\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary {arrows.meta,graphs,graphdrawing,positioning} \usegdlibrary {layered, trees}
\tikzset{nodes={text height=.7em, text width=2.8cm, align=center,
draw=black!50, thick, font=\footnotesize, fill=white},
>={Stealth[round,sep]}, rounded corners, semithick}

\begin{document}
\begin{tikzpicture}[on grid,y=1.2cm,x=3.2cm]
    \draw[fill=lightgray] (-1.7cm , 1.4cm) rectangle (1.7cm, -4.1cm);
    \draw[fill=lightgray] (-1.7cm,-4.3cm) rectangle (5cm, -8.0cm);
    \input{design_flow.tikz}
    \node (prep)[above=0.7 of pd, draw=none, fill=none] {Preliminary Design};
    \node (b)[right=0.5 of rd,fill=none, draw=none] {};
    \node (ridm)[below=0.7 of b, fill=none, draw=none,text width=5cm] {Rapid Iterative Design Method};
\end{tikzpicture}
\end{document}