Change tangential BCs and add rotating Ref test

Author: weymouth

src/Flow.jl

@@ -62,12 +62,10 @@ upperBoundary!(r,u,Φ,ν,i,j,N,::Val{true}) = @loop r[I-δ(j,I),i] -= Φ[CIj(j,I
 """
     accelerate!(r,t,g,U)
 
-Adds a space-time acceleration field g(i,x,t) and velocity field U(i,x,t) such that `rᵢ += gᵢ(i,x,t)+dUᵢ(i,x,t)/dt` at time `t=sum(dt)` to vector field `r`.
+Accounts for applied and reference-frame acceleration using `rᵢ += g(i,x,t)+dU(i,x,t)/dt`
 """
 accelerate!(r,t,::Nothing,::Union{Nothing,Tuple}) = nothing
-accelerate!(r,t,f) = for i ∈ 1:last(size(r))
-    @loop r[I,i] += f(i,loc(i,I,eltype(r)),t) over I ∈ CartesianIndices(Base.front(size(r)))
-end
+accelerate!(r,t,f::Function) = @loop r[Ii] += f(last(Ii),loc(Ii,eltype(r)),t) over Ii ∈ CartesianIndices(r)
 accelerate!(r,t,g::Function,::Union{Nothing,Tuple}) = accelerate!(r,t,g)
 accelerate!(r,t,::Nothing,U::Function) = accelerate!(r,t,(i,x,t)->ForwardDiff.derivative(τ->U(i,x,τ),t))
 accelerate!(r,t,g::Function,U::Function) = accelerate!(r,t,(i,x,t)->g(i,x,t)+ForwardDiff.derivative(τ->U(i,x,τ),t))

src/util.jl

@@ -203,8 +203,8 @@ function BC!(a,u_BC::Function,saveexit=false,perdir=(),t=0)
                 end
                 (!saveexit || i>1) && (@loop a[I,i] = u_BC(i,loc(i,I),t) over I ∈ slice(N,N[j],j)) # overwrite exit
             else    # Tangential directions, Neumann
-                @loop a[I,i] = a[I+δ(j,I),i] over I ∈ slice(N,1,j)
-                @loop a[I,i] = a[I-δ(j,I),i] over I ∈ slice(N,N[j],j)
+                @loop a[I,i] = u_BC(i,loc(i,I),t)+a[I+δ(j,I),i]-u_BC(i,loc(i,I+δ(j,I)),t) over I ∈ slice(N,1,j)
+                @loop a[I,i] = u_BC(i,loc(i,I),t)+a[I-δ(j,I),i]-u_BC(i,loc(i,I-δ(j,I)),t) over I ∈ slice(N,N[j],j)
             end
         end
     end

test/maintests.jl

@@ -358,6 +358,26 @@ end
         @test GPUArrays.@allowscalar all(sim.flow.u[1,:,1] .≈ sim.flow.u[end,:,1])
     end
 end
+
+@testset "Rotating reference frame" begin
+    function rotating_reference(N,x₀::SVector{2,T},ω::T,mem=Array) where T
+        function velocity(i,x,t)
+            s,c = sincos(ω*t); y = ω*(x-x₀)
+            i==1 ? s*y[1]+c*y[2] : -c*y[1]+s*y[2]
+        end
+        coriolis(i,x,t) = i==1 ? 2ω*velocity(2,x,t) : -2ω*velocity(1,x,t)
+        centrifugal(i,x,t) = ω^2*(x-x₀)[i]
+        g(i,x,t) = coriolis(i,x,t)+centrifugal(i,x,t)
+        udf(a::Flow,t) = WaterLily.@loop a.f[Ii] += g(last(Ii),loc(Ii,eltype(a.f)),t) over Ii in CartesianIndices(a.f)
+        simg = Simulation((N,N),velocity,N; g, U=1, T, mem) # use g
+        simg,Simulation((N,N),velocity,N; U=1, T, mem),udf
+    end
+    L = 4
+    simg,sim,udf = rotating_reference(2L,SA_F64[L,L],1/L,Array)
+    sim_step!(simg);sim_step!(sim;udf)
+    @test L₂(simg.flow.p)==L₂(simg.flow.p)<3e-3 # should be zero
+end
+
 @testset "Circle in accelerating flow" begin
     for f ∈ arrays
         make_accel_circle(radius=32,H=16) = Simulation(radius.*(2H,2H),