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I know you are seeking a universal answer here, but I don't think there is one.

The answer depends upon the objectives of the experiment, the requisite flow quality, spatial and financial constraints. Can you tell us more about what you are trying to do, what kind of scale you are thinking, etc?

Thanks Dr. Altman,

The research is for flow inserts inside a 50 mm pipe. I have put a fan in induced draft condition but the velocity profile is irregular during the test section. The repeatability is also very bad. We intend to have a fully developed flow in the test section. For these reasons I wish to ask if the forced draft configuration shall be better suited to get the already predicted velocity profile.

Do you have a tunnel inlet/nozzle with a contraction profile and ratio?

Does your inlet have any flow conditioning (screens, if so what mesh and how many, honeycomb)?

Does your draw-through fan have a set of stator vanes? How tightly controlled is the fan RPM?

What does the transition from the test section to the fan look like? What is the ratio of the diffuser length to inlet diameter and what is the maximum diffuser (half) angle?

I think maybe the answer to these questions is more important than the forced vs induced question you've posed.

Dear Dr. Altman,

I am very much thankful for the suggestions. We are in the trial basis but I shall definetely look into these things. Again many thanks.

Hi Muhammad,

I have worked quite a lot with both types of tunnel: the blow-down and suck-down (Eiffel) varieties. In my opinion, blow-down tunnels are far more practical, easy to use, change test sections, etc. You can also arrange it to have an over pressure in the test section, so that nothing gets inadvertently sucked in form holes in the walls, etc. Finally, you can modify them to operate as unsteady tunnels, which adds more flexibility, like I did. Please see: https://doi.org/10.2514/1.J054590.

Hi Muhammad,

In addition to what David said, the blowdown tunnel allows you dry the air to better control the moisture content of the air. If you're not careful a vacuum tunnel pulling from the free air can cause condensation inside the tunnel, which can be hard on your equipment long term. I think you're not talking about transonic flow, but condensation in the actual flow can happen in that case, changing the results. I'd suggest using a high-pressure source blowing into a larger diameter region, with at least double the diameter of your test section, but four times would be better. The larger section should be 2-3 diameters of the larger tube in length. At the beginning of this section you put your flow straighteners and sieves, and at the other end a nozzle. Even for a fairly condensed jet at the start, this should result in a very even, low turbulence flow at the end of the contracting nozzle.

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