Coming from GFSSP or SINDA/FLUINT

OpenFLUME solves the same class of problem as NASA's GFSSP and C&R Technologies' SINDA/FLUINT: networks of control volumes joined by flow elements, thermally coupled to lumped solid structure. Most of what you know from either code carries over; the differences and gaps are listed below.

Descriptions of the other codes use only their general, widely documented vocabulary. The OpenFLUME side of each mapping follows the user's manual.

No input-deck import

OpenFLUME cannot read GFSSP or SINDA/FLUINT input files. Rebuild the model in the editor or programmatically (see Rebuilding a model below).

Concept mapping: GFSSP

GFSSP	OpenFLUME	Notes
Boundary node	Boundary node (type: "boundary")	P plus T (or quality, for real fluids) imposed. Time histories via pressureSchedule / temperatureSchedule (manual §3.7).
Internal node	Internal node (type: "internal")	Needs a positive volume in transient runs; in steady runs, volume is ignored (§3.2).
Branch	Branch with a component	One mass-flow unknown per branch. Positive ṁ runs from → to (§1.4).
Resistance options (pipe, restriction, valve, pump, fittings, …)	Component types (19; Appendix A)	pipe (Darcy–Weisbach + Swamee–Jain), orifice (with gas expansibility and choking), valve (area·Cd·position, optional schedule), pump (ΔP–Q curve), checkValve, dynamicCheckValve, reliefValve, regulator, bend, areaChange, resistance (K), cavitatingVenturi, flowSource, heatedPipe.
Tabulated / user-defined resistance	dpTable, customResistance (constant K or K(Re)), userComponent	§4.3.16, §4.3.5, §4.3.17.
Solid node	Solid node	Lumped m·cp. cp can be a constant, a catalogue material, a table, an expression, or a time table (§3.4).
Ambient node	Ambient node	Fixed-temperature reservoir, optionally scheduled.
Solid-to-solid conductor	Conduction conductor (k, A, L)	k may be temperature-dependent or a catalogue material.
Solid-to-fluid conductor	Convection conductor	Exactly one fluid endpoint. Fixed h or a correlation: Dittus–Boelter, Miropolskii, Darr–Hartwig, TT-WF, custom expression (§4.4).
Solid-to-ambient convection	Conduction conductor sized so kA/L = hA	A convection conductor needs a fluid endpoint, so an h·A film to an ambient node is written as an equivalent conduction tie. Manual §3.6 uses conductances like this to clamp walls.
Radiation conductor	Radiation conductor (ε, A, view factor)	Endpoints are solid or ambient nodes.
Fluid inertia option	Pipe inertia (transient only)	Lumped (L/A)·dṁ/dt (§4.5.1). Trapped gas uses the internal node's gasCushion (§4.5.2).
User subroutines	User components (JavaScript), registers, logic rules, PID controllers, custom-h expressions	usercode.md. Code is trusted, not sandboxed.
Multiple fluids	Named fluids in fluids as isolated continua	Unlike fluids never mix at a node. They couple only through solid walls (§3.9). The single exception is a CEA reacting junction (§3.9.2).

Concept mapping: SINDA/FLUINT

SINDA/FLUINT	OpenFLUME	Notes
Tank (lump with volume)	Internal node with volume	Mass and internal-energy storage are integrated in time.
Junction (zero-volume lump)	Internal node	Fine in steady mode. In transient mode every internal node needs a positive volume, so there is no true zero-volume junction. Give it a small volume.
Plenum	Boundary node
Tube (path with inertia)	pipe with inertia: true	Transient only. Set Initial flow guess to the real starting flow, because for an inertial pipe it is the t = 0 condition (§6.5).
Connector / STUBE (quasi-steady path)	pipe without inertia, or another component	Branch momentum is quasi-steady by default.
Connector device models (losses, valves, pumps, check valves)	resistance, customResistance, valve, checkValve, pump, …	Match the ΔP(ṁ) relation (Appendix A, §4.3).
Tie (lump ↔ thermal node)	Convection conductor	Coupled implicitly: the fluid and solid energy residuals carry the same heat rate (§3.5).
Diffusion node	Solid node with mass and cp
Arithmetic node (no capacitance)	Solid node	Steady: mass/cp are not required. Transient: every solid node needs a positive mass, so give it a small one, which also gives it a short time constant.
Boundary node	Ambient node
Heater / source	heatInput on a solid or fluid node	Positive heat is added to the node (§1.4).
Linear conductor	Conduction conductor
Radiation conductor	Radiation conductor
Logic blocks	Registers, logic rules (including stop rules), PID controllers, user components	§3.8. Logic can write registers and stop a run, but cannot change component settings directly.
Twinned tanks (LTWIN)	No equivalent	Emulated with registers and logic in the LH2 tank no-vent fill example (manual §7.13).
Moveable ties (PUTTIE)	No equivalent	Conductors bind to two fixed node ids. Emulated in the same example by computing tie heat in logic and applying it as heatInput.

