Julia GPU FSI: Updating p0, v0, a0, t0 Each Step Without Non-isbits (Core.Box) Error

General Usage
1

using WaterLily
using Plots; gr()
using StaticArrays
using BenchmarkTools
using CSV, DataFrames
using Printf
using CUDA

function TwoCylWIV(Ur,mem=Array)

Re = 150; U = 1
p = 6; L = 2^p
nl = 12L; ml = 8L

# signed distance function to circle
radius = L/2
diameter = 2*radius

# fsi parameters
rho = 1.0 # density of fluid
mₐ = π*radius^2*rho # displacement mass
m = 2*mₐ # structural mass
fn = U/Ur/diameter # natural frequency Ur = U/(fn*D)
k = 4*π^2*m*fn^2 # spring constant

#first cylinder
centerFix = SA[2L,ml/2-1];
circleFix = AutoBody((x,t)->√sum(abs2, x .- centerFix) - radius)

#second cylinder
center = SA[6L,ml/2-1];
# initial condition FSI
p0=radius/3; v0=0; a0=0;  t0=0
# motion function uses global var to adjust
posx(t) = p0 + (t-t0)*v0
# motion definition
map(x,t) = x - SA[0, posx(t)]
# make a body
circle = AutoBody((x,t)->√sum(abs2, x .- center) - radius,map)

@show circle
bodies = AutoBody[]
push!(bodies, circleFix)
push!(bodies, circle)
# combine into one body
bodyCir = circleFix+circle
# @show bodyCir
#bodyCir = Bodies(bodies, repeat([identity], length(bodies)-1))

# generate sim
sim = Simulation((nl,ml), (U,0), diameter; ν=U*diameter/Re, body=bodyCir, mem=CuArray)


# get start time
duration=160; step=0.1; t₀=round(sim_time(sim))
t_series = Float64[]
p_series = Float64[]
cl_series = Float64[]
cd_series = Float64[]

@gif for tᵢ in range(t₀,t₀+duration;step)
    # update until time tᵢ in the background
    t = sum(sim.flow.Δt[1:end-1])
    while t < tᵢ*sim.L/sim.U
        # measure body
        measure!(sim,t)
        # update flow
        mom_step!(sim.flow,sim.pois)

        # pressure force
        forcePre = -WaterLily.pressure_force(sim.flow, sim.body)
        forceVis = -WaterLily.viscous_force(sim.flow, sim.body)
        force = forcePre + forceVis


        # compute motion and acceleration 1DOF
        Δt = sim.flow.Δt[end]
        accel = (force[2]- k*p0)/m
        p0 += Δt*(v0+Δt*accel/2.)
        v0 += Δt*accel
        a0 = accel

        # update time, sets the pos/v0 correctly
        t0 = t; t += Δt

        push!(t_series, t0)
        push!(p_series, p0/diameter)
        push!(cd_series, force[1]./(0.5sim.L*sim.U^2))
        push!(cl_series, force[2]./(0.5sim.L*sim.U^2))
    end
    # print time step
    @inside sim.flow.σ[I] = WaterLily.curl(3,I,sim.flow.u)*sim.L/sim.U
    flood(sim.flow.σ; shift=(-0.5,-0.5),clims=(-5,5))
    body_plot!(sim); plot!(title="tU/L $tᵢ")

    # print time step
    println("tU/L=",round(tᵢ,digits=4),", Δt=",round(sim.flow.Δt[end],digits=3))

    println("tU/L=",round(tᵢ,digits=4),", Δt=",round(sim.flow.Δt[end],digits=3),", AyD=",round(p0 / diameter,digits=3))

end


# Primary y-axis: displacement vs time
p = plot(t_series, p_series,
label = "Displacement",
xlabel = "Time",
ylabel = "Displacement",
legend = :topleft,
lw = 2)

display(p)
filename = @sprintf("output/01wiv_plot_Ur%.1f.png", Ur) |> x -> replace(x, "." => "_")
filename = filename |> x -> replace(x, "_png" => ".png")
savefig(p, filename)

df = DataFrame(
time = t_series,
displacement = p_series,
lift = cl_series,
drag = cd_series)
filename = @sprintf("output/01wiv_data_Ur%.1f.csv", Ur) |> x -> replace(x, "." => "_")
filename = filename |> x -> replace(x, "_csv" => ".csv")
CSV.write(filename, df)

#return t_series, p_series, cl_series, cd_series

end

elapsed = @elapsed TwoCylWIV(2)
println(“Elapsed time: $(elapsed) seconds”)

The above is my code. I need to update p0, v0, a0, and t0 at every time step, and define the body’s motion based on these variables. However, when using GPU computation, these variables appear as Core.Box types rather than isbits types. How should I modify my code to enable GPU computation? I look forward to your response.