# Aerospace Corelating Re number for model scale to actual scale

1. Feb 17, 2010

### 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.

2. Feb 17, 2010

### 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.

3. Feb 17, 2010

### 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).

4. Feb 17, 2010

### Cyrus

For aircraft, this is almost always impossible.