unset key         
set xtics auto        
set xrange [0:20]        
rho=1.225  		# Air density kg/m^3 at sea level    
radius=3.0  		# Rotor radius in meters    
area=pi*(radius**2)  	# Area swept by turbine rotors    
Cp=0.4  		# Power coefficient     
Ng=0.75  		# Generator efficiency     
Nb=0.9  		# Mechanical efficiency     
set dummy u         
Power (u)=area* (rho* (u**3)/2)/1000      
set xlabel “Wind speed, u_0/m.s^{-1}”    
set ylabel “Wind power density, P/kW.m^{-2}”   
plot Power(u)*Cp*Ng*Nb/area lw 2