Fluid friction is the component of the hydrodynamic force of the object in relation to the direction of motion of the object relative to the motion of the fluid or in the direction of the velocity of the incoming flow.
For pipe flow, the friction mainly occurs in the pipe wall, and the velocity of the fluid in the radial direction is different, and frictional resistance is also generated between the fluids. In addition to frictional resistance, differential pressure resistance is also a type of flow resistance that occurs with different pressures in the direction of flow. If there is a local obstacle (such as a valve or a pipe) on the flow channel or the cross-sectional area suddenly expands and contracts, causing a change in the flow velocity, the kinetic energy of the fluid is converted into a static pressure, and a pressure difference is generated, even if the flow rate is restored to the original flow rate, When the flow rate changes, eddy currents are generated, thus increasing the resistance. This type of differential pressure resistance is also called local resistance.
Fluid flow resistance: The flow of fluid in the piping system can be divided into flows in a uniform straight pipe, resulting in resistance along the surface friction; in various pipe fittings such as valves, bends, equipment import and export, etc. The flow causes local resistance mainly due to reverse pressure difference or eddy current due to flow path change, cross-sectional area change, flow path bifurcation convergence, and the like.
The force of an object at the flow boundary to the flowing fluid. It is opposite to the direction of fluid flow and is produced by momentum transfer. Flow resistance is one of the basic problems in the study of momentum transfer in viscous fluids. The reaction force of the flow resistance, that is, the force of the fluid on the object, is called drag. For pipe flow, the flow resistance is usually expressed by the pressure drop of the fluid. The mechanical energy (pressure energy) caused by this pressure drop can not be recovered, that is, part of the mechanical energy suffers losses, commonly known as resistance loss. For the flow around, pay more attention to the drag. As long as the incoming flow, ie the velocity of the fluid upstream of the object, is uniform, the flow of fluid around the stationary object is equivalent to the motion of the object in the stationary fluid. Therefore, a static model is often placed in the flowing fluid to simulate the motion of the object in the stationary fluid.
Fluid friction can be divided into friction resistance and differential pressure resistance. Frictional resistance is the flow resistance generated by the shear force of the surface of the object, and its direction is opposite to the direction of fluid motion. The differential pressure resistance is the resistance to fluid flow generated by the pressure perpendicular to the surface of the object, and the direction is also opposite to the direction of fluid motion. Both resistances often exist at the same time.
Taking the flow of a fluid around an object as an example, the relative magnitude of the two resistances depends on three factors:
1 The shape of the object. If the object is a bluff body like a ball, the boundary layer is separated earlier, and the differential pressure resistance is dominant. For streamlined objects, if the boundary layer is not separated or separated later, the differential pressure resistance is small and the frictional resistance is dominant.
2 The size of the Reynolds number determined by the feature length of the object, and the Reynolds number determines the flow state in the boundary layer. The turbulent boundary layer has a large frictional resistance, but due to the delay of separation, the pressure difference resistance is small; the laminar flow is reversed, the frictional resistance is small, and the differential pressure resistance is large.
3 The roughness of the surface of the object, the frictional resistance of the rough surface is large, but the rough surface can promote the deuteration of the boundary layer, delaying the separation, thereby reducing the differential pressure resistance.
Resistance calculation
The calculation formula of the resistance F when flowing around is: where Cd is the resistance coefficient; u is the inflow velocity; A is the projected area of the object in the direction perpendicular to the motion; ρ is the fluid density. The magnitude of the drag coefficient Cd depends on the shape of the object and the Reynolds number.
