\documentclass[a4paper,12pt]{article}

\usepackage[demo]{graphicx}
\usepackage{subfigure}
\usepackage{amsthm,amssymb,amsmath}

\usepackage[top=30mm, bottom=30mm, left=25mm, right=25mm]{geometry}

%\graphicspath{{images/}}


\begin{document}
%\section{Introduction}
%\section{Mumerical Method}
%\section{Results and Discussion}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clp1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdp1}
}
\subfigure{
\includegraphics[scale=0.3]{clp2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdp2}
}
\subfigure{
\includegraphics[scale=0.3]{clp3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdp3}
}
\caption{The effect of Gurney flap position on lift and drag coefficient of oscillating airfoil, a) Re=500, b) Re=1000, c) Re=2000}
\label{coe_re_pos}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clpk1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdpk1}
}
\subfigure{
\includegraphics[scale=0.3]{clpk2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdpk2}
}
\subfigure{
\includegraphics[scale=0.3]{clpk3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdpk3}
}
\caption{The effect of Gurney flap position on lift and drag coefficient of oscillating airfoil, a) k=0.125, b) k=0.25, c)k=0.5}
\label{coe_k_pos}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clcdp1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcdp2}
}
\subfigure{
\includegraphics[scale=0.3]{clcdp3}
}
\caption{The effect of Gurney flap position on aerodynamic efficiency of oscillating airfoil, a) Re=500, b) Re=1000, c) Re=2000}
\label{clcd_re_pos}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clcdpk1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcdpk2}
}
\subfigure{
\includegraphics[scale=0.3]{clcdpk3}
}
\caption{The effect of Gurney flap position on aerodynamic efficiency of oscillating airfoil, a) k=0.125, b) k=0.25, c)k=0.5}
\label{clcd_k_pos}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{cla1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cda1}
}
\subfigure{
\includegraphics[scale=0.3]{cla2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cda2}
}
\subfigure{
\includegraphics[scale=0.3]{cla3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cda3}
}
\caption{variation of aerodynamic coefficient for oscillating airfoil with different Gurney flap mountin angles, a) Re=500, b) Re=1000, c) Re=2000}
\label{coe_re_ang}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clak1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdak1}
}
\subfigure{
\includegraphics[scale=0.3]{clak2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdak2}
}
\subfigure{
\includegraphics[scale=0.3]{clak3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdak3}
}
\caption{variation of aerodynamic coefficient for oscillating airfoil with different Gurney flap mountin angles, a) k=0.125, b) k=0.25, c)k=0.5}
\label{coe_k_ang}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clcda1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcda2}
}
\subfigure{
\includegraphics[scale=0.3]{clcda3}
}
\caption{variation of aerodynamic efficiency for oscillating airfoil with different Gurney flap mountin angles, a) Re=500, b) Re=1000, c) Re=2000}
\label{clcd_re_ang}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clcdak1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcdak2}
}
\subfigure{
\includegraphics[scale=0.3]{clcdak3}
}
\caption{variation of aerodynamic efficiency for oscillating airfoil with different Gurney flap mountin angles, a) k=0.125, b) k=0.25, c)k=0.5}
\label{clcd_k_ang}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clh1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdh1}
}
\subfigure{
\includegraphics[scale=0.3]{clh2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdh2}
}
\subfigure{
\includegraphics[scale=0.3]{clh3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdh3}
}
\caption{variation of aerodynamic coefficient for oscillating airfoil with different Gurney flap's height, a) Re=500, b) Re=1000, c) Re=2000}
\label{coe_re_hei}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clhk1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhk1}
}
\subfigure{
\includegraphics[scale=0.3]{clhk2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhk2}
}
\subfigure{
\includegraphics[scale=0.3]{clhk3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhk3}
}
\caption{variation of aerodynamic coefficient for oscillating airfoil with different Gurney flap's height, a) k=0.125, b) k=0.25, c)k=0.5}
\label{coe_k_hei}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clcdh1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcdh1}
}
\subfigure{
\includegraphics[scale=0.3]{clcdh2}
}
\caption{variation of aerodynamic efficiency for oscillating airfoil with different Gurney flap's height, a) Re=500, b) Re=1000, c) Re=2000}
\label{clcd_re_hei}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clcdhk1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcdhk1}
}
\subfigure{
\includegraphics[scale=0.3]{clcdhk2}
}
\caption{variation of aerodynamic efficiency for oscillating airfoil with different Gurney flap's height, a) k=0.125, b) k=0.25, c)k=0.5}
\label{clcd_k_hei}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clcdhh1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcdhh2}
}
\subfigure{
\includegraphics[scale=0.3]{clcdhh3}
}
\caption{variation of aerodynamic coefficients for oscillating airfoil with different Gurney flap's height, Re=1000, k=0.5}
\label{clcd_c_hei}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clnon}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdnon}
}
\caption{variation of lift and drag coefficients for oscillating airfoil with 7\% height Gurney flap at Re=2000 and  k=0.5}
\label{cl-cd-non}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clhg1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhg1}
}
\subfigure{
\includegraphics[scale=0.3]{clhg2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhg2}
}
\subfigure{
\includegraphics[scale=0.3]{clhg3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhg3}
}
\caption{variation of aerodynamic coefficient for oscillating airfoil with different Gurney flap's horizontal gap, a) Re=500, b) Re=1000, c) Re=2000}
\label{coe_re_hg}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clvg1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdvg1}
}
\subfigure{
\includegraphics[scale=0.3]{clvg2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdvg2}
}
\subfigure{
\includegraphics[scale=0.3]{clvg3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdvg3}
}
\caption{variation of aerodynamic coefficient for oscillating airfoil with different Gurney flap's verticall gap, a) Re=500, b) Re=1000, c) Re=2000}
\label{coe_re_vg}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clhgk1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhgk1}
}
\subfigure{
\includegraphics[scale=0.3]{clhgk2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhgk2}
}
\subfigure{
\includegraphics[scale=0.3]{clhgk3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdhgk3}
}
\caption{variation of aerodynamic coefficient for oscillating airfoil with different Gurney flap's horizontall gap, a) k=0.125, b) k=0.25, c)k=0.5}
\label{coe_k_hg}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clvgk1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdvgk1}
}
\subfigure{
\includegraphics[scale=0.3]{clvgk2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdvgk2}
}
\subfigure{
\includegraphics[scale=0.3]{clvgk3}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{cdvgk3}
}
\caption{variation of aerodynamic coefficient for oscillating airfoil with different Gurney flap's vertical gap, a) k=0.125, b) k=0.25, c)k=0.5}
\label{coe_k_vg}
\end{figure}

\begin{figure}
\centering
\subfigure{
\includegraphics[scale=0.3]{clcdhg1}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcdhg2}
}
\hspace*{0.5mm}
\subfigure{
\includegraphics[scale=0.3]{clcdhg3}
}
\end{figure}

\end{document}