What is the same

• Staggered node/branch formulation. Mass and energy are conserved at nodes and momentum is solved on branches (manual §1.2), so junctions with any number of branches need no special handling.
• You choose the spatial resolution. One branch has one flow rate; to resolve a gradient along a line, discretize it yourself. Repeat… and Split… (§3.13) build such chains.
• Implicit solution. Steady state uses coupled Newton–Raphson with pseudo-transient continuation and trust-region globalization for hard cases. Transients use fully implicit backward Euler with fixed or adaptive steps (§4.6).
• Some GFSSP conventions are followed deliberately. The mass flux passed to convection correlations uses the GFSSP convention (§4.4), the default momentum-flux faces are "GFSSP-style" limited-upwind (§4.1.2), and Miropolskii film boiling is the correlation "as used by GFSSP for chilldown" (§8.5).

What is different

• It runs locally in a browser. The solver runs in a Web Worker, nothing is uploaded, and there is no license server. The same core runs in Node.js and from Python (pip install openflume; python.md).
• SI only in storage. Models, .fn files, and the scripting API are SI with no unit metadata. The Units ▾ presets (SI, Metric engineering, US customary) affect only what property fields display and accept (§1.3); the sweep workspace and Model Text always take SI.
• Text model files. .fn is a line-oriented, lossless text projection of the model (Appendix C) that can be diffed and kept in version control.
• Real fluids from CoolProp. 124 HEOS fluids (fluid-catalogue.md) sit alongside incompressible, ideal-gas, and thermal-expansion liquid models. Different equation-of-state classes can coexist in one network when coupled only through a wall.
• Provenance. Every run hashes the model that produced it, and results become stale as soon as the model changes (§8.7).
• Exploration tools. Single-parameter sweeps (§6.9), named simulation variants in the model file (§6.12), and run overlays (§6.8).
• Open source (MIT). You can read and change the closures; correlation constants are exposed in Setup → Physics → Closure calibration.

What is missing

These are listed in manual §1.7 or follow from the validation rules. Check them against your model before rebuilding.

• No shock capture. Quasi-1-D compressible duct flow (Fanno and Rayleigh choking, seeded converging–diverging nozzles) is available as an opt-in, with no Rankine–Hugoniot jump and no way to compute thrust.
• No acoustic wave propagation. Lumped inertia captures bulk surge, but water-hammer wave timing and reflections cannot be modeled.
• Two-phase flow is HEM only. No slip, flow-regime map, or Lockhart–Martinelli multiplier; pressure drop at high void fraction is under-predicted.
• Cavitation is checked only at cavitatingVenturi.
• Turbomachinery is a pump curve. No compressor or turbine maps and no shaft-work term in the energy equation.
• No general mixing of unlike fluids outside a declared CEA reacting junction. Species transport exists only for transient ideal-gas networks (§3.9).
• No zero-volume nodes in transient runs (fluid or solid); see the junction and arithmetic-node rows above.
• No twinned tanks or moveable ties. They can be emulated with logic, as in the LH₂ fill example, at some effort and with explicit coupling (lagged one step).
• Solid–fluid coupling at very high NTU per node. The LH₂ example's module header (src/ui/lh2StorageTank.ts) documents heat-exchanger ducts at NTU ≈ 29 per node that the segregated solid–fluid coupling could not solve, which is why that example's vapor-cooled shield and internal heat exchanger are not resolved as flow paths.
• Steady closed ideal-gas loops need a pressure anchor (a boundary node) or a transient run.
• No input-deck import from either code.

