docs/parameters.md

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