Fix positivity preserving WENO

src/Advection/positivity_preserving_tracer_advection_operators.jl

@@ -7,6 +7,8 @@ const ε₂ = 1e-20
 # Here in the future we can easily add UpwindBiased
 const BoundPreservingScheme = PositiveWENO
 
+@inline div_Uc(i, j, k, grid, advection::BoundPreservingScheme, U, ::ZeroField) = zero(grid)
+
 # Is this immersed-boundary safe without having to extend it in ImmersedBoundaries.jl? I think so... (velocity on immmersed boundaries is masked to 0)
 @inline function div_Uc(i, j, k, grid, advection::BoundPreservingScheme, U, c)
 

src/Advection/weno_reconstruction.jl

@@ -104,6 +104,10 @@ function WENO(FT::DataType=Float64;
 
     mod(order, 2) == 0 && throw(ArgumentError("WENO reconstruction scheme is defined only for odd orders"))
 
+    if !isnothing(bounds)
+        @warn "Bounds preserving WENO is experimental."
+    end
+
     if order < 3
         # WENO(order=1) is equivalent to UpwindBiased(order=1)
         return UpwindBiased(FT; order=1)

test/test_immersed_advection.jl

@@ -12,7 +12,8 @@ using Oceananigans.Advection:
         _biased_interpolate_yᵃᶠᵃ,
         FluxFormAdvection
 
-advection_schemes = [Centered, UpwindBiased, WENO]
+linear_advection_schemes = [Centered, UpwindBiased]
+advection_schemes = [linear_advection_schemes... WENO]
 
 @inline advective_order(buffer, ::Type{Centered}) = buffer * 2
 @inline advective_order(buffer, AdvectionType)    = buffer * 2 - 1
@@ -103,12 +104,19 @@ for arch in archs
         mask_immersed_field!(c)
         fill_halo_regions!(c)
 
-        for adv in advection_schemes, buffer in [1, 2, 3, 4, 5]
+        for adv in linear_advection_schemes, buffer in [1, 2, 3, 4, 5]
             scheme = adv(order = advective_order(buffer, adv))
 
             @info "  Testing immersed tracer reconstruction [$(typeof(arch)), $(summary(scheme))]"
             run_tracer_interpolation_test(c, ibg, scheme)
         end
+
+        for buffer in [2, 3, 4, 5], bounds in (nothing, (0, 1))
+            scheme = WENO(; order = advective_order(buffer, WENO), bounds)
+
+            @info "  Testing immersed tracer reconstruction [$(typeof(arch)), $(summary(scheme))]"
+            run_tracer_interpolation_test(c, ibg, scheme)
+        end
     end
 
     @testset "Immersed tracer conservation" begin