rustytorch/crates/specialized/rtx-cfd/tests/lbm_d3q19_tests.rs

//! Tests for D3Q19 Lattice Boltzmann Method implementation

use approx::assert_relative_eq;
use nalgebra::Vector3;
use rtx_cfd::solvers::lbm::{D3Q19Parameters, D3Q19Solver};

#[test]
fn test_d3q19_lattice_velocities() {
    let solver = D3Q19Solver::new(10, 10, 10, D3Q19Parameters::default());
    let velocities = solver.lattice_velocities();

    // D3Q19 should have 19 velocities
    assert_eq!(velocities.len(), 19);

    // Check specific velocity directions
    assert_eq!(velocities[0], Vector3::new(0, 0, 0)); // Rest
    assert_eq!(velocities[1], Vector3::new(1, 0, 0)); // +X
    assert_eq!(velocities[2], Vector3::new(-1, 0, 0)); // -X
    assert_eq!(velocities[3], Vector3::new(0, 1, 0)); // +Y
    assert_eq!(velocities[4], Vector3::new(0, -1, 0)); // -Y
    assert_eq!(velocities[5], Vector3::new(0, 0, 1)); // +Z
    assert_eq!(velocities[6], Vector3::new(0, 0, -1)); // -Z
}

#[test]
fn test_d3q19_weights() {
    let solver = D3Q19Solver::new(10, 10, 10, D3Q19Parameters::default());
    let weights = solver.weights();

    // D3Q19 should have 19 weights
    assert_eq!(weights.len(), 19);

    // Check weight values
    assert_relative_eq!(weights[0], 1.0 / 3.0, epsilon = 1e-12); // Rest particle
    assert_relative_eq!(weights[1], 1.0 / 18.0, epsilon = 1e-12); // Face neighbors
    assert_relative_eq!(weights[7], 1.0 / 36.0, epsilon = 1e-12); // Edge neighbors

    // Weights should sum to 1
    let sum: f64 = weights.iter().sum();
    assert_relative_eq!(sum, 1.0, epsilon = 1e-12);
}

#[test]
fn test_d3q19_equilibrium_distribution() {
    let solver = D3Q19Solver::new(10, 10, 10, D3Q19Parameters::default());
    let density = 1.0;
    let velocity = Vector3::new(0.1, 0.05, 0.02);

    let f_eq = solver.equilibrium_distribution(density, &velocity);

    // Should have 19 components
    assert_eq!(f_eq.len(), 19);

    // All components should be positive
    for &val in &f_eq {
        assert!(
            val > 0.0,
            "Equilibrium distribution component should be positive"
        );
    }

    // Sum should equal density
    let sum: f64 = f_eq.iter().sum();
    assert_relative_eq!(sum, density, epsilon = 1e-12);
}

#[test]
fn test_d3q19_equilibrium_at_rest() {
    let solver = D3Q19Solver::new(10, 10, 10, D3Q19Parameters::default());
    let density = 1.0;
    let velocity = Vector3::zeros();

    let f_eq = solver.equilibrium_distribution(density, &velocity);
    let weights = solver.weights();

    // For zero velocity, equilibrium should be density * weight
    for i in 0..19 {
        assert_relative_eq!(f_eq[i], density * weights[i], epsilon = 1e-12);
    }
}

#[test]
fn test_d3q19_macroscopic_variables() {
    let mut solver = D3Q19Solver::new(10, 10, 10, D3Q19Parameters::default());

    // Initialize with known state
    let density = 1.2;
    let velocity = Vector3::new(0.1, -0.05, 0.03);

    // Set distribution functions to equilibrium
    let f_eq = solver.equilibrium_distribution(density, &velocity);
    solver.set_distribution_at(5, 5, 5, &f_eq);

    // Extract macroscopic variables
    let macro_vars = solver.macroscopic_variables_at(5, 5, 5);

    assert_relative_eq!(macro_vars.density, density, epsilon = 1e-12);
    assert_relative_eq!(macro_vars.velocity.x, velocity.x, epsilon = 1e-12);
    assert_relative_eq!(macro_vars.velocity.y, velocity.y, epsilon = 1e-12);
    assert_relative_eq!(macro_vars.velocity.z, velocity.z, epsilon = 1e-12);
}

#[test]
fn test_d3q19_bgk_collision() {
    let mut solver = D3Q19Solver::new(10, 10, 10, D3Q19Parameters::new(0.6));

    // Initialize all cells with equilibrium
    let density = 1.0;
    let velocity = Vector3::new(0.1, 0.0, 0.0);
    solver.initialize_uniform(density, velocity);

    // Get original equilibrium distribution
    let f_eq = solver.equilibrium_distribution(density, &velocity);

    // Perturb from equilibrium by reducing a component
    let mut f = f_eq.clone();
    f[1] *= 0.8; // Reduce from equilibrium (80% of equilibrium value)

    solver.set_distribution_at(5, 5, 5, &f);

    let initial_value = f[1];

    // Apply collision step
    solver.collision_step();

    // Check that distribution moves toward equilibrium
    let f_after = solver.distribution_at(5, 5, 5);

    // With BGK collision: f_new = f - omega * (f - f_eq)
    // Since f[1] < f_eq[1], (f - f_eq) is negative, so f_new should increase
    assert!(
        f_after[1] > initial_value,
        "BGK should move towards equilibrium: {} > {}",
        f_after[1],
        initial_value
    );

    // Check that it's moving in the right direction (don't check exact value due to neighboring cell effects)
    let direction_to_equilibrium = f_eq[1] - initial_value;
    let actual_change = f_after[1] - initial_value;
    assert!(
        direction_to_equilibrium > 0.0 && actual_change > 0.0,
        "Should move towards equilibrium"
    );
}

#[test]
fn test_d3q19_streaming_step() {
    let mut solver = D3Q19Solver::new(5, 5, 5, D3Q19Parameters::default());

    // Initialize central cell with specific distribution
    let mut f = vec![0.0; 19];
    f[1] = 1.0; // Only +X component
    solver.set_distribution_at(2, 2, 2, &f);

    // Apply streaming step
    solver.streaming_step();

    // Check that the distribution has moved in +X direction
    let f_east = solver.distribution_at(3, 2, 2);
    assert_relative_eq!(f_east[1], 1.0, epsilon = 1e-12);

    // Original cell should have zero +X component
    let f_original = solver.distribution_at(2, 2, 2);
    assert_relative_eq!(f_original[1], 0.0, epsilon = 1e-12);
}

#[test]
fn test_d3q19_mass_conservation() {
    let nx = 8;
    let ny = 8;
    let nz = 8;
    let mut solver = D3Q19Solver::new(nx, ny, nz, D3Q19Parameters::default());

    // Initialize with uniform density
    solver.initialize_uniform(1.0, Vector3::new(0.05, 0.02, 0.01));

    let initial_mass = solver.total_mass();

    // Run simulation for several steps with periodic boundaries
    for _ in 0..50 {
        solver.step_periodic();
    }

    let final_mass = solver.total_mass();

    // Mass should be conserved
    assert_relative_eq!(final_mass, initial_mass, epsilon = 1e-12);
}

#[test]
fn test_d3q19_parameters_validation() {
    // Valid parameters
    let valid_params = D3Q19Parameters::new(0.6);
    assert!(valid_params.validate().is_ok());

    // Invalid relaxation time (too small)
    let invalid_params = D3Q19Parameters::new(0.4);
    assert!(invalid_params.validate().is_err());

    // Invalid relaxation time (too large)
    let invalid_params = D3Q19Parameters::new(2.1);
    assert!(invalid_params.validate().is_err());
}