flushright of text within align environment

Question

\begin{align}
\label{eq:3.1}u &= \frac{\partial r(x,t)}{\partial t} \qquad \text{(flow velocity)}
\end{align}

yields the following:

I desire the text "flow velocity" to be flushed to the right and attempted to do so with \tag{\text{flow velocity}}, but this took the place of my label which is undesired. How do I flush the text as described?

Answer 1

You want to make the description to be zero width (or some eye-computed width in case there's overlap)

\documentclass[leqno]{article}
\usepackage{amsmath}

\usepackage{kantlipsum}% for mock text
\usepackage[pass,showframe]{geometry}% to see the margins

\newcommand{\eqname}[2][0pt]{\makebox[#1][r]{(#2)}}

\begin{document}

\kant*[1][1-6]
\begin{flalign}
&& u &= \frac{\partial r(x,t)}{\partial t} & \eqname{flow velocity} \\
&& v &= a+b & \eqname{something else}
\end{flalign}
Just to check centering
\[u = \frac{\partial r(x,t)}{\partial t}\]

\end{document}

With the optional argument (a length), the equation would be pushed left: use it when the equation or the description is too long and forcing the equation to be centered would not be appropriate.

\begin{flalign}
&& u &= \frac{\partial r(x,t)}{\partial t} & \eqname[6em]{flow velocity} \\
&& v &= a+b & \eqname[6em]{something else}
\end{flalign}

Answer 2

You could use the flalign environment, which takes up the full horizontal width, although in order to keep the main aligned equation group near the center, rather than the left, you could put in a blank equation set to its left with & & before the start of the equation set in the middle:

\documentclass[leqno]{article}
\usepackage{amsmath}
\usepackage{lipsum}% for dummy text
\begin{document}

\lipsum[1]% dummy text so you can see where the margin is

\begin{flalign}
\label{eq:3.1} & & u &= \frac{\partial r(x,t)}{\partial t} & \text{(flow velocity)}
\end{flalign}
\end{document}

Answer 3

Here's another flalign-based solution, which places the equation close to the middle of the horizontal line (as opposed to, say, equidistant from the material at the left- and right-hand edges).

\documentclass[leqno]{article}
\usepackage{amsmath}
\counterwithin{equation}{section} % just for this example
\begin{document}
\setcounter{section}{3} % just for this example

\begin{flalign}\label{eq:3.1}
&&&&  u = \frac{\partial r(x,t)}{\partial t} && \text{(flow velocity)}
\end{flalign}
\end{document}