Specifying du/dx in pde toolbox error

I am trying to solve an elliptic PDE but it has a first order partial derivative w.r.t x, but in PDE specification, it has to be specified as f. But i don't know how to do that.

Instead, I tried to solve much simpler approach, and then I'll try to implement it to PDE I want to solve. I'm trying to solve an ODE using PDE toolbox (although it is so unefficient, but I hope it works).

Suppose I want to solve : u'' + u' + u = 0, with BC : u(0) = 5, u(1) = -0.2.

So I drew a rectangle with length 1, arbitrary width (in my case, I used 0.4 width).

and boundary conditions I used are : left = Dirichlet B.C., h = 1, r = 5; right = Dirichlet, h = 1, r = -0.2. up and bottom, Neumann = g = q = 0.

(PDE Specification) Elliptic type, c= -1, a = -1, and f = -ux ( I want to specify du/dx here).

When I run, I get this error : "Expression evaluates to wrong size. Must be scalar or row vector. In a system case, pass first or second row; for example u(2,:). In expression: '-ux' when evaluating pde coefficients."

I want to ask about how can I specify du/dx in PDE coefficient?

Thanks in advance.

Answer 1

[Bill Greene]

Hi,

Everything you have done is correct. But, because of the ux term in f, you also have to check the "Use nonlinear solver" box in the Solve/Parameters dialog. It is unfortunate that the error message is not very helpful in identifying this problem!

Bill