Add test for composite rheology

Author: danshapero

test/composite_rheology_test.py

@@ -0,0 +1,377 @@
+import pytest
+import numpy as np
+import sympy
+from sympy import lambdify, simplify, diff
+import firedrake
+from firedrake import (
+    as_vector,
+    max_value,
+    Constant,
+    Function,
+    derivative,
+    NonlinearVariationalProblem,
+    NonlinearVariationalSolver,
+    DirichletBC,
+)
+from icepack2 import constants
+from icepack2.model.minimization import (
+    viscous_power,
+    friction_power,
+    calving_terminus,
+    momentum_balance,
+    ice_shelf_momentum_balance,
+)
+
+
+sparams = {
+    "solver_parameters": {
+        "snes_type": "newtonls",
+        "snes_max_it": 200,
+        "snes_linesearch_type": "nleqerr",
+        "ksp_type": "gmres",
+        "pc_type": "lu",
+        "pc_factor_mat_solver_type": "mumps",
+    },
+}
+
+
+@pytest.mark.parametrize("degree", (1, 2))
+def test_composite_rheology_floating(degree):
+    lx, ly = 20e3, 20e3
+    Lx, Ly = Constant(lx), Constant(ly)
+    h0, dh = Constant(500.0), Constant(100.0)
+    u_inflow = Constant(100.0)
+
+    ρ_I = Constant(constants.ice_density)
+    ρ_W = Constant(constants.water_density)
+    g = Constant(constants.gravity)
+
+    # Use a combination of two different rheology exponents.
+    n_1 = firedrake.Constant(1.0)
+    n_3 = firedrake.Constant(constants.glen_flow_law)
+
+    τ_c = Constant(0.1)
+    ε_c = Constant(0.01)
+    A_1 = ε_c / τ_c ** n_1
+    A_3 = ε_c / τ_c ** n_3
+
+    # This is an exact solution for the velocity of a floating ice shelf with
+    # constant temperature and linearly decreasing thickness. See Greve and
+    # Blatter for the derivation.
+    def exact_δu(x, n, A):
+        ρ = ρ_I * (1 - ρ_I / ρ_W)
+        h = h0 - dh * x / Lx
+        P = ρ * g * h / 4
+        P_0 = ρ * g * h0 / 4
+        δP = ρ * g * dh / 4
+        return Lx * A * (P_0 ** (n + 1) - P ** (n + 1)) / ((n + 1) * δP)
+
+    errors, mesh_sizes = [], []
+    k_min, k_max, num_steps = 5 - degree, 8 - degree, 9
+    for nx in np.logspace(k_min, k_max, num_steps, base=2, dtype=int):
+        mesh = firedrake.RectangleMesh(nx, nx, lx, ly, diagonal="crossed")
+        x, y = firedrake.SpatialCoordinate(mesh)
+
+        # Make some function spaces.
+        cg = firedrake.FiniteElement("CG", "triangle", degree)
+        dg = firedrake.FiniteElement("DG", "triangle", degree - 1)
+        Q = firedrake.FunctionSpace(mesh, cg)
+        V = firedrake.VectorFunctionSpace(mesh, cg)
+        Σ = firedrake.TensorFunctionSpace(mesh, dg, symmetry=True)
+        Z = V * Σ
+        z = Function(Z)
+        z.sub(0).assign(Constant(u_inflow, 0))
+
+        expr = u_inflow + exact_δu(x, n_1, A_1) + exact_δu(x, n_3, A_3)
+        u_exact = Function(V).interpolate(as_vector((expr, 0)))
+
+        h = Function(Q).interpolate(h0 - dh * x / Lx)
+
+        # Make the boundary conditions.
+        inflow_ids = (1,)
+        outflow_ids = (2,)
+        side_wall_ids = (3, 4)
+
+        inflow_bc = DirichletBC(Z.sub(0), Constant((u_inflow, 0)), inflow_ids)
+        side_wall_bc = DirichletBC(Z.sub(0).sub(1), 0, side_wall_ids)
+        bcs = [inflow_bc, side_wall_bc]
+
+        # Make the model specification and solver.
