How to use comma around mathematical symbols?

I am having trouble determining the correct use of comma when specifying mathematical symbols, and the literature that I follow does not seem to be consistent. Which of the following would be correct usage of comma and why? The symbol C_D refers to drag coefficient.

• The enhancement of drag coefficient, C_D, is partly due to the offshore flow over limited fetch.

• The enhancement of drag coefficient C_D is partly due to the offshore flow over limited fetch.

• The enhancement of drag coefficient, C_D is partly due to the offshore flow over limited fetch.

The reason for specifying both the quantity name and its mathematical symbol is because the sentence references an element in a figure.

Solution 1:

I like the second one best, hands down. This choice comes up in non-mathematical contexts too: My brother Billy is coming with us. Yes, you could write My brother, Billy, is coming with us. But there's no need. In a more complex sentence, the commas can be helpful, but here, and in your sentence, they just drag us down.