I'm building a ducted fan and I would like it to:

  • have a uniform airflow at the output
  • have good static pressure
  • have a square section output (see figure: air enters the circular edge and exits from the square edge)

enter image description here

In my application the output section will be covered by a breathable membrane.

While doing some tests I noticed that the airflow in the center was much weaker than at the sides, with this blade design (counter-clockwise rotation):

enter image description hereenter image description here

How can I get more uniform airflow?

Here are some thoughts on how to improve the design, but since I don't know virtually anything about fan design and fluid dynamics I'm asking more competent folks here to judge whether these ideas could work:

  • adding more blades
  • make the blades flatter
  • make the blades wider
  • reducing the radius of the mounting cyclinder of the blades
  • make the output section smaller than the input section

Do you know if any of these would work? Do you have better suggestions?

  • $\begingroup$ Have you considered a matrix of smaller fans? Each fan would have its own center/slower throughput, but the total assembly would have the aggregate of the fans and would also "mix" more thoroughly. $\endgroup$
    – fred_dot_u
    Jul 14, 2020 at 15:29
  • $\begingroup$ @fred_dot_u Thank you for your comment. Unfortunately I cannot go for the matrix of smaller fans, so I can only try to improve this design... $\endgroup$
    – Lilla
    Jul 14, 2020 at 15:36
  • $\begingroup$ Did you check it with the duct in place, or was it in the open flow condition? The duct will massively change the downsteam jet characteristics. Also, check the area ratios of the round and square sections. Changes in area will cause changes to the velocity profile across the duct. You won't know what you've got until you have the whole thing built, but the breathable membrane should be tailored to achieve the uniform flow downstream of the membrane. $\endgroup$
    – Phil Sweet
    Jul 14, 2020 at 16:57
  • $\begingroup$ @PhilSweet With the duct in place. I added some sand on the breathable membrane and the sand would "bubble" only close to the edge of the duct. Talking about the ratio of the areas (say $\displaystyle \frac{\text{output}}{\text{input}}$): would you suggest to make it bigger or smaller than $1$? $\endgroup$
    – Lilla
    Jul 14, 2020 at 17:19
  • $\begingroup$ Look at how this is achieved in wind tunnels $\endgroup$ Jul 14, 2020 at 18:11

1 Answer 1


There are several possibilities:

A longer outlet tube to allow the velocity profile to develop but there will always be a profile

The output may have a rotational component, which you could reduce by using a flow straightener - think of a "block" of drinking straws end on.

To check what your profile is you could use a pitot tube and move it across the outlet section taking measurements to see the pressure profile. Then consider some baffles earlier in the output tube to reduce the maximums etc.

  • $\begingroup$ Thanks! I'll try to implement your solutions. The only thing is that I don't really have know what kind of baffle I should build: perhaps you have some image/reference I could look at to clear my mind? $\endgroup$
    – Lilla
    Jul 14, 2020 at 16:40
  • $\begingroup$ @Leonardo you will also need to enter that "set of straws" from a plenum so that the pressure at the inlet of each one is the same. The exit of a fan will never be uniform. $\endgroup$
    – Tiger Guy
    Jul 20, 2020 at 19:36

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