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Coordinate Transformation

  1. Oct 16, 2004 #1

    Clausius2

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    Gold Member

    Ok, I have a strong problem. It has to do with programming and concepts of mathematics.

    I have a coordinate transformation:

    [tex] x=x(\xi,\eta)[/tex]
    [tex] y=y(\xi,\eta)[/tex];

    and I want to calculate the metrics coefficients:

    [tex] \xi_x=\frac{\partial \xi}{\partial x}[/tex]
    [tex] \eta_x=\frac{\partial \eta}{\partial x}[/tex]
    [tex] \xi_y=\frac{\partial \xi}{\partial y}[/tex]
    [tex] \eta_y=\frac{\partial \eta}{\partial y}[/tex]

    But I have only the next (I only have the numeric transformation, I don't have any analytical formulae):

    x= a matrix of dimensions [tex] n_{\xi}, n_{\eta}[/tex];
    y= a matrix of dimensions [tex] n_{\xi}, n_{\eta}[/tex];

    [tex] \xi[/tex]= a vector of size [tex]n_{\xi}[/tex]
    [tex] \eta[/tex]= a vector of size [tex]n_{\eta}[/tex]

    Well, I want to compute the metrics coefficients using central diferences. Please, tell me what would you do in order to compute it. I'm goind to show you what i made in Matlab:

    for i=2:nxi-1;
    for j=2:neta-1;

    XI_x(i)=(XI(i+1)-XI(i-1))/(x(i+1,j)-x(i-1,j));
    ETA_x(j)=(ETA(j+1)-ETA(j-1))/(x(i,j+1)-x(i,j-1));
    XI_y(i)=(XI(i+1)-XI(i-1))/(y(i+1,j)-y(i-1,j));
    ETA_y(j)=(ETA(j+1)-ETA(j-1))/(y(i,j+1)-y(i,j-1));

    end
    end

    I'm not sure it works, because I'm not able to imagine the metrics coefficients just acting upon the functions x and y. I don't know If i'm taking correctly the variations in x and y in the denominators.

    Any advice will be greatly appreciated.
     
  2. jcsd
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