+        u, M = firedrake.split(z)
+        fields = {"velocity": u, "membrane_stress": M, "thickness": h}
+        boundary_ids = {"outflow_ids": outflow_ids}
+
+        L = (
+            viscous_power(**fields, flow_law_exponent=n_1, flow_law_coefficient=A_1) +
+            viscous_power(**fields, flow_law_exponent=n_3, flow_law_coefficient=A_3) +
+            ice_shelf_momentum_balance(**fields, **boundary_ids)
+        )
+
+        F = derivative(L, z)
+        qdegree = max(8, degree ** constants.glen_flow_law)
+        pparams = {"form_compiler_parameters": {"quadrature_degree": qdegree}}
+        problem = NonlinearVariationalProblem(F, z, bcs, **pparams)
+        solver = NonlinearVariationalSolver(problem, **sparams)
+
+        # Solve the problem using a continuation method -- step the flow law
+        # exponent from 1 to 3 in small intervals.
+        num_steps = 5
+        for exponent in np.linspace(1.0, constants.glen_flow_law, num_steps):
+            n_3.assign(exponent)
+            solver.solve()
+
+        u, M = z.subfunctions
+        error = firedrake.norm(u - u_exact) / firedrake.norm(u_exact)
+        δx = mesh.cell_sizes.dat.data_ro.min()
+        mesh_sizes.append(δx)
+        errors.append(error)
+        print(".", end="", flush=True)
+
+    log_mesh_sizes = np.log2(np.array(mesh_sizes))
+    log_errors = np.log2(np.array(errors))
+    slope, intercept = np.polyfit(log_mesh_sizes, log_errors, 1)
+    print(f"degree {degree}: log(error) ~= {slope:g} * log(dx) {intercept:+g}")
+    assert slope > degree + 0.9
+
+
+def test_manufactured_solution():
+    L, ρ_I, ρ_W, g = sympy.symbols("L ρ_I ρ_W g", real=True, positive=True)
+    A_1, A_3 = sympy.symbols("A_1 A_3", real=True, positive=True)
+    n_1, n_3 = sympy.symbols("n_1 n_3", integer=True, positive=True)
+
+    def strain_rate(M, A_1, A_3, n_1, n_3):
+        return A_1 * (M / 2)**n_1 + A_3 * (M / 2)**n_3
+
+    def sliding_velocity(τ, K_1, K_3, m_1, m_3):
+        return -K_1 * τ**m_1 - K_3 * τ**m_3
+
+    def stress_balance(x, h, s, M, τ):
+        return diff(h * M, x) + τ - ρ_I * g * h * diff(s, x)
+
+    def boundary_condition(x, u, h, s, M):
+        d = (s - h).subs(x, L)
+        τ_c = (ρ_I * g * h**2 - ρ_W * g * d**2) / 2
+        return (h * M - τ_c).subs(x, L)
+
+    # Create the thickness and surface elevation
+    x = sympy.symbols("x", real=True)
+    h0, δh = sympy.symbols("h0 δh", real=True, positive=True)
+    h = h0 - δh * x / L
+
+    s0, δs = sympy.symbols("s0 δs", real=True, positive=True)
+    s = s0 - δs * x / L
+
+    # The variable `β` is the fraction of the driving stress that membrane
+    # stress divergence will take up; this is chosen to eliminate some extra
+    # terms and make the rest of the algebra simpler. See the website also.
+    h_L = h0 - δh
+    s_L = s0 - δs
+    β = δh / δs * (ρ_I * h_L ** 2 - ρ_W * (s_L - h_L) ** 2) / (ρ_I * h_L**2)
+
+    ρ = β * ρ_I * δs / δh
+    P = ρ * g * h / 4
+    δP = ρ * g * δh / 4
+    P0 = ρ * g * h0 / 4
+
+    M = ρ * g * h / 2
+    τ = -(1 - β) * ρ_I * g * h * δs / L
+
+    # The exact velocity field looks similar to the floating case by using a
+    # careful choice of `β`.
+    u_0 = sympy.symbols("u_0", real=True, positive=True)
+    δu_1 = L * A_1 * (P0 ** (n_1 + 1) - P ** (n_1 + 1)) / ((n_1 + 1) * δP)
+    δu_3 = L * A_3 * (P0 ** (n_3 + 1) - P ** (n_3 + 1)) / ((n_3 + 1) * δP)
+    u = u_0 + δu_1 + δu_3
+
+    # Make the linear part of the rheology a much smaller effect than the
+    # nonlinear part -- at a stress of 100kPa, the nonlinear rheology gives
+    # a strain rate of 10 m/yr/km, while the linear part only 1 m/yr/km.
