Let's do friction a little better.
Friction Loss
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
There are several types of Friction.
Dry friction resists the relative lateral motion of two solid surfaces in contact.
Dry friction occurs in the oil pump between the gerotors and divider plate(s) / pump housing.
(if you don't oil the gerotors before installing the pump)
Fluid friction describes the friction between layers of a viscous fluid that are moving relative to each other.
Fluid friction is a force that restricts the movement either within itself or of another medium moving through the liquid.
Due to the movement of the molecules inside the fluid, internal friction occurs, and how the fluid interacts with other matter, external conflict occurs.
The internal resistance to flow is termed as viscosity. The less-dense the fluid, the greater is the ease for movement or frictional force.
If there is a wet surface between two thin glass plates, you will notice that plates get stuck and the bottom plate doesn’t fall when you hold only the top one.
When any object is dropped in a fluid, the extent of the splash is depended on the fluid friction of that particular fluid.
You find lighter dust particles move fast on the surface of a flowing river.
This is due to the high-velocity gradient at the top layer of water due to lower dynamic fluid friction at that layer.
Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid surfaces.
Lubricated friction occurs in the oil pump between the divider plate(s) / pump housing and gerotors.
It also occurs between bearings and races.
Skin friction is a component of drag, the force resisting the motion of a fluid across the surface of a body.
A fluid flowing through a pipe contacts the pipe wall.
The pipe wall has surface roughness. The amount of roughness affects the drag on the fluid.
Roughness is measured by the height of the projections sticking up from the pipe wall.
In the valleys between the projections, the fluid moves slowly. Above the projections it moves faster.
The drag between layers tears, or shears, them apart and each layer moves at a different speed.
The shear rate decreases as the distance from the wall increases. The velocity at the wall is zero and fastest at the center.
This means the central core of the fluid exits the pipe first.
So friction loss is the resistance of fluid to flow through a pipe by viscous shear stresses and turbulence that occurs along the internal pipe wall. 53)
These are dependent on the roughness of the pipe material.
This resistance is usually measured in feet or meters of head of the fluid, which is why it is also referred to as the head loss due to pipe friction.
The longer the run of pipe / hose, the higher the affects of friction on head pressure.
Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.
Internal friction is better defined in solid materials such as soil, rock, woods etc and not liquids and gases.
(as in particles of a soil body as two solid blocks sitting on top of each other)
Dry Friction
Skin Friction
The figures in the drawing are just hypothetical numbers.
But the point is that friction lowers incoming pressure to lower pressure on the outlet of a straight run of pipe / hose.
Viscosity and Density Effects
Liquids are categorized by their behaviors when undergoing shear. 56)
Those liquids that have a constant shear rate with change of velocity (like water and oil) are called Newtonian fluids.
Newton first developed the mathematical explanation for the phenomenon.
Those with shear rates that vary with changing velocity (like paint and blood) are Non-Newtonian.
The shear rate is a measure of a fluid’s viscosity or slipperiness.
The density of a fluid affects its viscosity.
Fluids with more mass per unit volume are heavier and require more energy to move them and shear less easily.
A temperature rise decreases the viscosity and density of liquids. The more viscous, or less slippery, a fluid the harder it is to get shearing between layers.
The high viscosity prevents rapid velocity changes occurring between layers.
The sub layer in viscous fluids is thicker than in low viscosity fluids.
Affects of Velocity
At low speeds the whole flow across a pipe is laminar and the fluid slides over itself. As the speed becomes faster, eddies start to form and cross the fluid layers.
A transition from laminar to turbulent flow develops.
At still higher velocities the flow in the core of the pipe becomes turbulent with swirling eddies throughout.
In Search of Fuel, Sunshine and a Steady Idle.
Reference the XLFORUM'S Sportsterpedia
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