# Parameter reference

These are the variables the UI exposes. They live in each example's
`in.sdpd_phase.2d` as `variable NAME equal X` lines and are patched at run
time when the user (or agent) supplies overrides.

## Run

| Name | Type | Meaning | Sensible range |
|------|------|---------|----------------|
| `prodRun` | int | Production-phase timesteps | 100 (smoke) — 250 000 (publication) |
| `preRun` | int | Equilibration timesteps before production | 0 — 20 000 |
| `preRunTemp` | int | Initial temperature-equilibration steps | 0 — 1000 |
| `dt` | float | Integration timestep | 1e-5 — 1e-3, smaller for stiffer EOS / smaller dh |

## Output

| Name | Type | Meaning |
|------|------|---------|
| `fqTher` | int | Print thermo line every N steps |
| `fqDump` | int | Write trajectory dump every N steps |
| `fqRes` | int | Write restart file every N steps |
| `fqV` | int | Re-apply velocity boundary conditions every N steps |

For a 100-step smoke test use `fqTher=10, fqDump=50` so you actually get
some output. For a 250 000-step production run, `fqTher=1000, fqDump=5000`
keeps disk usage in check.

## Geometry

| Name | Type | Meaning |
|------|------|---------|
| `dim` | int | 2 or 3 (every bundled example is 2) |
| `dh` | float | Particle spacing in lattice units. Smaller = finer = slower |
| `coreL` | float | Edge length of the simulation box |
| `dhN` | int | dh × dhN = sphere/droplet radius |
| `diaNx`, `diaNy` | int | Droplet diameter multipliers in X/Y |
| `squareL` | float | Half-length of a square sub-region |
| `lenNx` | int | Length-times multiplier for square regions |

The kernel cutoff `h = 4·dh` is hard-coded. If you halve `dh` (×2 resolution)
expect ~16× more particles in 2D and 4× more neighbours per particle.

## Fluid properties

| Name | Type | Meaning |
|------|------|---------|
| `rho` | float | Reference fluid density |
| `T` | float | kBT — thermal noise. Validation cases use ~1e-18 (effectively zero) |
| `Tbuf` | float | Temperature in buffer / reservoir regions |

## Multi-phase / Surface tension

| Name | Type | Meaning |
|------|------|---------|
| `sigmao` | float | Reference interfacial tension for the multi-phase term |
| `F` | int (1, 2, or 3) | Selector for the multi-phase free-energy functional |
| `fluid_type` | int | Phase identifier passed to the SDPD-mf pair style |
| `f2Frac` | float (0–1) | Volume fraction of phase 2 (the dispersed phase) |
| `randPos` | int (0/1/2) | 0=single droplet, 1=random, 2=square block |

For a Young–Laplace droplet test, vary `sigmao` and the droplet size
(`dhN`/`diaNx`); the pressure jump should follow Δp = σ/r in 2D.

## Boundary / driving

| Name | Type | Meaning |
|------|------|---------|
| `dVx`, `dVy` | float | Imposed wall velocities (Couette/Poiseuille setup) |
| `dSSx`, `dSSy` | float | Imposed shear strains |

## What's NOT exposed in the UI yet

These live in each example's `micro` file (or `eta_input` / `pair_input` /
`s_input` / `thixo_input` for cases that read them from data files):

- `eta11`, `eta22`, `eta12` — viscosity per type-pair
- `ksai11`, `ksai12` — multi-phase coupling constants
- `s11`, `s22`, `s12` — strength factors
- `wall_index*` — wall non-slip flag per type-pair
- `a_thixo`, `b_thixo`, `alpha_thixo`, `f0_thixo` — thixotropy coefficients

To change these, edit `Numerical examples/<case>/micro` directly (or its
`*_input` files) before running.

## What to vary in an optimization

Good choices for sweeps:

- `dh` (resolution) — affects accuracy and runtime
- `sigmao` (surface tension) — drives droplet shape recovery
- `dt` (timestep) — affects stability margin
- `f2Frac`, `randPos` — initial phase distribution

Not great choices:

- `prodRun` — varying it changes runtime, not physics
- `dim`, `fluid_type`, `F` — categorical selectors, only meaningful at
  specific values; use them as `categorical` not `int` in search space