Forced-dissipative barotropic QG beta-plane turbulence

A simulation of forced-dissipative barotropic quasi-geostrophic turbulence on a beta plane. The dynamics include linear drag and stochastic excitation.

Install dependencies

First let's make sure we have all required packages installed.

using Pkg
pkg"add GeophysicalFlows, CUDA, JLD2, CairoMakie, Statistics"

Let's begin

Let's load GeophysicalFlows.jl and some other packages we need.

using GeophysicalFlows, CUDA, JLD2, CairoMakie, Random, Printf

using Statistics: mean
using LinearAlgebra: ldiv!

parsevalsum = FourierFlows.parsevalsum
record = CairoMakie.record                # disambiguate between CairoMakie.record and CUDA.record

Choosing a device: CPU or GPU

dev = CPU()     # Device (CPU/GPU)

Numerical parameters and time-stepping parameters

      n = 128            # 2D resolution: n² grid points
stepper = "FilteredRK4"  # timestepper
     dt = 0.05           # timestep
 nsteps = 8000          # total number of timesteps
 save_substeps = 10      # number of timesteps after which output is saved

Physical parameters

L = 2π        # domain size
β = 10.0      # planetary PV gradient
μ = 0.01      # bottom drag

Forcing

We force the vorticity equation with stochastic excitation that is delta-correlated in time and while spatially homogeneously and isotropically correlated. The forcing has a spectrum with power in a ring in wavenumber space of radius $k_f$ (forcing_wavenumber) and width $δ_f$ (forcing_bandwidth), and it injects energy per unit area and per unit time equal to $\varepsilon$. That is, the forcing covariance spectrum is proportional to $\exp{[-(|\bm{k}| - k_f)^2 / (2 δ_f^2)]}$.

forcing_wavenumber = 14.0 * 2π/L  # the forcing wavenumber, `k_f`, for a spectrum that is a ring in wavenumber space
forcing_bandwidth  = 1.5  * 2π/L  # the width of the forcing spectrum, `δ_f`
ε = 0.001                         # energy input rate by the forcing

grid = TwoDGrid(dev; nx=n, Lx=L)

K = @. sqrt(grid.Krsq)            # a 2D array with the total wavenumber

forcing_spectrum = @. exp(-(K - forcing_wavenumber)^2 / (2 * forcing_bandwidth^2))
@CUDA.allowscalar forcing_spectrum[grid.Krsq .== 0] .= 0 # ensure forcing has zero domain-average

ε0 = parsevalsum(forcing_spectrum .* grid.invKrsq / 2, grid) / (grid.Lx * grid.Ly)
@. forcing_spectrum *= ε/ε0       # normalize forcing to inject energy at rate ε

We reset of the random number generator for reproducibility

if dev==CPU(); Random.seed!(1234); else; CUDA.seed!(1234); end

Next we construct function calcF! that computes a forcing realization every timestep. For that, we call randn! to obtain complex numbers whose real and imaginary part are normally-distributed with zero mean and variance 1/2.

function calcF!(Fh, sol, t, clock, vars, params, grid)
  randn!(Fh)
  @. Fh *= sqrt(forcing_spectrum) / sqrt(clock.dt)
  return nothing
end

Problem setup

We initialize a Problem by providing a set of keyword arguments. We use stepper = "FilteredRK4". Filtered timesteppers apply a wavenumber-filter at every time-step that removes enstrophy at high wavenumbers and, thereby, stabilize the problem, despite that we use the default viscosity coefficient ν=0.

prob = SingleLayerQG.Problem(dev; nx=n, Lx=L, β, μ, dt, stepper,
                             calcF=calcF!, stochastic=true)

Let's define some shortcuts.

sol, clock, vars, params, grid = prob.sol, prob.clock, prob.vars, prob.params, prob.grid
x,  y  = grid.x,  grid.y
Lx, Ly = grid.Lx, grid.Ly

First let's see how a forcing realization looks like. Note that when plotting, we decorate the variable to be plotted with Array() to make sure it is brought back on the CPU when vars live on the GPU.

calcF!(vars.Fh, sol, 0.0, clock, vars, params, grid)

fig = Figure()

ax = Axis(fig[1, 1],
          xlabel = "x",
          ylabel = "y",
          aspect = 1,
          title = "a forcing realization",
          limits = ((-Lx/2, Lx/2), (-Ly/2, Ly/2)))

heatmap!(ax, x, y, Array(irfft(vars.Fh, grid.nx));
         colormap = :balance, colorrange = (-8, 8))

fig

Setting initial conditions

Our initial condition is simply fluid at rest.

