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please how can I write gradient numerically? I have an eq. $\dfrac{\partial X}{\partial t}=\nabla^2 X + u\nabla X$ I wonder how write this in 3D. Many thanks for any idea... I can find how to solve the "pure" heat eq. but so I don't know..

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closed as not a real question by Christian Clason, Daniel Shapero, Ben, Jan, Bill Barth Jun 14 '13 at 18:45

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

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Hi lovis, and welcome to scicomp! I presume $X$ is a scalar quantity, right? –  Paul Feb 7 '13 at 17:32
    
yes, it is. Many thanks for your message. What can be done with this? –  lovis Feb 7 '13 at 17:39
    
Excellent. Are you using finite difference method to solve this equation as well? What order accuracy do you expect your solution to have? –  Paul Feb 7 '13 at 17:47
    
Do you only want to know how to discretize the gradient or how to solve the system? Can you rewrite your question to clarify? –  vanCompute Feb 7 '13 at 17:48
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Your equation is called advection-diffusion equation. You may want to google for that. –  Wolfgang Bangerth Feb 8 '13 at 0:39
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