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$$\mathrm{Re} = \frac{\rho \cdot v \cdot c}{\mu}$$

Where $\rho$ is the pressure, $v$ the velocity relative to the airflow, $c$ the chord of the airfoil and $\mu$ the air density.

Does that mean that an airfoil with chord $c_{\text{airfoil}}$ will have the same Reynolds number as a sphere with diameter $c_{\text{sphere}}$ and a cube with length $c_{cube}$, if $c_{\text{airfoil}} = c_{\text{sphere}} = c_{\text{cube}}$? (assuming angle of attack is zero and the cube has 4 plains parallel to the direction of velocity)

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    $\begingroup$ Yes. So...? In each case, the drag coefficient is a different function of the Reynolds number. $\endgroup$ May 20, 2017 at 20:37

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The value of Reynolds number is not something absolute. Even for a given body in a given flow, its value varies depending on what you choose for scales of length and velocity. However once you make a choice, you must consistently stick with it through the rest of your analysis for a given geometry. To answer your question if you choose your length scales such that they are identical for the three bodies, then the Reynolds number would be the same for all of them. But this in itself does not signify anything, nor does it permit application of results obtained for one of the shapes to others, because the geometry is different for the three cases.

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