Corelating Re number for model scale to actual scale

[deepacha]

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.

 

[Brian_C]

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.

 

[Cyrus]

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).

 

[Cyrus]

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.

 

[alphy]

Dear researcher,

Thank you for sharing your wind tunnel test results and your goal of correlating the Reynolds number for your model to the actual scale. This is an important consideration in many scientific studies and can greatly impact the accuracy and applicability of your results.

Firstly, it is important to note that the Reynolds number is a dimensionless number and is therefore independent of scale. This means that the Reynolds number for your model will be the same as the Reynolds number for the actual scale, as long as all other parameters (such as fluid density, velocity, and characteristic length) are properly scaled. The formula Re = pvD/u is still applicable, but the values for p, v, and u must be scaled appropriately.

In terms of correlating the Reynolds number for your model to the actual scale, there are a few approaches you can take. One option is to use the concept of dynamic similarity, which states that two systems are dynamically similar if they have the same dimensionless parameters (such as the Reynolds number). This means that if the Reynolds number for your model is the same as the Reynolds number for the actual scale, the flow behavior should also be the same.

Another approach is to use the concept of geometric similarity, which states that two systems are geometrically similar if they have the same shape and relative dimensions. In this case, you would need to ensure that the dimensions of your model are scaled proportionally to the dimensions of the actual scale. This can be achieved by using appropriate scaling laws and techniques.

In terms of your CFD simulation, it is important to properly scale your model and input parameters to ensure that the Reynolds number is accurately represented. You may also want to consider using a turbulence model that is appropriate for the Reynolds number range you are studying.

Overall, it is important to carefully consider the scaling and correlation of the Reynolds number for your model and the actual scale in order to draw accurate conclusions from your experiments and simulations. I hope this information helps and wish you success in your research.