As you know, there is no braille symbol assigned to an inverted capitalised Greek omega ℧, so you will need to create one and list it as a special symbol.
My first inclination is to create a symbol that hints at the meaning. I looked up Navier-Stokes equation on Wikipedia in order to get an idea what this symbol means, but only an upright Omega is used there. I looked up dimensionless velocity as well, with no luck. I asked a physicist. He's never seen this before.
You are free to create any symbol as long as it doesn't have another meaning in Nemeth. (Searched Appendix B of the Nemeth Code to be sure the new symbol is not listed there.) Since an upright Omega is (46, 6, 2456), it makes sense to me to make the upside-down version be (46, 4, 2456). By using the same base "2456" for omega, it will be easier for the reader to recognize within the formulae.
Let me know if this works for you.