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Basic Hydraulics: Flow, Pressure, Force, Area, Energy

  1. Nov 18, 2013 #1
    Hello, I am trying to get a grip on the basic relationships in hydraulics. I do not need the advanced math and engineering, compensation for frictional losses, and fluid viscosity and turbulence effects. Just a good basic understanding.

    The first concept that comes to mind is the concept of energy, I believe the total energy in a system is the sum of the potential and kinetic energies. And that energy within a system is the conserved property. Like the amount of work that can be done, has been done, and is being done if added up should always be the same in a closed system.

    Another concept is Pressure. I believe that pressure describes force distributed by a fluid over an area. Force is something that can be measured for a fixed area. Like if I have 100 PSI and apply it to something with .5 square inches, it generates 50 pounds of Force.

    I consider flow to be the mass of a fluid per unit time that moves past a point. I think that's the volumetric flow rate.

    A related concept is the velocity of the flow.

    Flow seems to be related to the pressure differential between two points.

    And by this point I've lost my grip.

    Using a basic example, if I open the faucet outside and let water flow out of a garden hose, it will come out steadily, if I then restrict the hose by placing my thumb over the end, the water gushes out violently.

    I don't believe that restricting the hose with my thumb changes the total energy in the system.

    I think that the water sprays out farther because its force increased, and its force increased because the same pressure was applied to a smaller area.

    Flow probably decreased. If I compare a faucet that is only slightly cracked open, it will just drip, compared to one fully open that is gushing, I think the flow is inversely related to the restriction.

    I'm lost.

    In the same system I can have high pressure, low flow output, or high flow, low pressure output. So I think flow * pressure = power (conserved).

    And I think flow is proportional to the difference in pressures and the area. A higher pressure differential should result in more flow for an orifice of the same area. And a larger orifice should increase flow. So flow = pressure1 / pressure2 * area.

    Thus putting my finger over the end of a garden hose reduces the effective area the pressure differential is applied across, which reduces the flow, and increases the pressure, result - spraying water ten feet.

    HELP! I need basic working theory!
  2. jcsd
  3. Nov 19, 2013 #2
  4. Nov 19, 2013 #3
    Mass of the fluid per unit time is mass flow rate. Volume of the fluid per unit time is the volumetric flowrate.

    I wrote a post in another thread a while back which might help a little, too. https://www.physicsforums.com/showthread.php?t=674896
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