Corelating Re number for model scale to actual scale

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Discussion Overview

The discussion revolves around the calculation and correlation of Reynolds number (Re) for a scaled wind tunnel model of a rectangular box designed for cross-ventilation. Participants explore the implications of dynamic similarity in wind tunnel testing and the challenges of ensuring accurate flow representation between the model and actual scale.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes conducting a wind tunnel test and seeks to calculate the Reynolds number for a scaled model, noting that the Re value will be small due to the model's size.
  • Another participant questions the timing of the inquiry, suggesting that understanding dynamic similarity should have been established prior to the wind tunnel test to ensure accurate flow representation.
  • A third participant emphasizes the importance of matching key dimensionless numbers, specifically the Reynolds number and geometric scaling, to achieve dynamic similarity.
  • This participant also cautions against applying laminar and turbulent flow thresholds universally, stating that these are specific to pipe flow and may not apply to other scenarios.
  • One participant reiterates the difficulty of achieving dynamic similarity in wind tunnel tests for aircraft, suggesting that it is often impossible to perfectly match conditions.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of achieving dynamic similarity in wind tunnel tests. While some emphasize the importance of matching Reynolds number and other parameters, others highlight the inherent challenges in doing so, particularly in the context of aircraft testing.

Contextual Notes

Participants note limitations related to the application of laminar and turbulent flow classifications, indicating that these may not be universally applicable outside of pipe flow scenarios. The discussion also reflects uncertainty regarding the correlation of Reynolds numbers between model and actual scale.

Who May Find This Useful

Individuals interested in wind tunnel testing, fluid dynamics, and the principles of dynamic similarity in experimental setups may find this discussion relevant.

deepacha
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Hello,

I did a wind tunnel test of a rectangular box with equal sized louvered window at the inlet and outlet. This box is a scale model and representative of a typical room with windows for cross-ventilation. I measured the velocity inside the room.

Now I want to calculate the Reynolds number for this scaled model. I know the formula for the calc. i.e Re= pvD/u.. But since the model is at a small scale, the Re value for this model will be small.

How do I co-relate the Re of model to Re of the actual scale. Is there a different Re scale for models like we have for the actual scale where Re<2300 = laminar and Re>4600 = turbulent.. Obviously the Re value for the model will not be this big.

what I want to do is find out the Re for the scale model with the measured velocity values. Do a CFD of the actual scale and find out it Re. I hope that the experiment and the simulation values would come close.

Any insight on this is appreciated.
 
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I'm not an expert on wind tunnels, but shouldn't you have figured this out before doing a wind tunnel test? The entire point of a wind tunnel is to exploit the concept of dynamic similarity. If the model size and wind tunnel conditions are not scaled to give the same Re and M as the actual structure, then the flow field will not be the same.
 
In order to match the flow dynamically, you must match the important Pi terms. For your situation, this will be the Reynolds number, and the geometric scaling of your model.

As for your stated numbers for laminar vs turbulent flow, that is only for pipe flow. Do not apply it universally.The simple answer is: if you cannot ensure dynamic similitude then there is no exact way to account for this. Therefore, every picture of an airplane you see in a subsonic wind tunnel is distorted in Re (including mine).
 
Brian_C said:
I'm not an expert on wind tunnels, but shouldn't you have figured this out before doing a wind tunnel test? The entire point of a wind tunnel is to exploit the concept of dynamic similarity. If the model size and wind tunnel conditions are not scaled to give the same Re and M as the actual structure, then the flow field will not be the same.

For aircraft, this is almost always impossible.
 

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