How to properly typeset multiletter quantities, e.g. Re or Nu (Reynolds, Nusselt numbers)
Update
I collected the ideas in the comments and answers and wrote myself some macros for the numbers I need. Maybe anybody else wants to use them. Works fine with sub- and superscripts and in multiplications a.s.o.
\documentclass{article}
\usepackage{mathtools}
\usepackage[%
,math-style=ISO
,bold-style=ISO
,sans-style=italic
]{unicode-math}
\newcommand{\Arch}{\operatorname{\mathit{A\kern-.06em r}}} % http://de.wikipedia.org/wiki/Archimedes-Zahl
\newcommand{\Biot}{\operatorname{\mathit{B\kern-.06em i}}} % http://de.wikipedia.org/wiki/Biot-Zahl
\newcommand{\Cauc}{\operatorname{\mathit{C\kern-.07em a}}} % http://de.wikipedia.org/wiki/Cauchy-Zahl
\newcommand{\Damk}{\operatorname{\mathit{D\kern-.06em a}}} % http://de.wikipedia.org/wiki/Damk%C3%B6hler-Zahl
\newcommand{\Eule}{\operatorname{\mathit{E\kern-.03em u}}} % http://de.wikipedia.org/wiki/Euler-Zahl
\newcommand{\Four}{\operatorname{\mathit{F\kern-.10em o}}} % http://de.wikipedia.org/wiki/Fourier-Zahl
\newcommand{\Frou}{\operatorname{\mathit{F\kern-.07em r}}} % http://de.wikipedia.org/wiki/Froude-Zahl
\newcommand{\Gras}{\operatorname{\mathit{G\kern-.05em r}}} % http://de.wikipedia.org/wiki/Grashof-Zahl
\newcommand{\Karl}{\operatorname{\mathit{K\kern-.11em a}}} % http://de.wikipedia.org/wiki/Karlovitz-Zahl
\newcommand{\Knud}{\operatorname{\mathit{K\kern-.11em n}}} % http://de.wikipedia.org/wiki/Knudsen-Zahl
\newcommand{\Lewi}{\operatorname{\mathit{L\kern-.05em e}}} % http://de.wikipedia.org/wiki/Lewis-Zahl
\newcommand{\Mach}{\operatorname{\mathit{M\kern-.10em a}}} % http://de.wikipedia.org/wiki/Mach-Zahl
\newcommand{\Nuss}{\operatorname{\mathit{N\kern-.09em u}}} % http://de.wikipedia.org/wiki/Nusselt-Zahl
\newcommand{\Pecl}{\operatorname{\mathit{P\kern-.08em e}}} % http://de.wikipedia.org/wiki/P%C3%A9clet-Zahl
\newcommand{\Pran}{\operatorname{\mathit{P\kern-.03em r}}} % http://de.wikipedia.org/wiki/Prandtl-Zahl
\newcommand{\Rayl}{\operatorname{\mathit{R\kern-.04em a}}} % http://de.wikipedia.org/wiki/Rayleigh-Zahl
\newcommand{\Reyn}{\operatorname{\mathit{R\kern-.04em e}}} % http://de.wikipedia.org/wiki/Reynolds-Zahl
\newcommand{\Schm}{\operatorname{\mathit{S\kern-.07em c}}} % http://de.wikipedia.org/wiki/Schmidt-Zahl
\newcommand{\Sher}{\operatorname{\mathit{S\kern-.07em h}}} % http://de.wikipedia.org/wiki/Sherwood-Zahl
\newcommand{\Stro}{\operatorname{\mathit{S\kern-.07em r}}} % http://de.wikipedia.org/wiki/Strouhal-Zahl
\newcommand{\Webe}{\operatorname{\mathit{W\kern-.14em e}}} % http://de.wikipedia.org/wiki/Weber-Zahl
\begin{document}
\begin{align*}
\Arch &=\frac{\increment\rho g L^3}{\rho\nu^2} \\
\Biot &= \frac{\alpha \cdot L}{\lambda_\mathup{s}} \\
\Cauc &= \frac{\rho \cdot \omega^2l^2}{E} \\
\Reyn &\approx \Damk^2\Karl^2 \\
\Lewi &= \frac{\Schm}{\Pran} \\
\end{align*}
\end{document}
This yields e.g.:
Please feel free to edit my kerning or to add more numbers!
The book Detailtypografie from Forssman and de Jong explains:
Symbols for physical (and technical) values are set italic. Likewise characteristics (dimensionless numbers), whose symbols consist of several letters, are set italic (here the text italic must be taken).
Loosely translated by me... sorry for bad English. Examples given by them are Eu, Re, Fr, Sr, Ma, and We.
Translated to LaTeX (the Math-chapter has been set in LaTeX by Johannes Küster), this would mean \textit{...}. Or am I wrong?
The problem however is, that the text italic changes with the surrounding font. I don't like this behavior, as it is hard to read sans serif symbols in serif formulas. For my taste, a symbol should stay the same in the whole document.
Then maybe I should use \mathit{...}? The problem is, there is no way to distinguish this from two multiplied variables.
DIN EN ISO 80000 says:
Symbols for characteristics, like the Mach-number, symbol Ma, are set with two letters from the Latin alphabet, always with a large initial letter. It is recommended that such two-letter-symbols are separated by a space to other symbols, when appearing in a multiplication.
I have set some versions I have seen until know. I also saw a calligraphic Re or a Ma with reduced distance between M and a. But can't find those right now.
The distance between indices and symbol is differing between math italic and text italic. Don't know, what would be better.
This is not really an answer but an appeal to everybody, that there are some more thoughts needed. Seems like there is still no real solution around.
When in doubt, I normally consult Knuth's publications (if I expect to find a paper that is likely to have an example of the issue at hand) and secondly the relevant Journal author guidelines (in that order).
Sometimes the latter will have some badly made templates, but most probably would have strict and stroppy editors. One caveat is that if you aiming at eventually publishing the document the editor is likely to have the final say and I would suggest it is unwise to get into an argument about typography with the editor.
I guess that the OP is dealing with Fluid Mechanics. The Journal of Fluid Mechanics has instructions as to how to typeset these dimensionless numbers, including a template and instructions.
Since these are essentially numbers I would recommend the approach that they are typeset in math italic font. It also looks better if you are describing any of these numbers inline.
The above cited journal has them defined as:
\newcommand\Rey{\mbox{\textit{Re}}} % Reynolds number
\newcommand\Pran{\mbox{\textit{Pr}}} % Prandtl number, cf TeX's \Pr product
\newcommand\Pen{\mbox{\textit{Pe}}} % Peclet number
which, I agree with egreg is not very wise. However, since they provide the .cls is not the end of the world and the template works.
Personally I recommend you use the \DeclareMathOperator from amsmath (if you worry about spacing-personally I wouldn't). Also my own preference is to use a notation such as,
N_{\mathrm{Re} rather than a double symbol such as Re.
As a final word, I think consistency is the key and that you should use the notation that is most familiar with your readers.
That depends on your tradition and what these are actually used for? If they are operators, I'd define them as such using \DeclareMathOperator (from amsmath). If it is just a name I'd use \mathrm or \mathit depending on tradition.
But I would also define macros for each of these such that I never write $\mathrm{Re}$ in the text.
BTW, if Re is just the real part operator, and you do not like the default look of \Re (most don't) just use
\renewcommand\Re{\operatorname{Re}}
as \Re should behave as an operator