How should I set boundary conditions of velocity for 2D channel flow?

[Leezhard]

Hi all,
I am currently doing a case study of 2D channel flow within x-z domain (with polymer stress and thermal stratification). Here is the code I wrote:

using Oceananigans
using Oceananigans.Units: minutes, hour, hours, day
using Oceananigans.BoundaryConditions: NoFluxBoundaryCondition

Nx = 64
Ny = 64
x_min = 0
x_max = 1
z_min = -1
z_max = 1

grid = RectilinearGrid(size=(Nx, Ny), x=(x_min, x_max), z=(z_min, z_max), topology=(Bounded, Flat, Bounded))

const dx = (x_max - x_min) / Nx
const dz = (z_max - z_min) / Ny

############################## Initial conditions ##############################

U0 = BackgroundField((x, z, t) -> 1-z^2)
W0 = BackgroundField((x, z, t) -> 0.0)

############################## Boundary conditions ##############################
# u
u_bcs = FieldBoundaryConditions(
    west   = ValueBoundaryCondition((x, z, t) -> 1 - z^2),
    east   = NoFluxBoundaryCondition(),        
    bottom = ValueBoundaryCondition(0.0),            
    top    = ValueBoundaryCondition(0.0)                
)

# w
w_bcs = FieldBoundaryConditions(
    west   = ValueBoundaryCondition(0.0), 
    east   = NoFluxBoundaryCondition(),
    bottom = ValueBoundaryCondition(0.0),  
    top    = ValueBoundaryCondition(0.0) 
)
# ############################## Model ##############################

const Re = 10000
ν = 1/Re
closure = ScalarDiffusivity(ν = ν)

model = NonhydrostaticModel(
    grid = grid,
    advection = WENO(),
    timestepper = :RungeKutta3,
    closure = closure,
    buoyancy = nothing,
    boundary_conditions = (u = u_bcs, w = w_bcs),
    background_fields = (u = U0, w = W0)
)

# Check model
println(model)

I got error:

ERROR: ArgumentError: Cannot specify east boundary condition FluxBoundaryCondition: Nothing on a field at Face()!

Ideally, an uniform flow of U0 = 1-z^2 comes in at west boundary and comes out at east boundary. Top and bottom boundaries are walls so non-slip conditon should apply there.

I am looking forward to your response and appreciate your support.

[glwagner]

Open boundaries are a bit tricky. You either need to use OpenBoundaryCondition or try with Periodic topology and use a sponger layer. Try searching PRs and issues to find more information about open boundaries. It's experimental so there is no official documentation.

[Leezhard]

Hi Gregory, thank you for your quick response. I will search and try. Also I am looking forward to the new boundary condition settings in oceananigans.