+    ε_1 = 0.001
+    ε_3 = 0.01
+    τ_c = 0.1
+
+    values = {
+        u_0: 100,
+        h0: 500,
+        δh: 100,
+        s0: 150,
+        δs: 90,
+        L: 20e3,
+        A_1: ε_1 / τ_c,
+        A_3: ε_3 / τ_c ** constants.glen_flow_law,
+        ρ_I: constants.ice_density,
+        ρ_W: constants.water_density,
+        n_1: 1,
+        n_3: constants.glen_flow_law,
+        g: constants.gravity,
+    }
+
+    # Check the momentum conservation law and boundary condition.
+    τ_b = lambdify(x, τ.subs(values), "numpy")
+    τ_d = lambdify(x, (-ρ_I * g * h * diff(s, x)).subs(values), "numpy")
+    τ_m = lambdify(x, simplify(diff(h * M, x)).subs(values), "numpy")
+    xs = np.linspace(0, values[L], 21)
+
+    tolerance = 1e-8
+    assert abs(boundary_condition(x, u, h, s, M).subs(values)) < tolerance
+
+    stress_norm = np.max(np.abs(τ_d(xs)))
+    assert np.max(np.abs(τ_m(xs) + τ_b(xs) + τ_d(xs))) < tolerance * stress_norm
+
+    # Check the constitutive relation.
+    ε_u = lambdify(x, simplify(diff(u, x).subs(values)), "numpy")
+    ε_M = lambdify(
+        x, simplify(strain_rate(M, A_1, A_3, n_1, n_3).subs(values)), "numpy"
+    )
+    assert np.max(np.abs(ε_u(xs) - ε_M(xs))) < tolerance * np.max(np.abs(ε_u(xs)))
+
+    # Check the sliding law.
+    α = 0.1
+    m_1 = 1
+    m_3 = constants.glen_flow_law
+    K_1 = α * u / abs(τ) ** m_1
+    K_3 = (u - K_1 * abs(τ) ** m_1) / abs(τ)**m_3
+
+    U = lambdify(x, simplify(u).subs(values), "numpy")
+    U_b = lambdify(
+        x, simplify(sliding_velocity(τ, K_1, K_3, m_1, m_3).subs(values)), "numpy"
+    )
+    assert np.max(np.abs(U(xs) - U_b(xs))) < tolerance * np.max(np.abs(U(xs)))
+
+
+@pytest.mark.parametrize("degree", (1, 2))
+def test_composite_rheology_grounded(degree):
+    lx, ly = 20e3, 20e3
+    Lx, Ly = Constant(lx), Constant(ly)
+    h0, dh = Constant(500.0), Constant(100.0)
+    s0, ds = Constant(150.0), Constant(90.0)
+    u_inflow = Constant(100.0)
+
+    # See the previous test for explanations of all the setup.
+    ρ_I = Constant(constants.ice_density)
+    ρ_W = Constant(constants.water_density)
+    g = Constant(constants.gravity)
+
+    n_1 = firedrake.Constant(1.0)
+    n_3 = firedrake.Constant(constants.glen_flow_law)
+
+    m_1 = firedrake.Constant(1.0)
+    m_3 = firedrake.Constant(constants.weertman_sliding_law)
+
+    τ_c = Constant(0.1)
+    ε_1 = Constant(0.001)
+    ε_3 = Constant(0.01)
+    A_1 = ε_1 / τ_c ** n_1
+    A_3 = ε_3 / τ_c ** n_3
+
+    h_L = h0 - dh
+    s_L = s0 - ds
+    β = dh / ds * (ρ_I * h_L**2 - ρ_W * (s_L - h_L)**2) / (ρ_I * h_L**2)
+
+    def exact_δu(x, n, A):
+        ρ = β * ρ_I * ds / dh
+        h = h0 - dh * x / Lx
+        P = ρ * g * h / 4
+        dP = ρ * g * dh / 4
+        P0 = ρ * g * h0 / 4
+        du = Lx * A * (P0 ** (n + 1) - P ** (n + 1)) / ((n + 1) * dP)
+        return du
+
+    errors, mesh_sizes = [], []
+    k_min, k_max, num_steps = 5 - degree, 8 - degree, 9
+    for nx in np.logspace(k_min, k_max, num_steps, base=2, dtype=int):
+        mesh = firedrake.RectangleMesh(nx, nx, lx, ly, diagonal="crossed")
+        x, y = firedrake.SpatialCoordinate(mesh)
+
+        # Make some function spaces.
