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Homework Help: Matlab plotting please help

  1. Apr 21, 2010 #1
    ψ4 + ψ2 U2(x2-y2+a2) = U4x2y2

    This is the stream line equation of a flow along a vertical plate!!
    i need to plot the stream lines using matlab!! ψ is the stream function
    U is constant and 'a' is the plate length
     
  2. jcsd
  3. Apr 21, 2010 #2

    Mark44

    Staff: Mentor

    Write some matlab code for these calculations and we'll help you with it, but we're not going to write the code for you.
     
  4. Apr 21, 2010 #3
    i tried to find the roots of the equation then i have to find the derivatives with respect to x and y to be able to draw them

    solve ('z^4+z^2*UINF^2*(x^2-y^2+a^2)-(UINF^4*x^2*y^2)=0','z')

    ans =

    1/2*(2*(x^4+2*x^2*y^2+2*x^2*a^2+y^4-2*y^2*a^2+a^4)^(1/2)-2*x^2+2*y^2-2*a^2)^(1/2)*UINF
    -1/2*(2*(x^4+2*x^2*y^2+2*x^2*a^2+y^4-2*y^2*a^2+a^4)^(1/2)-2*x^2+2*y^2-2*a^2)^(1/2)*UINF
    1/2*(-2*(x^4+2*x^2*y^2+2*x^2*a^2+y^4-2*y^2*a^2+a^4)^(1/2)-2*x^2+2*y^2-2*a^2)^(1/2)*UINF
    -1/2*(-2*(x^4+2*x^2*y^2+2*x^2*a^2+y^4-2*y^2*a^2+a^4)^(1/2)-2*x^2+2*y^2-2*a^2)^(1/2)*UINF

    one of the deravites is the folowwing (s)
    x=-5:1:5
    y=-5:1:5
    s=1/4/(-2*(x.^4+2*x.^2*y.^2+2*x.^2*a.^2+y.^4-2*y.^2*a.^2+a.^4).^(1/2)-2*x.^2+2*y.^2-2*a.^2).^(1/2)*UINF*(-1/(x.^4+2*x.^2*y.^2+2*x.^2*a.^2+y.^4-2*y.^2*a.^2+a.^4).^(1/2)*(4*x.^2*y+4*y.^3-4*y*a.^2)+4*y)
    a=1
    UINF=1
    plot(x,y,s)

    x =

    -5 -4 -3 -2 -1 0 1 2 3 4 5


    y =

    -5 -4 -3 -2 -1 0 1 2 3 4 5

    ??? Error using ==> mtimes
    Inner matrix dimensions must agree.



    i cant draw it why??
     
    Last edited: Apr 21, 2010
  5. May 17, 2011 #4
    If you follow the message given by MATLAB , YOU SEE THIS:

    ??? Error using ==> mtimes
    Inner matrix dimensions must agree. HERE BECAUSE YOU FORGET A DOTE OR SOMETHING
    You have missed to mesh you data
    Matlab does not recognise a paire of data without this code
    [x,y]=mesh(x=data, y=data) for example [x,y]=mesh(x=0:1:10, y=0:1:10) this should be written before the equations;
    Also you have missed something in the equation that will plot your graph
    for, example you x and y are varying and you devided 1/ by the rest of the equation you nedd to this ( 1./equation in your equation you missed the dote ) don't forget the dote after 1
    Try this. I think this will help you to solve your problem, GOOD LUCK
     
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