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How would air flow be effected in these scenarios with a fan

  1. Jul 2, 2014 #1
    I created a diagram of 5 scenarios in which I would like to understand how their variations would affect air flow and suction. Examples attached to post or link to examples: http://s29.postimg.org/kua3svg2f/understanding_airflow.png

    So onto the actual question, how will air flow and suction be effected in each of the scenarios?
    How can I maximize suction and speed at which the air is exhausted with the fan that I am using?

    My assumptions about each scenario is listed below. I could be wrong so please correct me or offer more information. Side note, I find fluid mechanics interesting however my knowledge on it is limited. I haven't ever taken a physics class but its a subject that I may take as a course at my university because of its practicality.

    Ex. 2: Since the outlet is narrowed at the end, then the velocity of the air stream would be increased as the tube narrows. Does the angle at which the frustum narrows effect it?
    Ex. 3: Since the inlet is narrowed at the beginning, then the suction strength is increased but the area of suction is decreased.
    Ex. 4: This will increase the velocity of the air flow to the same speed as in Ex. 2 and the suction strength will also be increased the same way as in Ex. 3.
    Ex. 5: I created this example to understand the Venturi effect a little more. In this example the fan pulls air in, which is then narrowed to increase the velocity of the air flow before being funneled into a straight cylinder at an angle. Following the Venturi effect this should create suction at the bottom of the cylinder and exhaust (for a lack of a better word) at the end. If that reaction occurs than what factors regarding the diagram design could be adjusted to increase suction (like lengths of the cylinder or angle of the T joint)?

    If anyone can offer easy to use software where I can simulate these events, that would be fantastic. However I lack terminology to operate and understand the CFD software that I have looked into.

    Attached Files:

  2. jcsd
  3. Jul 2, 2014 #2


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    Do you mean maximize the amount of air moved per second for a fixed size rotor? The Dyson blade-less fan moves a lot of air with a small hidden rotor, by funneling the stream the right way.
  4. Jul 2, 2014 #3
    Example 3 has the inlet and outlet of the fan funneled, picture a conical bifrustum with open ends and a fan in the center. How would that effect the airflow? Would that reduce the amount of air flow or would it focus the stream intake to a narrow hole so it sucks harder. Also if it sucks harder than does that mean that the air is moving faster?
  5. Jul 2, 2014 #4


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    EX 2 - since the outlet narrows, there's more resistance to the fan so the mass flow is reduced, but the exit pressure and velocity are increased.

    EX 5 - The induced flow would be the result of viscosity of the air, the stream from the fan will induce flow from the adjacent air.

    Not shown - a "velocity stack" like intake and outtake that tapered so that mass flow is constant without interacting with the surrounding air by reducing area as velocity increases would help. This type of shape is used in ducted fans, some of which are used in radio control models. The shape would be similar to the red and blue areas in this NASA article about propellers:


    Example of venturi based suction pump used to drain water from aquariums or other areas, using venturi effect on water from a tap. It wastes water if used for more that just initiating a syphon, but it works. The tap water flows from top to bottom through a venturi tube that exits into a chamber with reduced pressure, drawing in water from the side port. The shape of the exit nozzle is probably important, since the effective exit cross sectional area increases with radius, allowing the water velocity to slow down and increase pressure from below ambient to above ambient as the area increases while keeping mass flow constant. The exit nozzle can be raised and closed by turning, so that the tap water exits via the side port, used for filling aquariums or other areas.

    Last edited: Jul 2, 2014
  6. Jul 3, 2014 #5
    Ex. 5
    Some info on jet pump, venturi pump, and a few other uses, the configuration of actual parts.
    Your figure 5 might need some updating, as shown.
  7. Jul 3, 2014 #6
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