SingleLayerQG.set_q!(prob, device_array(dev)(zeros(grid.nx, grid.ny)))

Diagnostics

Create Diagnostic – energy and enstrophy are functions imported at the top.

E = Diagnostic(SingleLayerQG.energy, prob; nsteps, freq=save_substeps)
Z = Diagnostic(SingleLayerQG.enstrophy, prob; nsteps, freq=save_substeps)
diags = [E, Z] # A list of Diagnostics types passed to "stepforward!" will  be updated every timestep.

Output

We choose folder for outputing .jld2 files and snapshots (.png files).

filepath = "."
plotpath = "./plots_forcedbetaturb"
plotname = "snapshots"
filename = joinpath(filepath, "singlelayerqg_forcedbeta.jld2")

Do some basic file management,

if isfile(filename); rm(filename); end
if !isdir(plotpath); mkdir(plotpath); end

and then create Output.

get_sol(prob) = Array(prob.sol) # extracts the Fourier-transformed solution

function get_u(prob)
  vars, grid, sol = prob.vars, prob.grid, prob.sol

  @. vars.qh = sol

  SingleLayerQG.streamfunctionfrompv!(vars.ψh, vars.qh, params, grid)

  ldiv!(vars.u, grid.rfftplan, -im * grid.l .* vars.ψh)

  return Array(vars.u)
end

output = Output(prob, filename, (:qh, get_sol), (:u, get_u))

We first save the problem's grid and other parameters so we can use them later.

saveproblem(output)

and then call saveoutput(output) once to save the initial state.

saveoutput(output)

Time-stepping the Problem forward

We time-step the Problem forward in time.

startwalltime = time()

while clock.step <= nsteps
  if clock.step % 50save_substeps == 0
    cfl = clock.dt * maximum([maximum(vars.u) / grid.dx, maximum(vars.v) / grid.dy])

    log = @sprintf("step: %04d, t: %d, cfl: %.2f, E: %.4f, Q: %.4f, walltime: %.2f min",
    clock.step, clock.t, cfl, E.data[E.i], Z.data[Z.i], (time()-startwalltime)/60)

    println(log)
  end

  stepforward!(prob, diags, save_substeps)
  SingleLayerQG.updatevars!(prob)

  if clock.step % save_substeps == 0
    saveoutput(output)
  end
end

savediagnostic(E, "energy", output.path)
savediagnostic(Z, "enstrophy", output.path)

step: 0000, t: 0, cfl: 0.00, E: 0.0000, Q: 0.0000, walltime: 0.00 min
step: 0500, t: 25, cfl: 0.60, E: 0.0192, Q: 2.7431, walltime: 0.04 min
step: 1000, t: 50, cfl: 0.67, E: 0.0313, Q: 2.9683, walltime: 0.06 min
step: 1500, t: 75, cfl: 0.70, E: 0.0377, Q: 2.8375, walltime: 0.08 min
step: 2000, t: 100, cfl: 0.75, E: 0.0406, Q: 2.7452, walltime: 0.10 min
step: 2500, t: 125, cfl: 0.76, E: 0.0436, Q: 2.8703, walltime: 0.12 min
step: 3000, t: 150, cfl: 0.91, E: 0.0453, Q: 2.7849, walltime: 0.14 min
step: 3500, t: 175, cfl: 0.74, E: 0.0449, Q: 2.6579, walltime: 0.16 min
step: 4000, t: 200, cfl: 0.81, E: 0.0454, Q: 2.7357, walltime: 0.18 min
step: 4500, t: 225, cfl: 0.77, E: 0.0451, Q: 2.5594, walltime: 0.20 min
step: 5000, t: 250, cfl: 0.77, E: 0.0458, Q: 2.6418, walltime: 0.23 min
step: 5500, t: 275, cfl: 0.71, E: 0.0455, Q: 2.6214, walltime: 0.25 min
step: 6000, t: 300, cfl: 0.77, E: 0.0462, Q: 2.7571, walltime: 0.27 min
step: 6500, t: 325, cfl: 0.86, E: 0.0465, Q: 2.7127, walltime: 0.29 min
step: 7000, t: 350, cfl: 0.83, E: 0.0463, Q: 2.5924, walltime: 0.31 min
step: 7500, t: 375, cfl: 0.90, E: 0.0463, Q: 2.6251, walltime: 0.34 min
step: 8000, t: 400, cfl: 0.87, E: 0.0450, Q: 2.4965, walltime: 0.36 min

