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src-local/geometry.h

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geometry.h

Geometry helpers and initialization macros for drop impact simulations.

Provides

  • Drop geometry calculations
  • Initial grid refinement
  • Initial condition setup
  • Hooks for custom shapes

Author

Vatsal Sanjay ([email protected]) CoMPhy Lab, Durham University

#ifndef GEOMETRY_H
#define GEOMETRY_H

#include "params.h"

drop_distance_squared()

Squared distance from the drop center in axisymmetric coordinates.

Parameters

  • x: radial coordinate
  • y: axial coordinate
  • p: parameter structure containing drop_x, drop_y

Returns

Squared distance from the drop center.

Implemented as a macro for compatibility with Basilisk’s qcc preprocessor.

#define drop_distance_squared(x, y, p) \
    ((x - (p)->drop_x) * (x - (p)->drop_x) + (y - (p)->drop_y) * (y - (p)->drop_y))

is_inside_drop()

Return 1 if a point is inside the drop, 0 otherwise.

Parameters

  • x: radial coordinate
  • y: axial coordinate
  • p: parameter structure
static inline int is_inside_drop(double x, double y, const struct SimulationParams *p) {
    double r_sq = drop_distance_squared(x, y, p);
    double radius_sq = p->drop_radius * p->drop_radius;
    return (r_sq < radius_sq) ? 1 : 0;
}

Initialization Macros

These are macros rather than functions because they use Basilisk grid traversal constructs (refine, fraction, foreach) that must be expanded in the calling context.

REFINE_INITIAL_GRID()

Refine the initial grid around the drop interface. The refinement region extends slightly beyond the drop radius for a smoother interface.

Usage

REFINE_INITIAL_GRID(params);
#define REFINE_INITIAL_GRID(p) do { \
    const double _margin = 1.05; \
    const double _refine_r_sq = _margin * _margin * (p)->drop_radius * (p)->drop_radius; \
    refine(drop_distance_squared(x, y, (p)) < _refine_r_sq && level < (p)->MAXlevel); \
    fprintf(stderr, "Initial grid refinement complete (MAXlevel = %d)\n", (p)->MAXlevel); \
} while(0)

SETUP_INITIAL_DROP()

Initialize the VOF field and velocity field for the drop: - f = 1 inside the drop, 0 outside - u.x set to the impact velocity inside the drop - u.y set to zero everywhere

Usage

SETUP_INITIAL_DROP(params);
#define SETUP_INITIAL_DROP(p) do { \
    fraction(f, (p)->drop_radius * (p)->drop_radius - drop_distance_squared(x, y, (p))); \
    foreach() { \
        u.x[] = (p)->impact_velocity * f[]; \
        u.y[] = 0.0; \
    } \
    fprintf(stderr, "Initial drop setup complete:\n"); \
    fprintf(stderr, "  Drop center: (%g, %g)\n", (p)->drop_x, (p)->drop_y); \
    fprintf(stderr, "  Drop radius: %g\n", (p)->drop_radius); \
    fprintf(stderr, "  Impact velocity: %g\n", (p)->impact_velocity); \
} while(0)

Custom Shapes (optional)

Enable ENABLE_CUSTOM_SHAPES to supply a custom shape function for the drop. Potential extensions include ellipsoids, deformed drops, or multiple drops.

#ifdef ENABLE_CUSTOM_SHAPES
typedef double (*ShapeFunction)(double x, double y, const struct SimulationParams *p);

static inline void setup_custom_drop(ShapeFunction shape_func, const struct SimulationParams *p) {
    fraction(f, shape_func(x, y, p));

    foreach() {
        u.x[] = p->impact_velocity * f[];
        u.y[] = 0.0;
    }
}
#endif

#endif // GEOMETRY_H