I need to solve 4 coupled PDEs. Thanks to @xzczd's help, a few days ago I can obtain the solution of the first 2 equations with some proper boundary conditions, see this post. Now, these two equations have been coupled with the other two, in which I encountered some new issues.
L = 30; tmax = 20; c = 1/10; \[Epsilon] = 1/10; eqs = {D[Ar[x, t], t] + 1/4 (Ar[x, t]^3 + Ai[x, t]^2 Ar[x, t]) - D[Ar[x, t], {x, 2}] - 2 Ar[x, t] == 0, D[Ai[x, t], t] + 1/4 (Ai[x, t]^3 + Ar[x, t]^2 Ai[x, t]) - D[Ai[x, t], {x, 2}] - 2 Ai[x, t] == 0, D[Br[x, t], t] - D[Br[x, t], {x, 2}] + (Ar[x, t]^2 - Ai[x, t]^2)*(1/4*Br[x, t] + 10*x*D[Ar[x, t], x] + (50*x^2 - 1/4)* D[Ai[x, t], x]) + (Ar[x, t]^2 + Ai[x, t]^2)*(1/2*Br[x, t] + 10*x*D[Ar[x, t], x] - (50*x^2 + 3/4)*D[Ai[x, t], x]) + (1/2*Bi[x, t] + 20*x*D[Ai[x, t], x] - (100*x^2 - 1/2)*D[Ar[x, t], x])*Ar[x, t]*Ai[x, t] - 2*Br[x, t] - 2*D[Ai[x, t], {x, 3}] - 6*D[Ai[x, t], x] - 2*Ar[x, t] == 0, D[Bi[x, t], t] - D[Bi[x, t], {x, 2}] + (Ar[x, t]^2 - Ai[x, t]^2)*(-(1/4)*Bi[x, t] - 10*x*D[Ai[x, t], x] + (50*x^2 - 1/4)*D[Ar[x, t], x]) + (Ar[x, t]^2 + Ai[x, t]^2)*(1/2*Bi[x, t] + 10*x*D[Ai[x, t], x] + (50*x^2 + 3/4)*D[Ar[x, t], x]) + (1/2*Br[x, t] + 20*x*D[Ar[x, t], x] + (100*x^2 - 1/2)*D[Ai[x, t], x])*Ar[x, t]*Ai[x, t] - 2*Bi[x, t] + 2*D[Ar[x, t], {x, 3}] + 6*D[Ar[x, t], x] - 2*Ai[x, t] == 0}; subject to the boundary conditions:
bc = {Ar[-L, t] == Ar[L, t], Ai[-L, t] == -Ai[L, t], Br[-L, t] == Br[L, t], Bi[-L, t] == -Bi[L, t], \[Epsilon]*Derivative[1, 0][Ar][-L, t] - Ai[-L, t] == \[Epsilon]*Derivative[1, 0][Ar][L, t] - Ai[L, t], \[Epsilon]*Derivative[1, 0][Ai][-L, t] + Ar[-L, t] == -\[Epsilon]*Derivative[1, 0][Ai][L, t] - Ar[L, t], \[Epsilon]*Derivative[1, 0][Br][-L, t] - Bi[-L, t] == \[Epsilon]*Derivative[1, 0][Br][L, t] - Bi[L, t], \[Epsilon]*Derivative[1, 0][Bi][-L, t] + Br[-L, t] == -\[Epsilon]*Derivative[1, 0][Bi][L, t] - Br[L, t]}; The initial conditions are set randomly:
ic = {Ar[x, 0] == iniAr[x], Ai[x, 0] == iniAi[x], Br[x, 0] == iniBr[x], Bi[x, 0] == iniBi[x]}; iniAr = ListInterpolation[RandomReal[{-c, c}, 20], {{-L, L}}]; iniAi = ListInterpolation[RandomReal[{-c, c}, 20], {{-L, L}}]; iniBr = ListInterpolation[RandomReal[{-c, c}, 20], {{-L, L}}]; iniBi = ListInterpolation[RandomReal[{-c, c}, 20], {{-L, L}}]; As mentioned in the previous post, when solving it with NDSolve
sol = NDSolveValue[{eqs, ic, bc}, {Ar[x, t], Ai[x, t], Br[x, t], Bi[x, t]}, {x, -L, L}, {t, 0, tmax}] it returns an error
NDSolveValue::bcedge: Boundary condition Ai[-30,t]==-Ai[30,t] is not specified on a single edge of the boundary of the computational domain.
So I tried @xzczd's pdetoode
points = 200; domain = {-L, L}; difforder = 4; grid = Array[# &, points, domain]; ptoofunc = pdetoode[{Ar, Ai, Br, Bi}[x, t], t, grid, difforder]; odebc = Map[ptoofunc, bc, {2}] del = #[[2 ;; -2]] &; odeic = del /@ ptoofunc@ic; ode = del /@ ptoofunc@eqs; sollst = NDSolveValue[{ode, odeic, odebc}, Table[v[x], {v, {Ar, Ai, Br, Bi}}, {x, grid}], {t, 0, tmax}]; However, I got another error:
NDSolveValue::mconly: For the method NDSolve`IDA, only machine real code is available. Unable to continue with complex values or beyond floating-point exceptions.
What is machine real code? Could anyone help with solving the problem and explaining the error? Thank you in advance!
