Vertical Spacing using \subcaption package

Question

I have a simple problem. I'm trying to add a little bit of vertical spacing between the top two and bottom two subfigures. Here's my code:

\documentclass{article}
\usepackage[demo]{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\begin{document}
\begin{figure}
\centering
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[scale=0.2]{CHRe100t0_5.jpg}
  \caption{t = 0.5 s and Re = 100}
\end{subfigure}%
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[scale=0.2]{ChRe100t1.jpg}
  \caption{t = 1.0 s and Re = 100}
\end{subfigure}
\begin{subfigure}{.5\textwidth}
   \centering
   \includegraphics[scale=0.2]{CHRe100t1_5.jpg}
   \caption{t = 1.5 s and Re = 100}
\end{subfigure}%
\begin{subfigure}{.5\textwidth}
\centering
   \includegraphics[scale=0.2]{CHRe100t2.jpg}
   \caption{t = 2.0 s and Re = 100}
\end{subfigure}
\caption{y vs. u}
\end{figure}
\end{document}

Any suggestions would be greatly appreciated!

Answer 1

You could insert the instructions \par\bigskip between the second and third subfigure to create a bit of extra vertical space between the two sets of subfigures. If \bigskip is too much for your taste, try using \medskip.

\documentclass{article}
\usepackage[demo]{graphicx} % omit 'demo' option in real document
\usepackage{subcaption}
\begin{document}
\begin{figure}
\centering
\begin{subfigure}{.49\textwidth}
  \centering
  \includegraphics[scale=0.2]{CHRe100t0_5.jpg}
  \caption{t = 0.5 s and Re = 100}
\end{subfigure}
\begin{subfigure}{.49\textwidth}
  \centering
  \includegraphics[scale=0.2]{ChRe100t1.jpg}
  \caption{t = 1.0 s and Re = 100}
\end{subfigure}
\par\bigskip % force a bit of vertical whitespace
\begin{subfigure}{.49\textwidth}
   \centering
   \includegraphics[scale=0.2]{CHRe100t1_5.jpg}
   \caption{t = 1.5 s and Re = 100}
\end{subfigure}
\begin{subfigure}{.49\textwidth}
\centering
   \includegraphics[scale=0.2]{CHRe100t2.jpg}
   \caption{t = 2.0 s and Re = 100}
\end{subfigure}
\caption{y vs. u}
\end{figure}
\end{document}

Answer 2

The same result (as Mico) can be obtained with less typing:

\documentclass{article}
\usepackage[demo]{graphicx} % omit 'demo' option in real document
\usepackage{stackengine}
\begin{document}
\begin{figure}
\centering
\stackunder{\includegraphics[scale=0.2]{CHRe100t0_5.jpg}}{(a) t = 0.5 s and Re = 100}~~~~~~
\stackunder{\includegraphics[scale=0.2]{ChRe100t1.jpg}}{(b) t = 1.0 s and Re = 100}
\par\bigskip
\stackunder{\includegraphics[scale=0.2]{CHRe100t1_5.jpg}}{(c) t = 1.5 s and Re = 100}~~~~~~
\stackunder{\includegraphics[scale=0.2]{CHRe100t2.jpg}}{(d) t = 2.0 s and Re = 100}
\caption{y vs. u}
\end{figure}
\end{document}

Answer 3

Change the \belowcaptionskip length and use \par to denote new rows.

\begin{figure}
  \newcommand{\subfigurehspace}{.49\linewidth}
  \setlength{\belowcaptionskip}{0.5\baselineskip}
  \centering
  % %%% ROW 1 %%%
  \begin{subfigure}[b]{\subfigurehspace}
    \centering
    \includegraphics[width=\linewidth]{1.png}
    \caption{}
  \end{subfigure}
  \hfill
  \begin{subfigure}[b]{\subfigurehspace}
    \centering
    \includegraphics[width=\linewidth]{2.png}
    \caption{}
  \end{subfigure}
  \par
  % %%% ROW 2 %%% 
  \begin{subfigure}[b]{\subfigurehspace}
    \centering
    \includegraphics[width=\linewidth]{3.png}
    \caption{}
  \end{subfigure}
  \hfill
  \begin{subfigure}[b]{\subfigurehspace}
    \centering
    \includegraphics[width=\linewidth]{4.png}
    \caption{}
  \end{subfigure}
\end{figure}

If you don't want additional spacing after the last row, you can reset \belowcaptionskip to its original value before the final row. This should probably not be necessary, though.