Load saved output and visualize

We now have output from our simulation saved in singlelayerqg_forcedbeta.jld2 which we can load to create a time series for the fields we are interested in.

file = jldopen(output.path)

iterations = parse.(Int, keys(file["snapshots/t"]))
t = [file["snapshots/t/$i"] for i ∈ iterations]

qh = [file["snapshots/qh/$i"] for i ∈ iterations]
u  = [file["snapshots/u/$i"] for i ∈ iterations]

E_t = file["diagnostics/energy/t"]
Z_t = file["diagnostics/enstrophy/t"]
E_data = file["diagnostics/energy/data"]
Z_data = file["diagnostics/enstrophy/data"]

x,  y  = file["grid/x"],  file["grid/y"]
nx, ny = file["grid/nx"], file["grid/ny"]
Lx, Ly = file["grid/Lx"], file["grid/Ly"]

close(file)

We create a figure using Makie's Observables

n = Observable(1)

qₙ = @lift irfft(qh[$n], nx)
ψₙ = @lift irfft(- Array(grid.invKrsq) .* qh[$n], nx)
q̄ₙ = @lift real(ifft(qh[$n][1, :] / ny))
ūₙ = @lift vec(mean(u[$n], dims=1))

title_q = @lift @sprintf("vorticity, μt = %.2f", μ * t[$n])

energy    = Observable([Point2f(E_t[1], E_data[1])])
enstrophy = Observable([Point2f(Z_t[1], Z_data[1])])

fig = Figure(resolution=(1000, 600))

axis_kwargs = (xlabel = "x",
               ylabel = "y",
               aspect = 1,
               limits = ((-Lx/2, Lx/2), (-Ly/2, Ly/2)))

axq = Axis(fig[1, 1]; title = title_q, axis_kwargs...)

axψ = Axis(fig[2, 1]; title = "streamfunction ψ", axis_kwargs...)

axq̄ = Axis(fig[1, 2],
           xlabel = "zonal mean vorticity",
           ylabel = "y",
           aspect = 1,
           limits = ((-3, 3), (-Ly/2, Ly/2)))

axū = Axis(fig[2, 2],
           xlabel = "zonal mean u",
           ylabel = "y",
           aspect = 1,
           limits = ((-0.5, 0.5), (-Ly/2, Ly/2)))

axE = Axis(fig[1, 3],
           xlabel = "μ t",
           ylabel = "energy",
           aspect = 1,
           limits = ((-0.1, 4.1), (0, 0.055)))

axZ = Axis(fig[2, 3],
           xlabel = "μ t",
           ylabel = "enstrophy",
           aspect = 1,
           limits = ((-0.1, 4.1), (0, 3.1)))

heatmap!(axq, x, y, qₙ;
         colormap = :balance, colorrange = (-8, 8))

levels = collect(-0.32:0.04:0.32)

contourf!(axψ, x, y, ψₙ;
          levels, colormap = :viridis, colorrange = (-0.22, 0.22))
contour!(axψ, x, y, ψₙ;
         levels, color = :black)

lines!(axq̄, q̄ₙ, y; linewidth = 3)
lines!(axq̄, 0y, y; linewidth = 1, linestyle=:dash)

lines!(axū, ūₙ, y; linewidth = 3)
lines!(axū, 0y, y; linewidth = 1, linestyle=:dash)

lines!(axE, energy; linewidth = 3)
lines!(axZ, enstrophy; linewidth = 3, color = :red)

fig

We are now ready to animate all saved output.

frames = 1:length(t)
record(fig, "singlelayerqg_betaforced.mp4", frames, framerate = 16) do i
  n[] = i

  energy[]    = push!(energy[],    Point2f(μ * E_t[i], E_data[i]))
  enstrophy[] = push!(enstrophy[], Point2f(μ * Z_t[i], Z_data[i]))
end

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