. In these studies, a stream of water was used to which paint was added, and the nature of the movement of paint and water could be observed in the glass tube. At the same time, pressure, velocity and pressure of the liquid were measured.
The first mode of motion was observed at a low water velocity. In this case, the paint and water do not mix with each other and move together along the pipe. The speed and pressure are constant in time. This mode of fluid flow is called laminar.
If the speed of movement increases, then at a certain value, the picture of the motion of the liquid will change. The stream of paint begins to mix throughout the volume of the pipe, vortex-like formations and rotation of the liquid become visible. The measured values of the velocity and pressure of the liquid begin to pulsate. Such a movement is called turbulent. If the flow rate is reduced, then laminar motion is restored again.
With a laminar fluid flow, the hydraulic resistance is minimal, with turbulent it is much greater. Here it is necessary to clarify that there are still friction losses on the pipe walls. The velocity in the laminar flow is minimal at the pipe wall and maximum in the center of the flow, but the water flow moves smoothly along the entire pipe. In turbulent motion, the resulting turbulence creates obstacles to the movement of water and additional hydraulic resistance.
There is another phenomenon that contributes to losses. It is called cavitation. Cavitation is observed when a bottleneck appears when the fluid moves in the pipe. Then at such a place the speed increases and, according to Bernoulli's law, the pressure decreases. The decrease in pressure leads to the fact that the emission of gases dissolved in the liquid begins and the water begins to boil at the current temperature.
After passing through a narrow section, the flow velocity decreases, pressure increases and boiling disappears. Cavitation causes additional losses due to local disturbances in laminar flow. As a rule, it occurs in cranes, gate valves and other similar nodes. This phenomenon is considered extremely undesirable, because may damage the entire piping system.
Thus, it turns out that hydraulic resistance is a concept that is determined by several factors. These include the design features of the pipeline system (length, bends, valves and latches), including the material from which the pipes are made. Losses are also influenced by the nature of the fluid flow. This makes it possible to understand what a pipeline system should be and what should be avoided during its design and operation.
In the presented material, such a concept as hydraulic resistance with respect to the pipeline system is considered. The description of different modes of fluid flow and its behavior in pipes is given.