Definition wind tunnel blockage ratio

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Discussion Overview

The discussion revolves around the definition and implications of the blockage ratio in the context of designing a 2D wind tunnel model for experimental measurements. Participants explore how to determine the appropriate area of the model to consider in relation to the wind tunnel's cross-sectional area, particularly in light of the effects of model size on pressure distributions and flow characteristics.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks clarification on whether to use the projected frontal area of the model or the maximum thickness times the model span to calculate the blockage ratio.
  • Another participant asserts that the total frontal area of the test article should be used to express how much of the wind tunnel is free for airflow.
  • A question is raised about the effects on pressure distribution if the model is too large, indicating potential low pressures and flow issues.
  • Concerns are expressed that a larger model could create Venturi effects, leading to low pressures and difficulties in starting the wind tunnel.
  • One participant mentions that corrections can be applied for blockage effects and streamline curvature, but emphasizes that these corrections are limited if the model is excessively large.
  • A participant references Alan Pope's work, noting that he defines blockage ratio in terms of the frontal area of the model relative to the tunnel's cross-sectional area, but expresses uncertainty about the definition of "frontal area."
  • Another participant clarifies that the frontal area is determined by taking a perpendicular slice through the test section and projecting the model onto that plane.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the specific definition of "frontal area" as it pertains to calculating the blockage ratio. There are differing views on the implications of model size and the applicability of correction factors for blockage effects.

Contextual Notes

Participants acknowledge the limitations of their discussion, particularly regarding the definitions of area and the conditions under which correction factors can be applied. The discussion reflects a range of assumptions about the effects of model size on experimental outcomes.

BeSk
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Hello,
I’m designing a 2D wind tunnel model for my master thesis. It will be a profile equipped with a fixed hinged trailing edge flap. I’m going to measure at different angle of attacks and different flap settings at low speeds (about 70 to 100 m/s). The aim is to measure steady and unsteady pressure distributions. To calculate the maximum scale of the model I have to consider the maximum blockage ratio of 5% - 10% (which I found in the public literature).
My question is no which area of the model do I have to set in relation of the wind tunnel cross section area? Is it the in x-axis projected frontal area of the model (considering the angle of attack and the flap deflection) or is it the maximum thickness of the airfoil times model span?
The wind tunnel will has a closed test section and doesn’t have slotted walls.
I’m looking forward to your comments!
 
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Total frontal area of the test article. You are trying to express how much of the wind tunnel is actually free to pass through when traveling past the article.
 
Why? Maybe you can explain your opinion in detail. What effect would happen to my results (such as pressure distribution) if I would build a too big model?
 
Pressures would be low if you build too big. Using too large of a model essentially creates a pair of Venturi nozzles. Keeping blockage small keeps the flow from feeling the effects of the walls.

Also, I blockage is too high, some wind tunnels have trouble starting.
 
Just to add to what bonehead said. There are corrections you can apply to your data for blockage effects and other tunnel wall effects such as streamline curvature as long as your model is not too big. If the model is too big the tunnel walls can qualitatively change the flow over your model in which case you can't correct for that.

You should check out "low speed wind tunnel testing" by pope if you are unfamiliar with these things.
 
Thank you for your replies. I have the book from Alan Pope right here on my desk. I’m going to use these correction factors for solid- & wake blockage as well as streamline curvature. The Question is how big can I build the model considering the correction factors? Regarding the Blockage Ratio Pope says that it is the “frontal area” of the model in relation to the cross sectional area of the tunnel. For me it was not indisputable what he meant with “frontal area”.
 
It means take a perpindicular slice through your test section and then project your model onto that. The area on that plane is your frontal area.
 

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