# How to solve two coupled pde's

by keyns
Tags: coupled, solve
 P: 3 I'm new, hi all. I have two coupled equations, one of which is continuity. Basically, my problem comes down to the following system: (1) $u=f(v)$ (similarly, $v=g(u)$. Here, u and v are the components of a vector field, ie u=u(x,y) and v=v(x,y). (2) Continuity: $\nabla \cdot \textbf{u} = 0$ or $u_{x}+v_{y}=0$ From here, I can find the following expressions $u_{x} = -g_{y} \left( u \right)$ $v_{y} = -f_{x} \left( v \right)$ Which I think leaves an equation of the form $G \left( u,u_{x},u_{y} \right)=0$ and $F \left( v,v_{x},v_{y} \right)=0$ It seems to me my original problem has two variables and I have two equations. I think this should be solvable, but I don't know how. Any help please? Thanks in advance! --edit-- p.s. I'm looking for a numerical (discrete) solution.
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P: 1,391
 Quote by keyns I'm new, hi all. I have two coupled equations, one of which is continuity. Basically, my problem comes down to the following system: (1) $u=f(v)$ (similarly, $v=g(u)$. Here, u and v are the components of a vector field, ie u=u(x,y) and v=v(x,y).
If u = f(v) and v = g(u), don't you just have a (possibly nonlinear) system of equations to solve for v and u? What is the continuity condition (2) for? Do you get more than one solution by solving (1) and you need (2) to choose a solution of interest?
P: 3
 Quote by Mute If u = f(v) and v = g(u), don't you just have a (possibly nonlinear) system of equations to solve for v and u? What is the continuity condition (2) for? Do you get more than one solution by solving (1) and you need (2) to choose a solution of interest?
Actually I have only one relation for $u$ and $v$ that I can write it as $u(v)$ or $v(u)$. Sorry for the confusion. Otherwise you would be right. To clarify my equations:

(1) A relation for $u$ and $v$ (if I have $u$, I have $v$ and vice versa)
(2) A relation for $u_{x}$ and $v_{y}$ (if I have $u_{x}$, I have $v_{y}$ and vice versa

--edit-- Which then, after some rewriting, leads to two ODE's $G$ and $F$ as stated before. I just don't know how to solve those.

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