Fluids Mechanics Flows in bendsWatch
Why does the head loss vary in mitre bends, short bends and long bends? Why does the head loss increase with the Velocity head when plotting these in a graph?
Think of it visually. What happens when flow goes through a 90 degree mitre bend? It'll all impact against the perpendicular side of the bend. Now think of a short radius elbow: there'll be a bit less impact; the flow will be "guided" a bit more gently. The same effect will be even greater for a longer radius elbow.
It's the same with other fitting and components, too. For example, a venturimeter will have a lower head loss compared to an orifice plate.
The velocity head represents the kinetic energy of a fluid flow. More energy is lost through fittings, bends etc. when the flow has more kinetic energy. If you run into a wall, more kinetic energy would be lost if you did it faster rather than slower. Otherwise, you could just use really high flow velocities to negate the head losses from fittings, bends etc.