+        cg = firedrake.FiniteElement("CG", "triangle", degree)
+        dg = firedrake.FiniteElement("DG", "triangle", degree - 1)
+        Q = firedrake.FunctionSpace(mesh, cg)
+        V = firedrake.VectorFunctionSpace(mesh, cg)
+        Σ = firedrake.TensorFunctionSpace(mesh, dg, symmetry=True)
+        T = firedrake.VectorFunctionSpace(mesh, dg)
+        Z = V * Σ * T
+        z = Function(Z)
+        z.sub(0).assign(Constant((u_inflow, 0)))
+
+        # Make the exact velocity, thickness, surface elevation, and sliding
+        # coefficients.
+        u_expr = u_inflow + exact_δu(x, n_1, A_1) + exact_δu(x, n_3, A_3)
+        u_exact = Function(V).interpolate(as_vector((u_expr, 0)))
+
+        h = Function(Q).interpolate(h0 - dh * x / Lx)
+        s = Function(Q).interpolate(s0 - ds * x / Lx)
+
+        α = Constant(0.1)
+        τ_expr = (1 - β) * ρ_I * g * h * ds / Lx
+        K_1 = α * u_expr / τ_expr ** m_1
+        K_3 = (u_expr - K_1 * τ_expr ** m_1) / τ_expr ** m_3
+
+        # Create the boundary conditions.
+        inflow_ids = (1,)
+        outflow_ids = (2,)
+        side_wall_ids = (3, 4)
+
+        inflow_bc = DirichletBC(Z.sub(0), Constant((u_inflow, 0)), inflow_ids)
+        side_wall_bc = DirichletBC(Z.sub(0).sub(1), 0, side_wall_ids)
+        bcs = [inflow_bc, side_wall_bc]
+
+        # Create the model specification and solvers.
+        u, M, τ = firedrake.split(z)
+        fields = {
+            "velocity": u,
+            "membrane_stress": M,
+            "basal_stress": τ,
+            "thickness": h,
+            "surface": s,
+        }
+
+        rheology_1 = {
+            "flow_law_exponent": n_1,
+            "flow_law_coefficient": A_1,
+            "sliding_exponent": m_1,
+            "sliding_coefficient": K_1,
+        }
+
+        rheology_3 = {
+            "flow_law_exponent": n_3,
+            "flow_law_coefficient": A_3,
+            "sliding_exponent": m_3,
+            "sliding_coefficient": K_3,
+        }
+
+        boundary_ids = {"outflow_ids": outflow_ids}
+        L = (
+            viscous_power(**fields, **rheology_1) +
+            viscous_power(**fields, **rheology_3) +
+            friction_power(**fields, **rheology_1) +
+            friction_power(**fields, **rheology_3) +
+            calving_terminus(**fields, **boundary_ids) +
+            momentum_balance(**fields)
+        )
+
+        F = derivative(L, z)
+        qdegree = max(8, degree ** constants.glen_flow_law)
+        pparams = {"form_compiler_parameters": {"quadrature_degree": qdegree}}
+        problem = NonlinearVariationalProblem(F, z, bcs, **pparams)
+        solver = NonlinearVariationalSolver(problem, **sparams)
+
+        # Solve the problem using a continuation method.
+        num_steps = 5
+        for exponent in np.linspace(1.0, constants.glen_flow_law, num_steps):
+            n_3.assign(exponent)
+            m_3.assign(exponent)
+            solver.solve()
+
+        u, M, τ = z.subfunctions
+        error = firedrake.norm(u - u_exact) / firedrake.norm(u_exact)
+        δx = mesh.cell_sizes.dat.data_ro.min()
+        mesh_sizes.append(δx)
+        errors.append(error)
+        print(".", end="", flush=True)
+
+    log_mesh_sizes = np.log2(np.array(mesh_sizes))
+    log_errors = np.log2(np.array(errors))
+    slope, intercept = np.polyfit(log_mesh_sizes, log_errors, 1)
+    print(f"degree {degree}: log(error) ~= {slope:g} * log(dx) {intercept:+g}")
+    assert slope > degree + 0.9