Equation or list of equations expected instead of true in the first \ argument

Question

Problem :

I am facing an error for the BC in this equation:

a = NDSolve[{D[h[t, x], t] + Sin[x Degree] h[t, x]^2 D[h[t, x], x] + 2/3 Cos[x Degree] h[t, x]^3 == 0, h[0, x] == 1, D[h[t, 0], x] == 0}, h, {t, 0, 100}, {x, 0, 90}]

NDSolve::deqn: Equation or list of equations expected instead of True in the first argument {2/3 Cos[[Degree] x] h[t,x]^3+h[t,x]^2 Sin[[Degree] x] (h^(0,1))[t,x]+(h^(1,0))[t,x]==0,h[0,x]==1,True}. >>

Any tips to resolve it?

Answer 1

I had also faced the similar issue in the recent past. I did lot of research on it and found the solution on it. This is the very common problem with the people getting started.

The last condition as shown below:

D[h[t, 0], x] == 0

Is always True as you derivate on the constant. If what you mean is

D[ h[t,x], x]  /. x->0

The that's something else.

For the time being, just remove it (and if possible just find another sensible boundary condition compatible with a order of your equation).

NDSolve[{D[h[t, x], t] + Sin[x] h[t, x]^2 D[h[t, x], x] + 2/3 Cos[x] h[t, x]^3 == 0, h[0, x] == 1}, h, {t, 0, 100}, {x, 0, Pi/2}]

It works for me.