OpenFLUME is alpha software (0.3.1) by a single author, built with heavy AI assistance. Every validation report is regenerated from live solves.

Rebuilding a model

1. Inventory. List every lump or node, every path or branch, every thermal node, and every conductor or tie. Note where the original relies on a feature from the "missing" list above.

2. Choose the fluid model for each continuum in Setup → Fluids: a CoolProp name where the original used a property library, incompressible or ideal-gas where it used constant properties.

3. Translate each flow element by its ΔP(ṁ) relation, not its name. Manual §4.3 gives the governing equation for every component. If nothing fits, use dpTable (tabulated ΔP versus ṁ) or customResistance (K versus Re).

4. Convert to SI and check sign conventions: branch direction, elevationChange = z_to − z_from, and positive heat added to a node (§1.4).

5. Build it. Small models are quickest in the editor (manual §6.4; see the feed-system tutorial). For large or repetitive models, generate the configuration in a script; the schema is plain JSON (manual §3.0). From Python, for example:

   import openflume as of
   
   N = 10
   nodes = [{"id": "in", "type": "boundary", "x": 0, "y": 0,
             "pressure": 3e5, "temperature": 300}]
   nodes += [{"id": f"n{i}", "type": "internal", "x": 100 * i, "y": 0,
              "pressure": 2e5, "temperature": 300, "volume": 1e-4}
             for i in range(1, N)]
   nodes += [{"id": "out", "type": "boundary", "x": 100 * N, "y": 0,
              "pressure": 1e5, "temperature": 300}]
   ids = [n["id"] for n in nodes]
   branches = [{"id": f"p{i + 1}", "from": ids[i], "to": ids[i + 1],
                "component": {"type": "pipe", "length": 1.0,
                              "diameter": 0.02, "roughness": 1e-5}}
               for i in range(N)]
   
   net = of.Network.from_dict({
       "meta": {"name": "Line", "version": 2},
       "settings": {"mode": "steady", "tolerance": 1e-6,
                    "maxIterations": 200, "relaxation": 0.9},
       "fluid": {"model": "incompressible", "preset": "water"},
       "nodes": nodes,
       "branches": branches,
   })
   assert net.validate() == []
   net.save("line.fn")            # open in the web app with Load

6. Verify before comparing. Run a sub-case with a known answer, such as one branch against its hand calculation, before comparing the full model with the original code. Expect differences where the closures differ (friction or convection correlation, property source).

Code-to-code benchmarks

These published cases have been rebuilt and compared, and each serves as a template for the matching kind of model:

Case	Reference	Where
Compressible duct flow, 5 cases (Fanno, Rayleigh, combined, CD nozzle)	GFSSP TFAWS-2007 paper (NTRS 20070036728)	Compressible-flow report
Water-water counterflow heat exchanger	GFSSP Example 5 (v5 manual)	Example Water-water counterflow heat exchanger; manual §7.9, §8.3; architecture mirrored in the thermal-network report
Entrapped-air line	Lee & Martin, GFSSP Figure 10, plus independent RK4 rigid column	Example Entrapped-air line; manual §7.10; fluid-transient report
Conduction rod with convection	GFSSP Example 13 (NASA/TM-2011-216470 §6.13)	Manual §8.3
N₂–N₂ counterflow heat exchanger	JANNAF 2024, Majumdar & LeClair	Manual §8.3
LH₂ no-vent tank fill	SINDA/FLUINT Sample Problem F (TVS)	Example LH2 tank no-vent fill; manual §7.13
Cryogenic line chilldown	NBS Report 9264 (experiment); GFSSP predictions; SINDA/FLUINT validation case	Example Cryogenic line cooldown; manual §7.11, §8.4; chilldown tutorial

Manual §8.3 lists each case's tolerances. The GFSSP Example 5 outlet temperatures are reproduced to 0.44 K (hot) and 0.19 K (cold), and the SINDA/FLUINT Sample F fill fraction, pressure, vapor temperature, and vent flow agree within about 1 % at 15 minutes. Agreement with another code supports the implementation but inherits that code's assumptions (§8.1).