non orthogonal 2d grid
 

Hey there
I have a nozzle like the image
http://uupload.ir/files/c94v_grid.png

the vertical grid is called r and the grid along each streamline is called m as a meridional grid

I dont know what to do to compute derivative of some parameter along m direcion with backward difference method...
i mean what should be the denominator ?

Just consider the transformation from the physical (csi,eta) to the computational domain (x,y). In your case csi=x, you have only to consider the transformation along the vertical direction given by the law eta(csi).

thanks
so you mean say for taking derivative of like function F along each stream line dFdm
will be (Fi-F(i-1))/(x(i)-x(i-1) ?

Grid
 

From my experience with curvilinear grid (albeit still learning), you should calculate the metrics using central schemes. In this case

(F(i+1) - F(I-1))/(2*dx))