Jugular wrote:
The easiest way that I can think of to explain it is to think of a syringe without a needle on it compared to one with. The increased resistance that you feel on your thumb whilst pushing the plunger down with the one with a needle - that’s back pressure.
Difference in empting syringe with and without a needle is only in the exit cross section:
Case 1, pushing the plunger with the same force:
Exit flow velocities (v) are the same, outflow (Q) is smaller with a needle because of smaller orifice cross section (A): Q=v*A. Smaller outflow increase time to empty syringe.
Case 2, empting syringes in the same time:
In a syringe with a needle you must to increase pressure (force*plunger_cross_section) to increase exit flow velocity (v), to increase outflow Q.
The increased resistance that you feel on your thumb whilst pushing the plunger down with the one with a needle is caused by increased force that you used to push plunger (action - reaction law), not hydraulic "back pressure":
Newton's third law of motion:
"To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions."
Jugular wrote:
Sure it is. Just think of "back pressure" as Resistance.
Resistence is resistance to flow (tube convergence, divergence, turns, surface roughness and other physical properties will cause resistance to flow and affect the pressure drop - decrease in pressure from one point in a pipe or tube to another point downstream as the result of frictional forces on the fluid as it flows through the tube). No flow - no resistence!
And a pressure drop always affects the downstream flow, and never upstream flow.
So, "back pressure" is misleading term for something affecting upstream flow. But if you think of a local turbulences, all kind of the flow will happened there, back, front, up, down, left, right... but the river still flow downstream - to the sea ;)
M.
