Gradient of a surface

Hi all,

I have written the following code in order to determine the gradient of a function between certain X and Y values specified by the X and Y values limits of the graph I am plotting.

 K = 273.15 ; %constant

 [x, y] = meshgrid(1.4:0.1:5,70:5:200) ; %x & y values

 z = (x.^(0.4/1.4) - 1) ./ ( (y + K)/(25 + K) - 1) ; %function
 [Fx, Fy] = gradient(z) ; %gradient of the function

 figure
 contour(x, y, z, [0:0.1:1], 'ShowText', 'on')

 ax = gca ; 
 ax.XLim = [1.4, 5] ;
 ax.YLim = [70, 200] ;

 grid on

 xlabel('Pressure Ratio (-)')
 ylabel('Temperature (degC)')

Howver, when I try to plot the Fx and Fy, the plot comes out to be blank. Therefore, how do i find the gradient of the function between the desired X & Y values?

Thanks for your help in advance,

KMT.

Answer 1

[Walter Roberson]

>> max(Fx(:))
ans =
         0.145210103674753
>> min(Fx(:))
ans =
        0.0155307058204656
>> max(Fy(:))
ans =
      -0.00505640320134679
>> min(Fy(:))
ans =
        -0.386812925230569

The gradient is the change in values. It is to be expected that the change in values is often less than the range of values that you happen to be plotting. If you stick with the contour interval 0:.1:1 then only the 0.1 level has any data for Fx, and none of the Fy is in the range 0 to 1 so if you use 0:0.1:1 then as your level list then you would expect a blank plot.