# Periodic boundary conditions for PDE system using MethodOfLines.jl

Hi all! I’m working with ModellingToolkit and DiffEqOperators to solve a set of PDEs. In particular, I wanted to test out a 1D Kuramoto-Sivashinsky type of equation with periodic boundary conditions, but I’m having some issues with the boundary conditions:

``````@parameters x t
@variables u(..)
Dt = Differential(t)
Dx = Differential(x)
Dxx = Differential(x)^2
Dxxxx = Differential(x)^4

∇(u) = Dx(u)
∇²(u) = Dxx(u)
∇⁴(u) = Dxxxx(u)

# 2D PDE and boundary conditions
eqs  = Dt(u(x,t)) ~ - u(x,t) * ∇(u(x,t)) - ∇²(u(x,t)) - ∇⁴(u(x,t))

T = 1.0
xmin = 0.0
xmax = 1.0

μ0 = 0.3; σ0 = 0.05
f0 = x -> 0.6*exp(-(x - μ0)^2 / (2 * σ0^2))

bcs = [u(x,0.0) ~ f0(x),
u(xmin,t) ~ u(xmax,t)]

# Space and time domains
domains = [t ∈ IntervalDomain(0.0, T),
x ∈ IntervalDomain(xmin, xmax)]

# PDE system
pdesys = PDESystem(eqs, bcs, domains, [x,t], [u(x,t)])

# Method of lines discretization
N = 32
Δx = 1.0/(N+1)
discretization = MOLFiniteDifference([x => Δx], t; centered_order = 4)

# Convert the PDE problem into an ODE problem
prob = discretize(pdesys, discretization)

# Solving the problem
sol = solve(prob, Rodas4(autodiff=false); abstol=1e-6, reltol=1e-6)
``````

But I got the error `ERROR: InvalidSystemException: The system is unbalanced. There are 34 highest order derivative variables and 33 equations.` after running the `discretize` function.

A different set of BCs, such as:

``````bcs = [u(x,0.0) ~ f0(x),
u(xmin,t) ~ 0,
u(xmax,t) ~ 0]
``````

seems to be working fine. I wonder whether it’s the way I specified the periodic BCs was wrong. Any help is appreciated!

Are you using MethodOfLines.jl, or an older version of DiffEqOperators? I recommend retrying with the latest version of MethodOfLines

No I’m actually using the DiffEqOperators.

Please retry with MethodOfLines, the DiffEqOperators version has been moved out and expanded upon there, with better periodic condition handling.

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I see I’ll try it out and see what happens. Thanks!

Let me know how it goes!

MethodOfLines worked! Thanks again!

Glad to hear! Please give us a star on github if you’ve got time

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