Switch to interpreted units with siunitx
fluid-mechanics: Denote dimen. by BIPM standards

According to International Bureau of Weights and Measures (BIPM), the
seven base dimensions used in SI should be expressed as sans serif
upright capital letters, and they provide a list of which dimensions
correspond to which letters. See this page for more information:

http://www.bipm.org/en/si/si_brochure/chapter1/1-3.html