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Unit Tangent Vector in a Scalar Field

  1. Aug 27, 2015 #1
    Hello,
    I am attempting to calculate unit normal and tangent vectors for a scalar field I have, Φ(x,y). For my unit normal, I simply used:
    [itex]\hat{n}=\frac{\nabla \phi}{|\nabla \phi|}[/itex]​
    However, I'm struggling with using this approach to calculate the unit tangent. I need to express it in terms of the gradient of the scalar field but I am having a hard time visualizing this.
    Thanks!
     
  2. jcsd
  3. Aug 27, 2015 #2

    ShayanJ

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    Gold Member

    Take a linear combination of basis vectors with unknown coefficients. Equate its dot product with ## \hat n ## to zero. You should be able to find two independent vectors as solutions. Those will span the tangent space to each point.
     
  4. Aug 27, 2015 #3
    Thanks for your response! I think I tried this: I used [itex] \hat{n} \cdot \hat{T} = 0 [/itex], writing each in terms of an x- and y-component, and then solving for the x-component and y-component of the unit tangent [itex] \hat{T} [/itex]. My [itex] T_x [/itex] and [itex] T_y [/itex] are written only in terms of unit normal components [itex] n_x [/itex] and [itex] n_y [/itex] (by defining [itex] \sqrt{T_x^2 + T_y^2} = 1 [/itex]. When I do this however, and plot my unit tensor field, I don't get clean, tangential vectors. Some are tangential, but they can be a bit of a mess. I am doing this in FEA software, could some of my issues be due to my mesh size? Or am I going about it the wrong way?
     
  5. Aug 28, 2015 #4

    ShayanJ

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    One obvious problem I see, is that you're assuming the problem is two dimensional, Its not! You should work in three dimensions. Otherwise ##\hat n\cdot \vec T=0## and ##|\vec T|=1## will completely determine the components and you'll get only one vector as a solution which we know isn't right.
    In fact you should've known this from the start because you are considering a function of two variables which can only be a surface in three dimensions.
     
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